MSC.Fatigue Quick Start Guide - PDFCOFFEE.COM (2024)

C O N T E N T S MSC.Fatigue QuickStart Guide MSC.Fatigue QuickStart Guide

1 Introduction

Purpose of Guide, 2 ❑ Assumptions, 2 ❑ Organization of Guide, 2

Definitions, 3 ❑ The Fatigue “Five-Box Trick”, 4 ❑ Life Prediction Methods, 5 ❑ FE Analysis Methods, 5 ❑ Design Philosophies, 6 ❑ Life Estimation Process, 7

When to Use Which Method?, 8

Different Methods of FE Import, 9 ❑ MSC.Nastran FE Model and Results, 9 ❑ ABAQUS (or Advanced FEA) FE Results, 11 ❑ ANSYS FE Results, 12 ❑ MSC.Marc FE Results, 12 ❑ PATRAN Neutral and Result Files, 13 ❑ I-DEAS Master Series Universal Files, 19 ❑ MSC.Patran FEA Result Files, 20

Directory Structure, 21

MSC.Fatigue Modules, 22

MSC.Fatigue Output Files, 26

Problem Description, 30 ❑ Objective, 30

Geometry, 31 ❑ Import the Model, 31 ❑ View the Stress Results, 32

Set Up the Fatigue Analysis, 33 ❑ Solution Parameters, 34 ❑ Material Information, 35 ❑ Loading Information, 38

2 A Simple S-N Analysis

Main Index

Run the Fatigue Analysis, 42

Review the Results, 43 ❑ View the Life Contour Plot, 43 ❑ Tabular Listing, 44 ❑ What If?, 44

Concluding Remarks, 46

Multiple Mean Stress Curve Support, 47 ❑ Set Up the Fatigue Analysis, 47 ❑ Run the Fatigue Analysis, 49

Problem Description, 54 ❑ Objective, 54

Set Up the Fatigue Analysis, 55 ❑ Load the Previous S-N Analysis Parameters, 55 ❑ Loading Information, 56

Run the Fatigue Analysis, 60 ❑ Rainflow Cycle Counting, 60 ❑ Damage Summation, 60 ❑ Speeding up the Analysis, 61

Review the Results, 62 ❑ Tabular Listing, 62 ❑ Histogram Matrix, 62 ❑ Effect of Mean Stress, 64 ❑ Probabilistic Nature of Fatigue, 65

Concluding Remarks, 67

Problem Description, 70 ❑ Objective, 70

Geometry, 71 ❑ Import the Model and Results, 71 ❑ View the Stress Results, 72

Set Up the Fatigue Analysis, 73 ❑ Solution Parameters, 74 ❑ Material Information, 74 ❑ Loading Information, 80

3 Rainflow Cycle Counting

4 Component S-N Analysis

Main Index

Run the Fatigue Analysis, 83

Review the Results, 84 ❑ View the Life Contour Plot, 84 ❑ Tabular Listing, 85 ❑ Design Optimization, 85

Concluding Remarks, 88 ❑ Batch Operations, 88

Problem Description, 90 ❑ Objective, 90

Geometry, 91 ❑ Import the Model, 91 ❑ View the Stress Results, 91

Set Up the Fatigue Analysis, 92 ❑ Solution Parameters, 93 ❑ Material Information, 94 ❑ Loading Information, 100

Run the Fatigue Analysis, 104 ❑ Rainflow Cycle Counting, 104 ❑ Elastic-Plastic Correction, 104

Review the Results, 106 ❑ View the Life Contour Plot, 107

Concluding Remarks, 108 ❑ Other Notch Corrections, 108 ❑ Stresses vs. Strains, 108

Problem Description, 112 ❑ Objective, 112

Geometry, 113 ❑ Import the Model, 113 ❑ View the Stress Results, 113

Set Up First Fatigue Analysis, 115 ❑ Solution Parameters, 115 ❑ Material Information, 115 ❑ Loading Information, 116

5 A Simple e-N Analysis

6 Residual Stress

Main Index

❑ ❑

Run the Fatigue Analysis, 117 Review the Results, 117

Set Up Second Fatigue Analysis, 118 ❑ Include the Residual Stress, 118 ❑ Run the Fatigue Analysis, 119 ❑ Review the Results, 119

Investigate Mean Stress, 120

Investigate Surface Finish/Treatment, 123

Concluding Remarks, 125

Problem Description, 128 ❑ Objective, 128

Geometry, 129 ❑ Import the Model, 129 ❑ Define a Compliance Function, 129

Set Up the Fracture Analysis, 133 ❑ Solution Parameters, 134 ❑ Material Information, 135 ❑ Loading Information, 138

Run the Fracture Analysis, 140 ❑ Cycle by Cycle Growth, 140 ❑ The Fatigue Crack Propagation Rectangle, 141

Review the Results, 142 ❑ Tabular Listing, 142 ❑ Interactive Operation, 142 ❑ Optimization, 144

Concluding Remarks, 146 ❑ Analysis without an FE Model, 146 ❑ MSC.Fatigue Files, 147

Problem Description, 150 ❑ Objectives, 150

S-N Analysis of Lug Weld, 152 ❑ Solution Parameters, 152 ❑ Material Information, 152

7 Introduction to Crack Growth

8 Design Philosophies

Main Index

❑ ❑ ❑

Loading Information, 154 Job Control, 154 Results - Factor of Safety Analysis, 154

e-N Analysis of Lug, 157 ❑ Solution Parameters, 157 ❑ Material Information, 157 ❑ Loading Information, 157 ❑ Job Control, 157 ❑ Results, 158

LEFM Analysis of Lug, 159 ❑ Solution Parameters, 159 ❑ Material Information, 161 ❑ Loading Information, 161 ❑ Job Control, 161 ❑ Results, 161

Concluding Remarks, 163

Problem Description, 166 ❑ Objectives, 166

S-N Analysis of Engine Mounting Lug, 168 ❑ Solution Parameters, 168 ❑ Material Information, 168 ❑ Loading Information, 170 ❑ Job Control, 173 ❑ Results, 174

Crack Growth Analysis of Engine Lug, 177 ❑ Solution Parameters, 177 ❑ Material Information, 178 ❑ Loading Information, 179 ❑ Job Control, 180 ❑ Results, 180

Problem Description, 182 ❑ Objectives, 182

Geometry, 184 ❑ Import FE Model and Results, 184 ❑ Post/Create Groups, 185

9 Multiple Loads

10 A Multiaxial Assessment

Main Index

View the Stress Results, 186

Determine the Critical Location, 188 ❑ Solution Parameters, 189 ❑ Material Information, 190 ❑ Loading Information, 190 ❑ Job Control, 194

Evaluate Results, 197 ❑ Biaxiality - a Multiaxial Assessment, 198

Concluding Remarks, 205

Introduction, 210

Problem Description for Spot Weld Analysis with Spot Welds Modeled as Stiff BARS, 211 ❑ Objective, 211

Geometry and FE Results, 212

Define a Group of CBARS, 214

Spot Weld S-N Analysis, 215 ❑ Solution Parameters, 215 ❑ Material Information, 215 ❑ Loading Information, 219 ❑ Job Control, 221 ❑ Results Evaluation, 222

Problem Description for Spot Weld Analysis with Spot Welds Modeled as CWELDS, 226 ❑ Objective, 226 ❑ Reading in the Model and CWELD Results, 226 ❑ Set Up the Spot Weld Analysis, 227 ❑ Run the Spot Weld Analysis, 229

Problem Description for Spot Weld Analysis with Spot Welds Modeled with CHEX/MPC, 230 ❑ Objective, 230 ❑ Reading in the Model and CHEX/MPC Results, 230 ❑ Convert CHEX/MPC to Equivalent BARs, 231 ❑ Set Up the Spot Weld Analysis, 232 ❑ Run the Spot Weld Analysis, 233

Concluding Remarks, 236

Problem Description for a Seam Weld Analysis, 237 ❑ Objective, 237

11 Welding

Main Index

Geometry and FE Results, 238 ❑ Creating a Weld Group, 238 ❑ Viewing the Stress Results, 239

Setting up the Seam Weld Analysis, 240 ❑ Material Information, 241 ❑ Loading Information, 243 ❑ Listing the Fatigue Results, 244

Concluding Remarks, 248

Problem Description, 250

Geometry and FE Results, 251 ❑ Viewing the Stress Results, 252

Setting Up the Wheels Analysis, 254 ❑ Solution Parameters, 254 ❑ Material Information, 254 ❑ Loading Info, 256

Fatigue Analysis and Results, 259 ❑ Plotting the Fatigue Results, 259 ❑ Wheels Interactive Menu, 260

Concluding Remarks, 265

Problem Description, 268 ❑ Objective, 268

Geometry and FE Results, 269 ❑ The Gauge Tool, 269 ❑ FE Results Extraction, 274

Time History Extraction, 276 ❑ Fatigue Analysis Setup, 276 ❑ Solution Parameters, 277 ❑ Material Information, 277 ❑ Loading Information, 279 ❑ Job Control, 281 ❑ Run Soft S/G (SSG), 282

Correlation Techniques, 283 ❑ Overlays and Cross Plots, 283 ❑ Signal Statistics, 284

12 Wheels Module Analysis of Rotating Structures

13 A Software Strain Gauge

Main Index

❑ ❑ ❑

Rosette Analysis, 285 Single Location Uniaxial Life Analyzer, 288 Single Location Multiaxial Life Analyzer, 289

Concluding Remarks, 292

Introduction, 294 ❑ Objective, 294

Analysis Using Transient Results, 296 ❑ Transient Keyhole Job, 296 ❑ Static Keyhole Job, 299

Modal Superposition Method, 300

Vibration Fatigue, 302 ❑ Definitions, 303 ❑ Frequency Domain Life Estimation - General Procedure, 303 ❑ Vibration Fatigue Analysis Setup, 306 ❑ Additional Job Setups - Multiple Load Inputs, 313 ❑ Results, 318

Comparison Studies, 324 ❑ Pseudo-Static vs. PSD Approach, 324 ❑ FE Model and Analysis, 324 ❑ Pseudo-static Fatigue Analysis Setup, 328 ❑ Results, 330 ❑ Dynamic Transient vs. PSD Approach, 336 ❑ Plot Mode Shapes, 337 ❑ Run Vibration Analyses, 337 ❑ Run Transient Analyses, 338 ❑ View and Compare Results, 339 ❑ Recreate the Transfer Function, 341 ❑ Plot the Stationarity, 344 ❑ Random Vibration FE Results, 345

Temperature Corrected MSC.Fatigue Analysis, 348 ❑ Objective, 348 ❑ Import the Model, 349 ❑ Set Up the Fatigue Analysis, 350 ❑ Run the Fatigue Analysis, 354

14 Dynamic Fatigue

15 Temperature Corrected Fatigue Analysis

Main Index

16 Aerospace Spectrum File Support

Aerospace Spectrum File Support, 358 ❑ Objective, 358

File Definitions, 359 ❑ Spectrum file, 359 ❑ Example Spectrum File, 359 ❑ Load Control file, 360 ❑ Example Load Control File, 360

Example Problem, 361 ❑ Solution Parameters, 361 ❑ Material Information, 361 ❑ Loading Information, 362 ❑ Run Fatigue Analysis, 363

Conclusion, 364

Introduction, 366

Example Problem, 367 ❑ Solution Parameters, 367 ❑ Material Information, 367 ❑ Loading Information, 368 ❑ Sequence Information, 369 ❑ Event Information, 370 ❑ Entering Load Data, 371 ❑ Managing the Duty Cycle Form, 374 ❑ Job Control, 375 ❑ Review Results, 375

Conclusion, 377

Problem Description, 380

Fatigue Preprocessing, 381 ❑ Low Damage Removal, 381

Material Management, 383 ❑ Materials Database Manager - PFMAT, 383 ❑ ASCII Materials File Reader, 383

Advanced Loading Utilities, 384

17 Multiple Fatigue Analysis (Duty Cycle Analyzer)

18 Fatigue Utilities

Main Index

❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑

Main Index

Arithmetic Manipulation - MART, 384 Multi-Channel Editor - MCOE, 385 Rainflow Cycle Counter - MCYC, 387 Formula Processor - MFRM, 391 File Cut and Paste - MLEN, 393 Multi-File Manipulation - MMFM, 394 Peak-Valley Extraction - MPVXMUL, 395 Simultaneous Values Analysis DAC/RPC - MSIMMAX, 395 Amplitude Distribution - MADA, 397 Auto Spectral Density - MASD, 398 Fast Fourier Filtering - MFFF, 398 Butterworth Filtration - MBFL, 400 Frequency Response Analysis - MFRA, 400 Statistical Analysis - MRSTATS, 401 Header/Footer Manipulation - MFILMNP, 402

Advanced Fatigue Utilities, 403 ❑ Single Location S-N Analysis - MSLF, 403 ❑ Single Location e-N Analysis - MCLF, 404 ❑ Cycle and Damage Analysis - MCDA, 405 ❑ Cycles File Lister - MCYL, 406 ❑ Time Correlated Damage - MTCD, 407 ❑ Single Location Vibration Fatigue - MFLF, 408 ❑ Stress-Strain Analysis - mSSA, 409 ❑ Multi-Axial Life Analysis - MMLF, 409 ❑ Crack Growth Data Analysis - MFCG, 410 ❑ Kt/Kf Evaluation - MKTAN, 411

Graphical Display Utilities, 412 ❑ Graphical Editing-mGED, 412 ❑ Multi-File Display - mMFD, 412 ❑ Two Parameter Display - mTPD, 414 ❑ Polar Display - mPOD, 414 ❑ Three Dimensional Display - mP3D, 415 ❑ Plot File (.plt) Display - MQPLOT (for UNIX), 415 ❑ Plot File (.plt) Display - MWNPLOT (for Windows), 416 ❑ Printer and Device Setup - MPLTSYS, 416 ❑ Plot/Pen Colors Utility - MNCPENS, 418

File Conversion Utilities, 420 ❑ Convert Binary .dac to ASCII - MDTA and Convert ASCII to Binary .dac - MATD, 420 ❑ Signal Regeneration - MREGEN, 420 ❑ Convert RPC File to .dac - MREMDAC and Convert .dac to RPC file - MDACREM, 421 ❑ Cross Platform Conversion - MCONFIL, 422 ❑ Waterfall File Create - MWFLCRE, 422

Other Utilities, 423 ❑ Environment Settings - MENM, 423

Problem Description, 426

Element Centroidal Calculations, 427

Group Averaging, 428

Extracting Time Histories, 429

Identify Critical Location, 430

Defining Histogram Matrices, 431

Constant Amplitude Zero Mean Time Histories, 434

Glossary Terms, 436

Material Types, 456

Material Listing, 459

Alternative Names, 467

Where to Get Help, 474 ❑ Accessing Help from a Form, 474

Technical Support Centers, 475 ❑ Preparing to Call the Hotline, 476

MSC Offices, 477

19 Miscellaneous Features

A Glossary of Terms

B Material Listing

C Support

INDEX

Main Index

MSC.Fatigue QuickStart Guide, 479

Main Index

MSC.Fatigue QuickStart Guide

CHAPTER

1

Introduction

■ Purpose of Guide ■ Definitions ■ When to Use Which Method? ■ Different Methods of FE Import ■ MSC.Fatigue Modules ■ Directory Structure

Main Index

2

1.1

Purpose of Guide Welcome to MSC.Fatigue. MSC.Fatigue is an advanced fatigue life estimation program for use with finite element analysis. When used early in a development design cycle it is possible to greatly enhance product life as well as reduce testing and prototype costs, thus ensuring greater speed to market. It is jointly developed in close cooperation between MSC.Software Corporation and its fatigue technology partner, nCode International, Ltd. of Sheffield, England. The purpose of this manual is to provide you with typical example problems to demonstrate proper usage of the program. Each example is designed to show certain aspects and help to convey various principles of fatigue life estimation. The intent is to get you up to speed as quickly as possible without a steep learning curve or hours sifting through a thick manual.

Assumptions This guide makes certain assumptions of the reader. The basic assumptions made are, a good knowledge of basic computer skills and terminology, and a working knowledge of finite element analysis. This manual does not deal with creation of finite element models or any aspects of actual finite element analyses except where necessary to achieve proper fatigue life estimations. This manual assumes that the user has little or no experience with fatigue analysis in general and therefore makes every effort to explain principles of fatigue life estimation from example to example. It is not meant to be an exhaustive course on fatigue analysis however. For this we refer you to the MSC.Fatigue User’s Guide and the many references sited therein.

Organization of Guide All chapters but this one, serve as tutorials to learn the basics of MSC.Fatigue. First read this chapter in its entirety and then after a successful installation, it is highly suggested that you start at the first example and work your way sequentially. Each exercise introduces concepts that build on each other from exercise to exercise.

Main Index

CHAPTER 1 Introduction

1.2

Definitions The first concept to understand before embarking on this tutorial is the definition of the term fatigue within the confines of this guide. Very often the terms fatigue, fracture, and durability are used interchangeably. Each does, however, convey a specific meaning. Note: Throughout this manual, when a new term or concept is introduced or mentioned for the first time, it is highlighted in blue italics. This means that a definition is provided in Glossary of Terms (App. A). What is Fatigue? Although many definitions can be applied to the word, for the purposes of this manual, fatigue is failure under a repeated or otherwise varying load which never reaches a level sufficient to cause failure in a single application. It can also be thought of as the initiation and growth of a crack, or growth from a preexisting defect, until it reaches a critical size, such as separation into two or more parts. Fatigue analysis itself usually refers to one of two methodologies: either the stresslife or S-N method, commonly referred to as total life since it makes no distinction between initiating or growing a crack, or the local strain or strain-life (ε-N) method, commonly referred to as the crack initiation method which concerns itself only with the initiation of a crack. Fracture specifically concerns itself with the growth or propagation of a crack once it has initiated. Durability is then the conglomeration of all aspects that affect the life of a product and usually involves much more than just fatigue and fracture, but also loading conditions, environmental concerns, material characterizations, and testing simulations to name a few. A true product durability program in an organization takes all of these aspects (and more) into consideration. Note: Fatigue cracks initiate and grow as a result of cyclic plastic deformation. Without plasticity there can be no fatigue failure. All attempts are made in this guide to explain how plasticity is taken into account when determining fatigue life from linear elastic finite element analysis.

Main Index

3

4

The Fatigue “Five-Box Trick” Almost without exception, each exercise is constructed around the concept of the fatigue “five-box trick.” The illustration to the right depicts this well. For any life analysis whether it be fatigue or fracture there are always three inputs. The first three boxes are these inputs:

Materials

Loading

The Fatigue “Five-Box Trick”

Analysis

Results

Geometry

1. Cyclic Material Information: Materials behave differently when they are subject to cyclic as opposed to monotonic loading. Monotonic material properties are the result of material tests where the load is steadily increased until the test coupon breaks. Cyclic material parameters are obtained from material tests where the loading is reversed and cycled until failure at various load levels. These parameters differ depending on the fatigue analysis type involved. 2. Service Loading Information: The proper specification of the variation of the loading is extremely important to achieve an accurate fatigue life prediction. The loading can be defined in various manners. Whether it be time based, frequency based, or in the form of some sort of spectra depends on the fatigue analysis type to be used. When working with finite element models the loading can be force, pressure, temperature, displacement, or a number of other types. Loading in the test world usually refers to the acquisition of a response measurement, usually from a strain gauge. 3. Geometry Information: Geometry has different meanings depending on whether you are working from a finite element model or from a test specimen. In the testing world, the geometry input is the Kt (stress concentration factor) since the point of failure is usually away from the actual point of measurement. Therefore a geometry compensation factor (Kt) is defined to relate the measured response to that at the failure location. You can think of this as a fudge factor. With a finite element model the local stresses and strains are known at all locations (Kt=1 at all locations). The FE geometry gives us the entire stress distribution needed for fatigue life calculations. For crack growth analysis the geometry definition takes on yet another form as a compliance function. The correctness and accuracy of each of these inputs is important in that any error in any of these will be magnified through the fatigue analysis procedure, the fourth box, since this process is logarithmic. A ten percent error in loading magnitude could result in a 100% error in the predicted fatigue life.

Main Index

CHAPTER 1 Introduction

Do not worry too much about this, because the fifth box is the postprocessing or results evaluation. This can take on the form of color contours on a finite element model or a tabular listing but also quite often leads back into the three inputs to see what effect variations of these inputs will have on the life prediction. This is referred to as a sensitivity or a “what if” study. This is extremely useful at times when you are not quite sure about the accuracy of one of the inputs. The software denotes this as “optimization” in places.

Life Prediction Methods MSC.Fatigue uses three life prediction methods as already mentioned earlier. These are total life, crack initiation, and crack propagation. Total life is aptly named in that only the total life of the component is of concern and not when a crack will initiate or how quickly it will grow.

Total Life

Crack Initiation

=

Nf

Crack Growth

+

Ni

Np

The three methods are related to each other by the fact that the total number of cycles to failure, Nf, equals the number of cycles to initiate a crack, Ni, plus the number of cycles to propagate that crack, Np. The three methods have grown out of different needs over the decades using different techniques and having different degrees of accuracy. So in theory this equation is true, but in practice when applying the three methods to the same problem, rarely, if ever does it add up. In reality however, rarely are all three methods used on the same problem, mainly because different industries adopt different analysis methods depending on the driving design philosophy. See Design Philosophies (p. 6).

FE Analysis Methods In addition to the three life prediction methods, MSC.Fatigue also supports use of these methods using stress/strain response results from different finite element analysis techniques. The table below summarizes which FE analysis types are applicable to which life prediction methods in this release of the software.

Life Prediction Methods versus FE Analysis Results Total Life Crack Initiation Linear Static Linear Static Linear Transient Linear Transient Frequency Response Random Vibration

Main Index

Crack Growth Linear Static Linear Transient

5

6

Design Philosophies There are three main fatigue design philosophies. Each centers around one of the fatigue life estimation methodologies. To illustrate the three consider the design of a stool. Safe Life The safe life philosophy is a philosophy adopted by many, but especially the ground vehicle industry. Products are designed to survive a specific design life. Full scale tests are usually carried out with margins of safety applied. In general, this philosophy results in fairly optimized structures such as a stool with three legs. Any less than three legs and it would fall over. This philosophy adopts the crack initiation method and is used on parts and components that are relatively easy and inexpensive to replace and not life threatening if failure were to occur. Most of the life is taken up in the initiation of a crack. The propagation of that crack is very rapid and short in comparison. Fail Safe On the other end of the spectrum of design philosophies is that of fail safe. This is where a failure must be avoided at all costs. And if the structure were to fail it must fall into a state such that it would survive until repairs could be made. This is illustrated with our stool now having six legs. If one leg were to fail, the stool would remain standing until repairs could be made. This philosophy is heavily used in safety critical items such as in the aerospace or offshore industries. Damage Tolerant The middle ground philosophy is that of damage tolerance. This philosophy, adopted heavily in the aerospace community and nuclear power generation, relies on the assumption that a flaw already exists and that a periodic inspection schedule will be set up to ensure that the crack does not propagate to a critical state between inspection periods. As implied, this philosophy adopts the crack growth method. This is illustrated using our stool (now with four legs) but with someone inspecting it. This particular design philosophy is generally used in conjunction with the fail safe philosophy, first to design for no failure. and then to assume that, for whatever reason, a flaw exists and must be monitored.

Main Index

CHAPTER 1 Introduction

Life Estimation Process The life estimation process really centers around two major relationships. 1. The first relation is that of the loading environment to the stresses and strains in the component or model. This load-strain or load-stress relation is determined using finite element modeling and running linear elastic FE analysis. It is dependent on the characterization of the material properties and in some instances requires that a notch correction procedure take place. For the purposes of this discussion a notch correction is simply a way to compensate for plasticity from a linear FE analysis. 2. The second relation is that of the stresses or stains to the life of the component or model. This is accomplished by using damage modeling. Each fatigue life method has its own techniques to determine and sum damage which shall be explained as you progress through the example problems.

The Life Estimation Process Loading Environment

Load-Strain Relationship

Stresses Strains

Linear Elastic FE Material Characterization Notch Correction

Strain-Life Relationship

Life

Damage Modeling

Note: All example problems were created on the Windows platform as well as all graphical dumps of screen shots. The graphics may appear slightly differently on UNIX platforms; however, all operations are identical unless specified otherwise. In some situations, you may need to double click the mouse on the Windows platform whereas on UNIX, a single click performs the operation.

Main Index

7

8

1.3

When to Use Which Method? Of the three fatigue methods used to predict life, it is important to understand when to use which. This will become more evident as you proceed through this manual and work each exercise. As a quick answer to this question, the following guidelines are presented. S-N (Total Life)

• Long life fatigue problems where there is little plasticity since the S-N method is based on nominal stress

• Components where crack initiation or crack growth modeling is not appropriate, e.g., composites, welds, plastics, and other non-ferrous materials

• Situations where large amounts of pre-existing S-N data exist • Components which are required by a control body to be designed for fatigue using standard data such as MIL handbook data.

• Spot weld analysis and random vibration induced fatigue problems Crack Initiation

• Mostly defect free, metallic structures or components • Components where crack initiation is the important failure criterion - safety critical components

• Locating the point(s) where cracks may initiate, and hence the growth of a crack should be considered

• Evaluating the effect of alternative materials and different surface conditions • Components which are made from metallic, isotropic ductile materials which have symmetric cyclic stress-strain behavior

• Components that experience short lives - low cycle fatigue - where plasticity is dominant Crack Growth

• Pre-cracked structures or structures which must be presumed to be already cracked when manufactured such as welds

• Pre-prediction of test programs to avoid testing components where cracks will not grow

• Planning inspection programs to ensure checks are carried out with the correct frequency

• To simply determine the amount of life left after crack initiation • Components which are made from metallic, isotropic ductile materials which have symmetric cyclic stress-strain behavior Main Index

CHAPTER 1 Introduction

1.4

Different Methods of FE Import There are various methods of results import or access to FE results available in MSC.Fatigue. The most common methods, and the one most often used in the exercises described in the following chapters, is the import of Output2 files. Note:

Although the method of import is discussed in this section for various analysis codes and file types, once the results are imported into the database they are all treated the same by MSC.Fatigue. This means that any linear static or transient stress or strain tensor results that exist or can be imported into the database are supported by MSC.Fatigue. Thus, the supported analysis codes are not necessarily limited to those discussed here and can include customer customization of proprietary or in-house codes within the MSC.Patran environment.

To prepare to run each of these exercises in this section, create a fully reversed unit load using PTIME, the same as was done in the very first exercise in the manual. Call it unitload.

MSC.Nastran FE Model and Results You will need the following files to perform this mini-exercise: keyhole.bdf, keyhole.op2, keyhole.xdb, nastran_op2.fin, nastran_xdb.fin To use MSC.Nastran results you must set the Analysis Preference to MSC.Nastran. This is done either when you open a new database and are asked for the New Model Preferences | Analysis Code or you can change it anytime from the Preference | Analysis pulldown selection. Open a new database called keyhole, and set the Analysis Preference to MSC.Nastran. Overwrite any old database if necessary. There are two types of files that can be used from MSC.Nastran: Output2 Files The first type is the Output2 file. This file is very convenient to use because it generally contains both the finite element results and the model FE connectivity information. To import an Output2 file: 1. In Pre&Post, select the Import application switch on the main form. (This is the Analysis switch in MSC.Patran.) 2. Set the Action to Access Results, the Object to Read Output2, and the Method to Both.

Main Index

9

10

3. Select the Output2 file, keyhole.op2, using the file browser from the Select Results File... button and then press Apply.

XDB Files The second type of MSC.Nastran file supported is the XDB file which is a MSC.Nastran result database. The results contained in this file are not actually imported into the database but are retained in the XDB file. A direct access attachment is made between the database and the XDB file. To access XDB results: 1. In Pre&Post, select the Import application switch on the main form. (This is the Analysis switch in MSC.Patran.) 2. Set the Action to Access Results, the Object to Attach XDB, and the Method to Result Entities. Select the XDB file, keyhole.xdb, using the file browser from the Select Results File... button and then press Apply. Now we have read results and the model data from an Output2 file and have attached an XDB file. The exact same results are available from the Results application from the two different data sources in our example. You should note that we read the model data from the Output2 file. This was unnecessary to do since this could have been done from the XDB file also. When you reopen the database and access any result cases associated with an XDB file, it will be reattached as long as you have not moved or deleted it. Input Files Another mechanism for importing model data only from MSC.Nastran is to read the input file. This is done either from File | Import with the Object set to Model and the Source set to MSC.Nastran Input; or this is done from the Import application (Analysis application in MSC.Patran) with the Action set to Read Input File. You can try this with the keyhole.bdf file in a new database if you wish. Note:

Main Index

When you read an MSC.Nastran input file and then import the results from an Output2 or XDB file, but be sure to set the Method to Results Entities only.

CHAPTER 1 Introduction

Summary With the database now containing the model and two sets of FE results, run a fatigue analysis using either the file, nastran_op2.fin or nastran_xdb.fin. Go to the main MSC.Fatigue setup form and in the Jobname databox type nastran_op2 or nastran_xdb depending on which one you wish to run and press the carriage return to read in the job parameters. Investigate the job setup if you wish; then open the Job Control... form and submit the analysis. Successful completion of this exercise requires that the keyhole.op2 file be read in first and then the keyhole.xdb file attached to the database. If for some reason the jobs do not run properly, check carefully the Loading Info... form to make sure the correct Result Cases are selected. Close the database when you are finished. In summary, MSC.Nastran FE results and model information can be used in MSC.Fatigue in the following ways: 1. Read both the model and results information directly into the database from an Output2 file. 2. Attach an XDB file to the database to access the results and specify to import the model information from the XDB file into the database. 3. Read the model information from an MSC.Nastran input deck into the database and use the Output2 or XDB methods to access the results information only.

ABAQUS (or Advanced FEA) FE Results You will need the following files to perform this mini-exercise: keyhole.fil, abaqus.fin. To use ABAQUS or Advanced FEA results you must set the Analysis Preference to ABAQUS or MSC.Advanced FEA.This is done either when you open a new database and are asked for the New Model Preferences | Analysis Code or you can change it anytime from the Preference | Analysis pulldown selection. Open a new database called keyhole, and set the Analysis Preference to ABAQUS. Overwrite the old database if necessary. To read in the results and model data from ABAQUS: 1. In Pre&Post, select the Import application switch on the main form. (This is the Analysis switch in MSC.Patran.) 2. Set the Action to Read Results and the Object to Both. 3. Select the results file, keyhole.fil, using the file browser from the Select Results File... button and then press Apply.

Main Index

11

12

4. Go to the main MSC.Fatigue setup form and in the Jobname databox type abaqus and press the carriage return to read in the fatigue job parameters from the abaqus.fin file. Investigate the job setup if you wish; then open the Job Control... form and submit the analysis. Close the database when you are finished.

ANSYS FE Results You will need the following files to perform this mini-exercise: keyhole.rst, ansys.fin. To use ANSYS results you must set the Analysis Preference to ANSYS.This is done either when you open a new database and are asked for the New Model Preferences | Analysis Code or you can change it anytime from the Preference | Analysis pulldown selection. Open a new database called keyhole, and set the Analysis Preference to ANSYS 5 and overwrite any old database if necessary. To read in the results and model data from ANSYS: 1. In Pre&Post, select the Import application switch on the main form. (This is the Analysis switch in MSC.Patran.) 2. Set the Action to Read Results and the Object to Both. 3. Select the results file, keyhole.rst, using the file browser from the Select Results File... button and then press Apply. 4. Go to the main MSC.Fatigue setup form and in the Jobname databox type ansys and press the carriage return to read in the fatigue job parameters from the ansys.fin file. Investigate the job setup if you wish; then open the Job Control... form and submit the analysis. Close the database when you are finished.

MSC.Marc FE Results You will need the following files to perform this mini-exercise: keyhole.t16, marc.fin.

Main Index

CHAPTER 1 Introduction

To use MSC.Marc results you must set the Analysis Preference to MSC.Marc. This is done either when you open a new database and are asked for the New Model Preferences | Analysis Code or you can change it anytime from the Preference | Analysis pulldown selection. Open a new database called keyhole, and set the Analysis Preference to MSC.Marc. Overwrite any old database if necessary. To read in the results and model data from MSC.Marc: 1. In Pre&Post, select the Import application switch on the main form. (This is the Analysis switch in MSC.Patran.) 2. Set the Action to Read Results and the Object to Both. 3. Select the results file, keyhole.t16, using the file browser from the Select Results File... button and then press Apply. 4. Go to the main MSC.Fatigue setup form and in the Jobname databox type marc and press the carriage return to read in the fatigue job parameters from the marc.fin file. Investigate the job setup if you wish; then open the Job Control... form and submit the analysis. Close the database when you are finished.

PATRAN Neutral and Result Files You will need the following files to perform this mini-exercise: key.out, keyhole.nod, keyhole.els, keyhole.res_tmpl, patran_nod.fin, patran_els.fin, external.fin. PATRAN Neutral files contain model information and PATRAN Result files contain FE results. Both are simple ASCII files with standard formats that have been used for years in the CAE community. Many proprietary and in-house codes use these standard formats. Because of the simplicity of these files it is simple to create them from any source for use with MSC.Fatigue. There are two types of PATRAN Result files: nodal and elemental. PATRAN Neutral Files Only the nodes and elements are of interest or even necessary from a PATRAN Neutral file for use with MSC.Fatigue. The format of this file is made up of various packets. The pertinent packets necessary for MSC.Fatigue are (in order): 25 26 1 2 21 99

Main Index

File title Summary data Node data Element data Named components (group information - optional) End of file flag

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14

The format of each of these packets is (see the file keyhole.out as an example): Packet Type 25: Title Card Header Card 25 ID IV ID =0 Not applicable User Title Card TITLE

Format (I2,8I8) KC IV =0 Not applicable KC=1

Format (20A4) = Identifying title may contain up to 80 Characters

Packet Type 26: Summary Data Header Card 26 ID ID =0 n/a IV =0 n/a KC=1

IV

Summary Data Card DATE TIME DATE TIME VERSION

Format (I2,8I8) KC N1 N2 N3 N1=Number of Nodes N2=Number of Elements

N4

N5

Format (3A4, 2A4, 3A4) VERSION = Date neutral file was created = Time neutral file was created = PATRAN release number - not necessary

Packet Type 01: Node Data Header Card 1 ID ID =Node ID

Format IV KC IV =0 n/a KC=2

(I2,8I8)

Data Card 1 Format (3E16.9) X Y Z X =X Cartesian Coordinate of Node Y =Y Cartesian Coordinate of Node Z =Z Cartesian Coordinate of Node Data Card 2 Format (I1, 1A1, I8, I8, I8, 2X, 6I1) ICF GTYPE NDF CONFIG CID PSPC None of these parameters are necessary but the card must exist.

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CHAPTER 1 Introduction

Packet Type 02: Element Data Header Card Format (I2,8I8) 2 ID IV KC N1 N2 ID =Element ID IV =Shape (2 = bar, 3 = tri, 4 = quad, 5 = tet, 7 = wedge, 8 = hex) KC=1 + (NODES + 9)/10 + (N1 +4)/5(for text files) N1=Number of associate data values N2=ID of node in XY-plane (bar only) Data Card 1 Format (I8, I8, I8, I8, 3E16.9) θ2 θ3 NODES CONFIG PID CEID θ1 NODES=Total number of nodes, all other parameters are not necessary. Data Card 2 Format (10I8) LNODES=Element corner nodes followed by additional nodes Data Card 3 Format (5E16.9) ADATA=Associate data values (will not be present if N1 is zero) Packet Type 21: Named Components Header Card 21 ID IV ID =Component number KC=1 + (IV + 9)/10

Format (I2,8I8) KC IV =2 times the number of data pairs

Data Card 1 Format NAME =Component Name

(A12)

Data Card 2 Format (10I8) NTYPE(1)* ID(1) NTYPE(2) ID(2) NTYPE(5) ID(5) (NTYPE(i), ID(i)) =Data pairs of type and ID number of each item in (i = 1, IV/2)component. * NTYPE meanings: 5-node 6-bar 7-triangle

8-quadrilateral 9-tetrahedron 11-wedge 12-hexahedron

NTYPEs 6-12 may have 100 or 200 added to the basic NTYPE. The number of hundreds is usually the number of midside nodes.

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Packet Type 99: End of Neutral File Header Card 99 ID IV ID =0 not/applicable

Format

(I2, 8I8) KC IV =0 not/applicable KC=1

PATRAN Nodal Result Files These files contain results at nodes and are formatted as such: Record 1: Record 2: (2I9 E15.6, 2I9) Record 3: Record 4: Record 5 to n+4: (5E13.7))

TITLE (80A1) NNODES,MAXNOD,DEFMAX,NDMAX,NWIDTH SUBTITLE1 (80A1) SUBTITLE2 (80A1) NODID,(DATA(J), J=1, NWIDTH)(I8,

where Parameter

Description

TITLE

80A1 title stored in an 80 word real or integer array

SUBTITLE1

Same format as TITLE

SUBTITLE2

Same format as TITLE

NNODES

Number of nodes (integer)

MAXNOD

Highest node ID number (integer)

DEFMAX

Maximum absolute displacement (real)

NDMAX

ID of node where maximum displacement occurs (integer)

NWIDTH

Number of columns after NODID for nodal information (integer)

NODID

Node ID number (integer)

DATA

Result quantities organized by column index (real)

PATRAN Elemental Result Files These files contain results at element centroids and are formatted as such: Record 1: Record 2: Record 3: Record 4: Record 5 To N+4: (2I8, /, (6E13.7))

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TITLE (80A1) NWIDTH (I5) SUBTITLE1 (80A1) SUBTITLE2 (80A1) ID, NSHAPE, (DATA(J), J=1,NWIDTH)

CHAPTER 1 Introduction

where Parameter

Description

TITLE

80A1 Title Stored In An 80 Word Real Or Integer Array

SUBTITLE1

Same format as TITLE

SUBTITLE2

Same format as TITLE

NWIDTH

Number Of Columns Of Data Stored In The File (Integer)

ID

Element Identification Number (Integer)

NSHAPE

Essential Shape Code (Bar = 2, Tri = 3, Quad = 4, Tet = 5, Pyr = 6, Wedg = 7, Hex = 8; Int.)

DATA

Result Quantities Organized By Column Index (Real)

Import the Files All of these files can be imported into the database. 1. Open a new database called keyhole. Overwrite old database if necessary. 2. Import the Neutral file (key.out) first (File | Import - Object=Model, Source=Neutral). Keep the Analysis Preference set to MSC.Nastran and ignore any error/warning messages. The FE model is now in the database and should be visible from the graphics screen. Now import the FE results. There are two files to import, a nodal results file and an element centroidal results file that contain stress components. 3. From File | Import set the Object to Results and the Format to PATRAN2 .nod.... Two file browsers will appear, one asking for a template file and the other asking for the actual results file. The template file is called keyhole.res_tmpl and you will have to locate it from the browser which defaults to a standard installation directory. Once you have found and selected the template file, select the result file (keyhole.nod) and press the Apply button to have it imported. The template is a file that defines how the columns of a PATRAN Results file will be translated and stored in the database. MSC.Fatigue requires stress or strain tensors when imported into the database. Therefore the template file defines which 6 columns compose the 6 components of the tensor. 4. Repeat this procedure with the elemental results file (keyhole.els) but set the Format to PATRAN2 .els....

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5. Go to the main MSC.Fatigue setup form and in the Jobname databox type patran_nod or patran_els and press the carriage return to read in the fatigue job parameters from the patran_nod.fin or patran_els.fin file. Investigate the job setup if you wish; then open the Job Control... form and submit the analysis. Success of this exercise requires that the .nod file be read first and the .els file second. If the jobs fail, check that the proper Results Cases are called out in the Loading Info... form. Note that the patran_els setup uses Element as the Results Location. External File Access 6. Before proceeding make a copy of the file keyhole.nod from a system window or DOS prompt and call it keyhole1.nod. 7. The PATRAN Results files can also be accessed directly by MSC.Fatigue instead of through the database. Read the job setup file external.fin by typing external in the Jobname databox and then pressing enter. 8. Open the Loading Info... form and note that the Results From optionmenu is set to External. 9. On the right side of the form the name of the external file is specified by putting a # in the place of the load case ID. Even for one load case this is necessary. This is why you renamed or copied the file to include a number in it. The spreadsheet Load Case ID corresponds to the external file number that will be used for accessing the results, e.g., Load Case ID 10 will use file filename10.nod if filename#.nod is specified as the External File Name. 10. Submit the job from the Job Control... form if you so desire. Note:

Be careful as you import strains from external PATRAN Results files. Strains accessed directly from the database are required to be true or tensor strains, and not engineering strains. MSC.Fatigue will convert them to engineering strains (by multiplying the shear components by two) in order to properly calculate strain combination parameters such as von Mises. If you import a PATRAN Results file that contains strains make sure they are true strains and not engineering strains. If they are engineering strains then you must access them externally. You can specify whether strains are tensor or engineering strains from the Strain Type selection on the Loading Info... form only if the access is External.

Creating External Files With the ability of MSC.Fatigue to access external results files in this manner and with the ability of Pre&Post and MSC.Patran to write external results files, virtually any tensor result that exists in the database can be accessed by MSC.Fatigue. In the Results application, with the Object set to Report, you can specify the results to output and the format in which to write them such as an .nod or an .els file. For example, to write out an .nod file from the Results application: 1. Set the Action to Create, the Object to Report, and the Method to Overwrite File. Main Index

CHAPTER 1 Introduction

2. Select the Result Case and the Result to output and specify the 6 components of the tensor to be output from the Selected Quantities. 3. Under Target Entities, make sure that the Addlt. Display Control is set to Nodes. 4. Under Display Attributes enter a file name and set the Report Type to Data Only. 5. Open the Format... form. Set the File Width to 80 and blank out all other databoxes. Set the Alignment of the Title to Left. Enter four lines for the Title as such: TITLE $NNODES$MAXNOD$DEFMAX$NDMAX$NWIDTH SUBTITLE1 SUBTITLE2

Set the Value Format of the Entity ID to %I8% and all of the Components to %E13.7% except for the YZ Component which should be %E13.7%%1N%. 6. Press Apply to create the result file which can then be accessed externally by MSC.Fatigue. To create an element centroidal results file you follow the same procedure except the Addlt. Display Control must be set to Element Centroids; you must select NSHAPE and the 6 components in that order from the Select Results mode of the form; the format of the second line of the Title must be only $NWIDTH and the NSHAPE column Value Format must be %I8%%1N% (all others should be components should be %E13.7%). For more information on creating report files, see the MSC.Patran User’s Guide or access the on-line help from Pre&Post.

I-DEAS Master Series Universal Files Note:

This type of Model and Results import is only available in the Pre and Post version. It is not available in MSC.Patran.

Open a new database called keyhole, and set the Analysis Preference to anything. Overwrite the old database if necessary. You will need the following files for this mini-exercise: keyhole.unv, universal.fin. To import model and results data from a Universal file using Pre&Post: 1. Select File | Import. 2. Set the Object to Model and the Source to Universal File. 3. Select the results file, keyhole.unv and press Apply.

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4. Go to the main MSC.Fatigue setup form and in the Jobname databox type universal and press the carriage return to read in the fatigue job parameters from the universal.fin file. Investigate the job setup if you wish; then open the Job Control... form and submit the analysis. Close the database when you are finished. Note:

Both model and results are read even though we only specified that the Model be read. Also you can achieve the same by typing uf_reader.select_file( "keyhole.unv", "OPEN" ) in the MSC.Patran command window.

MSC.Patran FEA Result Files One last type of results file can also be accessed by MSC.Fatigue which is a file produced by the MSC.Patran FEA analysis code which produces a .res results file. There are two ways to access it, either by importing its contents into the database or by accessing it externally. Import the neutral file key.out as you did in PATRAN Neutral and Result Files (p. 13). Then import the results into the database by using the File | Import mechanism with the Object set to Results and the Format set to P/FEA 2 .res. To access the results directly from the file itself, on the Loading Info... form set the Results From pick to MSC.Patran FEA. Then type in the name of the file on the right side of the form or use the Select File button to use a file browser. If you would like to experiment with these operation, copy these files to your directory: keyhole.res, patran_fea.fin, patran_res.fin. Open a new database called keyhole and import the file and use the two setup files to run the fatigue jobs.

Main Index

CHAPTER 1 Introduction

1.5

Directory Structure After a successful installation of MSC.Fatigue, there will be the following directory structure under fatiguexx or patranxx (where xx is the version number):

fatiguexx or patranxx

app-defaults

bin

icons

helpfiles

mscfatigue_files test_files

res_templates

schema

executables

bin

Motif resource files

database schema files

on-line help files MSC.Fatigue Pre&Post or MSC.Patran executables

help central material database

user interface icons

mats central environment

User’s Guide QuickStart Guide

result file templates installation test files

nssys central loading database

ptime example problem files for all problems in this manual

examples

Note: If you are a Patran customer your MSC.Fatigue documentation is delivered on the MSC.Patran documentation CD. For standalone customers the MSC.Patran documentation CDs are delivered in the delivery kit.

Main Index

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1.6

MSC.Fatigue Modules MSC.Fatigue is made up of a number of actual executable modules. Below is a list of MSC.Fatigue program modules with brief descriptions of their function. The use of most of these modules is presented throughout this manual. The list is given here for reference. Table 1-1 MSC.Fatigue Modules and Descriptions Module Name

Description

MSC.Fatigue Pre&Post

Allows for import of finite element (FE) model and stress/strain results data, with graphical, form driven setup of fatigue analysis jobs, graphical evaluation of FE stress and fatigue life results, and access to all other modules of MSC.Fatigue. This same functionality is also found in MSC.Patran.

Analysis Modules:

(in bin directory)

FEFAT

S-N and crack initiation analysis from FE static and transient stress/strain data including multi-axial assessments and factorof-safety analysis.

FEMLF

Multi-axial crack initiation analysis from FE static and transient stress/strain data including factor-of-safety analysis.

FEVIB

Random vibration induced fatigue analysis from FE frequency response and random vibration stress results. This module uses the S-N method.

SPOTW

Spot weld fatigue analysis using the S-N method and FE results from MSC.Nastran bar and beam elements which simulate the spot welds.

PCRACK

Crack growth analysis using FE stress results from static or transient analysis.

SEAMW

Seam weld analysis using the stress (cubic) results from a MSC.Nastran run.

WHEELS

Wheels analysis using FE stress results.

SSG

Software strain gauge analysis using FE strains results.

Data Management:

Main Index

PAT3FAT

Translates FE analysis stress/strain or force results from MSC.Patran or MSC.Fatigue Pre&Post databases into a MSC.Fatigue analysis input file.

PCPOST

Crack growth results viewer and tabular listings.

CHAPTER 1 Introduction

Table 1-1 MSC.Fatigue Modules and Descriptions Module Name

Main Index

Description

PFMAT

Materials database manager.

PFPOST

Results tabulator for basic S-N and crack initiation analysis, multi-axial assessment, factor-of-safety, and vibration induced fatigue results.

PKSOL

Compliance function library and generator for crack growth analysis.

PTIME

Loading database manager for time and frequency signals.

Graphical Display:

(in bin directory)

MGED

Graphical time and frequency signal editor.

MGRAPHIC

Batch graphical plotter (UNIX only).

MMFD

Multi-file time and frequency signal display.

MP3D

Histogram and waterfall (3D), three parameter (x, y, z) display.

MPLTSYS

Plotter and printer definition setup (UNIX only).

MPOD

Polar display plots for critical plane and spot weld analysis.

MQLD

Quick look display of single parameter (y-only) time and frequency signals.

MTPD

Two parameter display of x-y data sensitivity plots.

MQPLOT

Displays and prints plot files with slide show capability (UNIX only).

MWNPLOT

Displays and prints plot files with slide show capability (Windows only).

MNCPENS

Utility program for modifying plot colors (curves, background, text, grid lines, etc.).

Load Manipulation:

(in bin directory)

MADA

Amplitude distribution analysis of time domain signals.

MART

Arithmetic manipulation of loading files.

MASD

Auto-spectral density function creation from time domain signals.

MBFL

Butterworth filtration of time domain signals.

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Table 1-1 MSC.Fatigue Modules and Descriptions Module Name

Main Index

Description

MCOE

Multi-channel creator/editor for loading signal files.

MFFF

Fast Fourier filtering of time domain signals.

MFILMNP

Load signal header/footer manipulation.

MFRA

Frequency response analysis of time domain signals.

MCYC

Rainflow cycle counter processing of a time series signal.

MFRM

Formula processor for load signal files.

MLEN

File length manipulation.

MMFM

Multi-file manipulation (addition, subtraction, division, multiplication).

MPVXMUL

Peak/valley slicing routine for multiple time signals.

MRSTATS

Running statistics of time signals.

Fatigue Utilities:

(in bin directory)

MCDA

Cycle and damage analysis display.

MCLF

Single shot crack initiation analyzer for stress or strain data.

MCYL

Cycles file lister/tabulator.

MFCG

Crack growth data analyzer.

MFLF

Single shot vibration fatigue analyzer for stress response power spectral density information.

MKTAN

Stress concentration library for use with MCLF and MSLF.

MMLF

Single shot multi-axial fatigue analyzer for stress/strain rosette data.

MSLF

Single shot S-N analyzer for stress data.

MSSA

Stress-strain analysis including elastic-plastic corrections, and rosette analysis.

MTCD

Time correlated damage analysis for crack initiation runs.

File Conversion:

(in bin directory)

MCONFIL

Cross-platform file translation.

MDACREM

RPC to DAC file translation.

CHAPTER 1 Introduction

Table 1-1 MSC.Fatigue Modules and Descriptions Module Name

Description

MREMDAC

DAC to RPC file translation.

MDTA

Binary to ASCII load signal translation.

MATD

ASCII to binary load signal translation.

MREGEN

Regeneration of time signal from cycle files.

MWFLCRE

Waterfall plot (3 parameter) creation from multiple single parameter files and separation of waterfall plots into multiple single parameter files.

General Utilities:

Main Index

FASTAN

Manages fast analysis executions from MSC.Patran or MSC.Fatigue Pre&Post. Runs in batch only.

FEFTRN

Translates FE data directly from MSC.Nastran xdb files and old I-DEAS universal files into a MSC.Fatigue analysis input deck.

MENM

MSC.Fatigue environment manipulator.

PFSETFONT

Sets or changes fonts for the Mask driven user interface (UNIX only).

UNVFES

Translates old I-DEAS universal files into MSC.Fatigue analysis input decks. Works only in batch mode and is called from FEFTRN.

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1.7

MSC.Fatigue Output Files MSC.Fatigue creates a number of files during an analysis. Every attempt is made to explain the use of these files throughout the examples. A listing of the basic files using generic names are shown in the table below along with a brief description. Table 1-2 MSC.Fatigue Output Files File

Main Index

Description

name.db

This is the database containing the FE model and its results created by Pre&Post or MSC.Patran.

jobname.fin

This is the job control file that is written by Pre&Post or MSC.Patran when you press the Apply button in the Job Control... form. The following Actions create this form: Full Analysis, Partial Analysis, Translate Only, Save Job Only. If you take a look at this file it contains Parameter=keyword entries. It defines the analysis set up as specified when you fill out the various forms. You can read this file in under the Job Control... form also when the Action is set to Read Saved Job.

jobname.fes

This is the fatigue analysis input file. It is a binary file and contains all information necessary to run a complete fatigue analysis using the FEFAT analyzer. It is created by the PAT3FAT and FATTRANS translators which read the jobname.fin file and the FE results information from the database name.db. You can view the contents of this file with the Utilities... option in FEFAT. The Action, Translate Only on the Job Control... form will create this file and then stop.

jobname.fpp

This binary file is created by FEFAT after preprocessing. It is the result of the rainflow cycle count. The Action, Partial Analysis on the Job Control... form will create all files up to this point and then stop.

jobname.fef

This is the results file of a fatigue analysis created by FEFAT when a Full Analysis is requested. It is an ASCII file and can be read back into Pre&Post or MSC.Patran to create life contour plots. It is also read by the MSC.Fatigue module PFPOST to do tabular listings of results. A jobname.fef_tmpl file is also created which is a template used when read back into Pre&Post or MSC.Patran defining the meaning of each column of results data in the jobname.fef file.

CHAPTER 1 Introduction

Table 1-2 MSC.Fatigue Output Files File

Main Index

Description

jobname.msg

This is the message file containing all messages during an analysis. If a job does not run properly for some reason, this is the file to look in first to find clues as to the problem.

jobname.sta

This is a one line status file read by the Monitor action from the Job Control Form... which is updated constantly as the analysis proceeds.

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Main Index

MSC.Fatigue QuickStart Guide

CHAPTER

2

A Simple S-N Analysis

■ Problem Description ■ Geometry ■ Set Up the Fatigue Analysis ■ Run the Fatigue Analysis ■ Review the Results ■ Concluding Remarks ■ Multiple Mean Stress Curve Support

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30

2.1

Problem Description In this first example problem we start with a very simple model to introduce some fatigue analysis concepts by investigating the total life of the component shown to the side. For the purpose of this exercise we will refer to it as the keyhole model as it is a keyhole shape notched component. Due to symmetry only the top half of the keyhole was modeled.

Objective To introduce the S-N fatigue life prediction method, commonly referred to as the “total life” method. All files necessary to perform this and subsequent examples are found in /mscfatigue_files/examples (UNIX) x:\mscfatigue_files\examples (Windows)

Where, is the installation top level directory such as /msc/fatigueXX (/msc/patranXX) or z:\msc\fatigueXX (z:\msc\patranXX), z is the drive letter for Windows workstations, and XX is the version number. The is commonly referred to as P3_HOME and, as such, can be set as an environment variable as explained in the MSC.Fatigue Installation and Operations Guide under the section called User Environment. Each chapter will have a table in this section indicating which files are necessary for proper execution. Table 2-1 Chapter 2 Necessary Files File P3_HOME/mscfatigue_files/examples/simpleSN.op2 Copy the file simpleSN.op2 to a clean working directory to begin.

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CHAPTER 2 A Simple S-N Analysis

2.2

Geometry A linear static finite element analysis has been performed already with a load magnitude of 10,000 Newtons. To begin, read this model and results information into a new database using MSC.Fatigue Pre&Post (referred to as Pre&Post from here on) or use MSC.Patran. From the system prompt or a DOS window in a clean directory invoke Pre&Post or MSC.Patran. or fatXX or fatigue where XX is the version number p3 or patran fXX

Note: Pre&Post or MSC.Patran can also be invoked from the Start menu on Windows workstations. In all cases, be sure that Pre&Post or MSC.Patran is running from the working directory. After the graphical interface starts open a new database from File | New and call it keyhole. The model was run through an MSC.Nastran analysis, so keep the Analysis Preference set to MSC.Nastran when asked.

Import the Model Select the Import toggle switch (Analysis in MSC.Patran) on the main form. When the form appears, set the Action to Access Results, the Object to Read Output2, and the Method to Both (model and results). Press the Select Results File button and select the file simpleSN.op2. Press the Apply button. The model will then appear and you are ready to set up a fatigue analysis.

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View the Stress Results Before moving on to the fatigue analysis, first press the Results application switch on the main form to view the stress results from the MSC.Nastran analysis. The Create | Quick Plot form is displayed. Go to the “Select Fringe Result” listbox and select Stress Tensor . Set the Quantity Option menu to Maximum Principal 2D. Press the Apply button and note the areas of high stress. The maximum principal stress appears to be about 333 MPa. When you are done, press the Results switch again to close down the Results application form.

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CHAPTER 2 A Simple S-N Analysis

2.3

Set Up the Fatigue Analysis To begin setup for a fatigue analysis, select the Analysis switch in Pre&Post (or from the Tools pulldown menu in MSC.Patran, select MSC.Fatigue and then Main Interface). This will bring up the MSC.Fatigue main form from which all parameters, loading and materials information, and analysis control are accessed.

Access from MSC.Fatigue Pre&Post

Access from MSC.Patran

Once the form is open, set the General Setup Parameters as follows: 1. Analysis: S-N 2. Results Loc.: Node This simply means that the fatigue lives will be determined at the nodes of the model. 3. Nodal Ave.: Global Accept the default which simply means element nodal stresses will be averaged to the nodes for all element contributions. 4. F.E. Results: Stress S-N analyses require stresses; you do not have a choice. 5. Res. Units: MPa Model dimensions are millimeters and forces are in Newtons, therefore stress units are MPa. 6. Jobname: simple_sn 7. Title: Simple S-N Analysis

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Solution Parameters Now open the Solution Params... form. On this form, set only these parameters: 1. Mean Stress Correction: None The time signal we are using is fully reversed, R=-1. The S-N curve itself was generated by testing numerous polished test specimens at different constant amplitude, fully reversed (R=-1) loading conditions. (The parameters (power law) that defines the S-N curve was determined by regression analysis of the raw data.) Therefore no mean stress correction is required since there is no mean stress to speak of. Note: Acceptance values for Mean Stress Correction are Goodman, Gerber, Multiple Mean, or None. In Section 2.7, we shall demonstrate, the Multiple Mean Curve Method. 2. Stress Combination: Max. Abs. Principal This is the stress parameter that will be used in the fatigue analysis. The stress tensor from the FE analysis results will be extracted at each node. However only a single stress value can be looked up on the S-N curve. So the six component values of the stress tensor will be resolved to the maximum absolute principal value which will be used as the stress look up parameter. Press the OK button to continue.

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CHAPTER 2 A Simple S-N Analysis

Material Information Now press the Material Info... button on the main MSC.Fatigue form.

Select an S-N Curve A spreadsheet appears whose cells need to be filled in. We will specify an S-N curve, a material surface finish and treatment, and a region on the model to which this combination will apply. 1. Material: MANTEN_MSN Select the first cell of the spreadsheet with the cursor. A listbox appears at the bottom of the form from which you select a material (S-N curve). Select MANTEN_MSN. 2. Surface Finish: No Finish The next cell becomes active and a pulldown menu appears. Select No Finish. 3. Surface Treatment: No Treatment The next cell becomes active and a pulldown menu appears. Select No Treatment. 4. Region: default_group

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The next cell becomes active and a listbox appears. Select default_group. This is a default group of entities defined in the database. It contains all the nodes and elements of the model. This defines the area of the model (the entire thing) to which this combination of material, finish, and treatment are to be assigned.

View the S-N Curve It is of interest to view the actual S-N curve that will be used to look up damage, and ultimately, calculate a fatigue life from the stresses of the model. Press the Materials Database Manager button. This will launch PFMAT, the materials database manager. First load the material by pressing or double clicking on the Load switch, selecting data set 1 from the optionmenu that pops up, and then selecting MANTEN_MSN from the list. You can then press or double click the Graphical Display switch to view the S-N curve. Note: Again, Pre&Post or MSC.Patran will be suspended until PFMAT is closed so that any newly created materials are recognized by the Pre&Post or MSC.Patran graphical interface.

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Note: The dashed line portion of the S-N curve indicates a region where the S-N curve is invalid. The S-N fatigue analysis method is generally only good for high cycle fatigue problems, meaning that the number of cycles to failure is generally very high. Note that this invalid region is below about 104 cycles. Another region of the curve is the “cut-off” region where the endurance limit is defined (108). Anything above this limit will be reported back as being beyond the “cut-off” (infinite life). The material information is complete. Select File | Exit to close the plot and eXit to quit PFMAT. Press the OK button to close the Material Info... form.

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Note: An S-N curve is based on the principle of similitude. This simply means that if we can reproduce the same stress as that experienced in, say, Tower Bridge as shown to the right, in a test laboratory specimen made of the same material, then we can expect the life of the two to be about the same, if subjected to the same levels of stress.

Snom

Loading Information Now in order to do a fatigue analysis using linear static FE results we must define how the load varies with time. This is easily done in MSC.Fatigue using the Loading Database Manager, PTIME. Open the Loading Info... form. Then press the Time History Manager button. This will launch PTIME. The load will be defined as a constant amplitude, fully reversed loading. This will have the effect of oscillating the 10,000 Newton load from +10,000 to -10,000 newtons. Note: Pre&Post or MSC.Patran will be suspended during this operation until PTIME is closed. This is indicated by the blue busy signal in the top right corner. Since PTIME is a separate process, this suspension is necessary to make Pre&Post or MSC.Patran’s graphical interface recognize any new time signals.

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Define a Unit Load - Fully Reversed When PTIME comes up, select Enter X-Y points as the method of input (on Windows you will have to double click on this option or select it and press the OK button). A form will appear that will ask for a file name, description and other information. Enter the following leaving defaults for those not mentioned: 1. Filename: UNITLOAD 2. Description 1: Constant Amplitude, Fully Reversed Unit Load 3. Fatigue equivalent units: Cycles We are defining a single occurrence of this fully reversed, constant amplitude signal as one cycle of the loading. Press the OK button to go on. Next you will be prompted to enter the XY points. We actually only enter Y points as the X points are taken as evenly spaced intervals with the sample rate set to one. Enter the following numbers with a carriage return after each: 0, 1, -1, 0. End by putting in a blank entry and then press the End button.

Plot the Time History PTIME returns to its main menu where you can select Plot an entry and press the OK button. A new form is displayed showing the Database Entry to plot. Accept the default file, UNITLOAD.

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Note: The mean of this signal is zero. In fatigue analysis, constant amplitude loading is usually accompanied by a description of the mean, commonly referred to as the R-ratio. The R-ratio is the minimum value of the signal divided by the maximum value and is a measure of the signals mean value. In this case R=-1 signifying a fully reversed load where the maximum and minimum absolute magnitudes are identical. Select File | Exit to close the plot and press or double click the eXit switch in PTIME. Associate the FE Load to its Time Variation Now back on the Loading Info... form you must associate the time variation of the load that you just created to the FE load case. This is done via a spreadsheet. Three pieces of information must be input to the spreadsheet in the center of the form with all other parameters using their default settings. 1. Load Case ID: 1.1-3.1-2Place the cursor in the cell in the first column and click the mouse button. This selects the cell. A number of listboxes, buttons, and pulldown menus appear below the spreadsheet. This is where you specify the FE analysis results that you will use in the fatigue analysis. They appear empty at first. To fill them, press the Get/Filter Results... button. On this form turn the Select All Results Cases toggle ON and press the Apply button. This will fill the listbox on the left with the only result load case that exists. Select it, and

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select Stress Tensor from the second listbox and then press the Fill Cell button. This will fill the cell with the internal IDs of the selected load case and its stress results. This is the significance of the numbers 1.1-3.1-2-. They are internal IDs only but are necessary to identify the results. Note: The actual load case ID numbers you see may differ from those shown here. What you want to select is the DEFAULT, Static Subcase and the corresponding Stress Tensor at layer Z1. 2. Time History: UNITLOAD The middle cell becomes active after successfully selecting a FE load case. Another spreadsheet (with one row) appears at the bottom of the form from which you select the previously created time history file. Click on the UNITLOAD row anywhere with the mouse. This will fill the cell with the time history file name. 3. Load Magnitude: 1.0 The next cell becomes active and a databox appears below the spreadsheet. Simply accept the default, which is unity. A specification of unity here signifies that the stresses from the FE analysis will be used “as-is” in the fatigue analysis and the time variation loading that we defined will be used to scale the stresses up or down as needed. You must press a carriage return (Return or Enter) to accept the value in the databox and fill in the cell in the spreadsheet. A common error is to forget to do this. The time variation of the loading is now associated to the static FE results. Press the OK button to close the Loading Info... form.

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2.4

Run the Fatigue Analysis You are ready to run the fatigue analysis. Open the Job Control... form. Set the Action to Full Analysis and press the Apply button. The database will close momentarily as the results information is extracted. When the database reopens, the job will have been submitted. You can then set the Action to Monitor Job and press the Apply button from time to time to view the progress. When the message Fatigue analysis completed successfully

appears, the analysis is complete. Close down the Job Control... form when done.

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2.5

Review the Results Open the Results... form on the main MSC.Fatigue setup form (not to be confused with the Results application switch on the main Pre&Post or MSC.Patran form). With the Action set to Read Results press Apply. The fatigue analysis results will then be read into the database.

View the Life Contour Plot Just as you viewed the stresses earlier, you can view the life plot. Select the Results application switch on the main Pre&Post or MSC.Patran form. The Create | Quick Plot form will appear. On this form select the Total Life, simple-snfef item in the Select Result Cases listbox and the Log of Life (Cycles) item in the Select Fringe Result listbox and then press Apply. Note that the smallest life reported is approximately 5.65. This is a log base(10) value. So the actual life value is 10 5.65 . Reporting life values in log units tends to spread the contour bands out for better results interpretation. Since such a large spread of results values can occur (from finite to infinite at locations where no damage occurs), it is not really practical to plot pure life values. Press the Results switch again to close the Results application.

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Tabular Listing Press the Analysis switch to bring the main MSC.Fatigue form back (this is not necessary in MSC.Patran). On the MSC.Fatigue Results... form, change the Action to List Results and press Apply. This will start the module PFPOST which tabularly lists the fatigue analysis results. Accepting the jobname and the default filtering values, by pressing OK a couple of times, will get you to the main menu. Press or double click the Most damaged nodes switch to view a tabular listing. Note the life value of approximately 10 5.65 =4.5E5 cycles on Node 1. Press Cancel to quit the listing and press or double click eXit to leave PFPOST.

What If? As one small exercise to introduce the concept of “what if” analyses, change the Action to Optimize and press Apply (you do not need to enter a node number) on the Results... form. This will launch the module FEFAT in its design optimization mode. FEFAT is the FE-fatigue analyzer used to calculate fatigue life. It can be run in both batch and interactive modes. When it comes up, press Worst Case to automatically select the node with the lowest life prediction. Enter a Design Life of 1E6 (a million)

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cycles. Press the OK button. The analyzer will re-analyze the fatigue life at Node 1 and will report the life value to you. Pressing the End button will put you into the main optimization menu.

Change the material from MANTEN_MSN to RQC100_MSN to see the effect of a different material on the fatigue life. Do this by pressing or double clicking the Material optimization switch and selecting or typing in the new material name. Press OK and then press or double click the Recalculate switch to report the new life. Note that the life is bettered by almost an order of magnitude. Hint:

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When you change materials, they must be the same types of materials (steel vs. steel, aluminum vs. aluminum, etc.) If you wish to change from steel to aluminum then the Young’s modulus changes would invalidate the results. There are some general guidelines on how to do this properly, however, in the MSC.Fatigue User’s Guide.

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Concluding Remarks This was a very simple analysis, the results of which should be obvious. The lowest life was naturally predicted at the highest stressed location. Because the loading was simple, perhaps a detailed fatigue analysis as performed here, was not necessary. In fact you could have simply extracted the highest principal stress (333 MPa) and gone directly to the S-N curve using PFMAT to assess the life. This, of course, starts to become very impractical with anything much more complicated as you will see in subsequent examples. As an exercise, go back to the Material Info... form and invoke the materials database manager, PFMAT, again and plot the S-N curve as done before. With the S-N curve plotted you can use the left mouse button positioned on the curve to read off coordinate values (reported in the lower left corner). You can also use the right mouse button to zoom in on the curve (click once on one side of the curve and again on the other side to zoom). To restore the curve, select the View | Full Plot option. You can read the life value right from the curve. Hint:

To read the correct life value from the curve for this exercise, you must multiply the maximum principal stress at Node 1 by two (666 MPa) since the total range of the signal is twice the stress determined by the FE analysis since it is experiencing full reversal.

Note: Note about plasticity: as mentioned in Introduction (Ch. 1), fatigue cannot occur without some local plasticity. The S-N method makes no effort to define the amount of plasticity or compensate for it in any specific manner. All plasticity is built into the S-N curve itself. The S-N curve used in this exercise is known as a material S-N curve. This is significant because you must know beforehand what the S-N curve you use actually represents. In this case the S-N curve is representative of the actual material and relates local stress (σ) to life. That is, the monitored stress used to create the S-N curve is the stress at the actual failure location. This will become more clear when we discuss another type of S-N curve (component S-N) in a later exercise. Exit from Pre&Post when finished with this exercise. Keep the files and directory for use in the next exercise.

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2.7

Multiple Mean Stress Curve Support This section describes the multiple mean stress curve support in MSC.Fatigue. Multiple mean stress curve analysis uses empirical data to account for mean stress effects rather than analytical methods such as Gerber and Goodman. Multiple mean stress curve analysis is for S-N analysis only. Temperature corrections, certainty of survival, Optimization and Fast Analysis are not available for Multiple Mean Stress Curve S-N analysis.

Set Up the Fatigue Analysis We will use the model in the previous exercise to run this test case. Leave all settings on the general setup form as is except for the Jobname and the Title. Set these as shown: 1. Jobname: multi_mean 2. Title: Multiple Menu Stress Curve

Solution Parameters Open the Solution Params form. Set the Mean Stress Correction optionmenu to Multiple Mean Curves. Use the default values for all the other widgets. Press OK to close the form.

Material Information Open the Material information form. It is identical to the standard S-N analysis, except:

• An ASCII materials database is used (extension .mnd) instead of the standard Materials database (.mdb).

• Selecting the Materials Database Manager button brings up a text editor instead of PFMAT.

• Even though the Surface Finish, Treatment and Kf cells are visible they are not available for input.

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Click on the Materials Database Manager button to view the file containing the Multiple Mean Stress Curve data.

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Select the Material MANTEN from the listbox and pick the default_group for analysis. Click OK.

Loading Information Open the Loading information form. This form should be still filled out from the previous example. Press OK to accept the inputs.

Run the Fatigue Analysis Open the Job Control form. Set the Action to Full Analysis and press the Apply button. Monitor the job and once it has completed close the form. Open the Results form. Set the Action to List Results and press Apply. The PFPOST module is now displayed. Select the Jobname multi_mean and press OK. Accept the defaults on the next form by just pressing OK. Now select the detail Information switch. The form that is displayed shows that a life of 1.86E4 repeats, read off the zero mean MANTEN curve, is reported at Node 1. Press End to close the form. Press eXit to exit the PFPOST module.

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Lets take a closer look at the stress time history at this node. In order to do this the Action needs to be changed to Extract Time History. Enter Node 1 in the databox and press Apply. The MFATFE module is now displayed. On the first form select the Utilities switch. Next select the Node/Element options switch and press OK. On the next form press OK to accept the Result Filename, then select the User input switch and set the Node/Element ID to 1. Press OK. Select the Time History Extraction switch and press OK. This will bring up a table that shows the maximum and minimum stress values for Node 1. Press Cancel to close this form and bring up the graphical representation of the data as shown below.

To close this picture select File | eXit. Select Return, then select return to Main menu, and finally select eXit to close the MFATFE module. Make a note of this life as we will compare this life with and offset time history to demonstrate the Multi Mean Stress Curve concept. Verification: On the General Setup form, change the Jobname to multi_mean_offset. Offset the Time History on the loading form by applying an offset of 0.3 that will yield an offset range mean of 100 Mpa. A 100 Mpa mean stress S-N curve exists in the database for MANTEN, the material used in the analysis above.

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Run the Job and list the results. The life at the same location (Node 1) drops to 2.3E3 repeats. The offset stress time history at Node 1 is shown below.

For a stress range of 666 Mpa, the 100 Mpa mean stress curve yields the life calculated.

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MSC.Fatigue QuickStart Guide

CHAPTER

3

Rainflow Cycle Counting

■ Problem Description ■ Set Up the Fatigue Analysis ■ Run the Fatigue Analysis ■ Review the Results ■ Concluding Remarks

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3.1

Problem Description This example is an extension of the previous example where the simple constant amplitude loading is replaced with a more complex randomly varying time signal.

Invoke Pre&Post or MSC.Patran by typing the following symbols at the system prompt or from a DOS window: fXX p3

or fatX or fatigue where XX is the version number or patran

If you have not already, open the same database that you created in the previous example working in the same directory from the File | Open menu. The name of the database should be keyhole.

Objective • • • • • •

To predict the life of the keyhole subject to a varying load signal. To understand how to normalize the FE stresses. To introduce the concept to rainflow cycle counting. To introduce the concept of damage summation. To investigate the effect of mean stress. To investigate the probabilistic nature of fatigue.

Note: The geometry and materials information are identical to that of the previous exercise.

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3.2

Set Up the Fatigue Analysis To begin setup for a fatigue analysis press the Analysis switch in Pre&Post (or from the Tools pulldown menu in MSC.Patran, select MSC.Fatigue and then Main Interface). This will bring up the MSC.Fatigue main form from which all parameters, loading and materials information, and analysis control are accessed.

Access from MSC.Fatigue Pre&Post

Load the Previous S-N Analysis Parameters Instead of defining all the analysis parameters again, let us begin from the last analysis. Once the form is open, type the jobname of the previous example in the Jobname databox (simple_sn) and issue a carriage return (Return or Enter). You will be prompted to read in an old analysis setup file (it detects a file called simple_sn.fin in your local directory and reads in the parameters). Hint:

You can do the same thing in the Job Control... form with the Action set to Read Saved Job.

Now change the jobname and the title: 1. Jobname: rf_cycle 2. Title: Simple S-N Analysis, Variable Loading

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Loading Information Open the Loading Info... form. Then press the Time History Manager button. This will launch PTIME. The time variation of the load will be defined by a signal called SAETRN which is stored in the loading central database in the MSC.Fatigue installation directory.

Copy SAETRN from the Central Database When PTIME comes up, select Add an entry... and then Copy from central as the method of input. A form will appear that will ask for a name. Use the List button to select SAETRN from the central database. Scale the Time History Load From the PTIME main menu, select Change an entry... and then Polynomial transform. We are going to scale up the time history to represent the actual loading applied to the component. You will be asked for the Database Entry to transform and a new target file. Use the same name (SAETRN) for both and allow

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overwrite. The transformation from will then appear. We simply want to scale the load up so all that is needed is to input a scale factor of 10 in the second databox. Press OK when done.

Finally a form appears allowing you to change any details associated with this time history. Enter the following: 1. Description 1: Leave as is 2. Description 2: Blank this out 3. Load type: Force 4. Units: Newtons 5. Number of fatigue equivalent units: 1 6. Fatigue equivalent units: Repeats Life results will be reported as the number of Repeats of this entire loading sequence and not as individual stress cycles as in the previous exercise. Plot the Time History PTIME returns to its main menu where you can select Plot an entry. Accept the default file, SAETRN. Note that the maximum value is close to 10,000 Newtons. As a comparison to the previous example, which oscillated in a fully reversed fashion between positive 10kN and negative 10kN, this signal varies significantly with a very positive mean and only occasionally reaches or nears the 10kN maximum. We therefore would expect this loading to be less damaging with all else the same.

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Select File | Exit to close the plot and press or double click the eXit switch in PTIME. Associate the FE Load to its Time Variation Now back on the Loading Info... form you must associate the time variation of the load that you just created to the static FE load case. Go to the spreadsheet as was done in the previous example. Two things need to be changed on this form.

1. Time History: SAETRN Select the middle cell to make it active. Another spreadsheet (now with two rows) appears at the bottom of the form from which you select the time history file. Click on the SAETRN row anywhere with the mouse. This will replace the cell with the new time history file name. 2. Load Magnitude: 10,000 The next cell becomes active and a databox appears below the spreadsheet. Change this entry to 10,000. You must press a carriage return (Return or Enter) to accept the value in the databox and fill the cell in the spreadsheet. Forgetting to do this is a common error. The time variation of the loading is now associated to the static FE results. Press the OK button to close the Loading Info... form. Note: In the previous example we entered unity for the Load Magnitude accepting the FE load as being the true representation of the load and thus the stresses. The time history, UNITLOAD, scaled the stress distribution between 1 and 1 to signify the time variation of the loading. This time the time history SAETRN is used to define the actual loading as it changes with time. The FE load magnitude is therefore simply an arbitrary number used to obtain the stress distribution. The stresses in the FE analysis need to be normalized by this FE load magnitude of 10kN, to simulate the stress distribution due to a unit load.

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The load magnitude acts as a divisor to normalize the stresses to obtain a stress distribution due to a unit load as in the equation σij(t)=P(t)σij/Pfea, where σij and Pfea are the stress tensor and load magnitude from the FE analysis, P(t) is the externally defined time variation of the loading, and σij(t) is the resulting time variation of the stress tensor (at any particular location in the component). This can be done because the analysis is linear elastic. Using linear elastic FE analysis and associating an external time variation of the loading for fatigue analysis is called the “pseudo-static” method. “It might be said that all stress analyses are basically fatigue analyses, the differences lying in the number of cycles of applied stress.” - quote from Carl C. Osgood, Fatigue Design (1982).

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3.3

Run the Fatigue Analysis You are ready to run the fatigue analysis. Open the Job Control... form, set the Action to Full Analysis and press the Apply button. The database will close momentarily as the results information is extracted. When the database reopens, the job will have been submitted. You can then set the Action to Monitor Job and press the Apply button from time to time to view the progress. When the message Fatigue analysis completed successfully

appears, the analysis is complete. Close down the Job Control... form when done.

Rainflow Cycle Counting This analysis takes a few minutes to run to completion. The reason it takes longer than the previous example is due to the complex nature of the time signal. The program is performing a procedure called rainflow cycle counting, referred to as “preprocessing” in MSC.Fatigue. Cycle counting is a mechanism to extract and count the number of stress cycles in a signal. The term Rainflow is attributed to two Japanese gentlemen, Matsuishi and Endo, who invented the method. It is based on the concept of rain drops flowing off Japanese style pagoda roofs. Time history signals are stood on end and rain is visualized to run off of each peak or valley. Various rules were adopted to count cycles and reversals which is beyond the scope of this text; but suffice it to say that the end result of rainflow cycle counting is a set of constant amplitude signals and a count of the number of cycles in each. Cycle counts can be visualized as probability density functions (PDF) or as 3-dimensional histogram matrices as you will see later.

Damage Summation It is important to break up a variable signal into a number of constant amplitude signals in order to assess the life from the S-N curve. The curve itself is created by a series of constant amplitude tests. So for each cycle in the signal you must look up the proper stress from the S-N curve. What stress to look up is the job of rainflow cycle

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counting. The next challenge to tackle is the summation of the damage from each cycle in order to report a total life due to all cycles. This is accomplished by way of the Palmgren-Miner linear damage summation law. This states that damage can be summed by determining the ratio of Total Damage= ∑ N i ⁄ N f i the number of cycles experienced to the ∆D 1 = N 1 ⁄ N f number of cycles to failure for a given 1 ∆D 2 = N 2 ⁄ N f 2 stress range or level and then summing all the ratios for every stress range. When this number, known as Miner’s Constant, reaches unity, failure is said to have occurred. The predicted life is then determined by summing the percentage of life used by each stress level for the entire time signal. Life is then reported back as to the number of times the given time signal can be applied before failure. Hint:

This is where user-defined fatigue equivalent units come in handy, because rarely does one want life reported in “repeats” of the time signal, but rather in more meaningful units such as hours, miles, years, laps, missions, etc. This is accomplished by defining these user-defined units in the PTIME, loading database manager, utility. Use the Change an entry | Edit details option.

Speeding up the Analysis There are two ways that you could speed up this analysis. 1. First, since we already know where the failure location will be (at the point of highest stress) because of the simplicity of this model, we could have defined a Group with only this node (Node 1) and specified it in the Materials Info... form. This however, would only calculate life at this one node and would ignore the rest of the model. 2. Second, on the Job Control... form you can turn on the Simplified Analysis toggle. As an exercise after you finish this problem, turn this toggle ON, change the Jobname to something else and re-run the problem. Note how much faster the analysis proceeds relative to the first time. What is happening is that for a normal analysis, the rainflow procedure is being applied to each location once its stress time variation is determined. When the Simplified Analysis toggle is turned ON, the rainflow procedure is applied to the loading time history first and the FE stresses are used to scale the rainflow histogram matrix. This speeds up the analysis significantly for a complex time signal for a single load. It does however, produce slightly less accurate results. Notice the slight variation in predicted life when you do this.

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3.4

Review the Results Open the Results... form on the main MSC.Fatigue setup form (not to be confused with the Results application switch on the main Pre&Post or MSC.Patran form). With the Action set to Read Results, press Apply. The fatigue analysis results have been read into the database. You can review the life contour plot as you did in the previous exercise if you wish. The contour will look similar but the magnitudes will be different.

Tabular Listing On the MSC.Fatigue Results... form, change the Action to List Results and press Apply. This will start the module PFPOST which tabularly lists the fatigue analysis results. Accepting the jobname and the default filtering values by pressing OK a couple of times will get you to the main menu. Press or double click the Most damaged nodes switch to view a tabular listing. Note the life value of approximately 105.26=184,000 repeats of the signal on Node 1. This is significantly less damaging than the previous example considering the life is reported in repeats of the time history and not as individual cycles. To get the number of cycles, we would have to multiply the life result by the rainflow cycle count. Press Cancel to quit the listing and press or double click eXit to leave PFPOST.

Histogram Matrix Let us take a look at the results of a rainflow cycle count. From the Results... form, change the Action to Optimize and press Apply (you do not need to enter a node number) on the Results... form. This will launch the module FEFAT in its design optimization mode. When it comes up, press Worst Case to automatically select the node with the lowest life prediction. Enter a Design Life of 1E6 (a million) repeats.

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Press the OK button. The analyzer will re-analyze the fatigue life at Node 1 and will report the life value to you. Pressing the End button will put you into the main optimization menu.

Select results Display and then plot Cycles histogram. This will display a histogram plot showing the results of the rainflow cycle count for the critical location on the model. It looks a little bit like a city skyline. Note that there are quite a few cycles that have low stress ranges and that there are fewer with high stress ranges. The height of each tower represents the number of cycles at that particular stress range and mean. Each tower is used to look up damage on the S-N curve and damage is summed over all towers. A histogram cycle plot from our first example would yield only a single tower of unit height with a mean of zero. Hint:

The accuracy of the fatigue calculation is dependent on the number of towers allowed in the rainflow histogram. Typically it is broken up into what are called bins which is the matrix size. These bins can be 32x32, 64x64, or 128x128. If you want to increase the accuracy, you can run FEFAT interactively at the critical location and specify a larger bin size.

Now convert the cycle histogram plot to a damage histogram plot. This is done by either returning to the main menu and selecting results Display | plot Damage histogram or with the cycle histogram plot still displayed, select Plot_type | Damage.

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Now you can see the damage caused by each bin. Notice that the lower stress ranges produced zero damage. All damage came from cycles in the higher stress range, which is to be expected. Select File | Exit when done viewing the graphics.

Effect of Mean Stress Now let us investigate the effect of mean stress on the fatigue life predictions. First remember that the S-N curve we are using was produced for an R-ratio of minus one, or no mean stress in other words. The time history used in this example has a predominately tensile mean. The initial life prediction did not take into consideration this mean stress and therefore could perhaps be giving a somewhat non-conservative answer. From FEFAT’s design optimization menu, select Sensitivity analysis | Mean stress correction (all) then press or double click the Recalculate switch. A listing showing no correction plus two mean stress correction methods appear: Goodman and Gerber. Note that both of them give more conservative answers.

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How is mean stress compensated for in the S-N analysis? The simple way to explain this is that for both the Goodman and Gerber methods, knowing the ultimate tensile strength (Su) and the actual stress amplitude (σa) and mean (σm), an equivalent stress range with zero mean is determined. Goodman and Gerber follow these equations: σa σm ------ + ------- = 1 Goodman Se Su σa σm 2 ------ +  ------- = 1 Gerber Se  Su 

Graphically this looks like the plot to the right where, at least for Goodman, if you draw a line connecting Su to the intersection of σa and σm and then continue it on to the stress amplitude axis, this will indicate the equivalent stress Se with zero mean. This stress is then used to look up damage on the S-N curve. Note: As a stress range of a cycle becomes larger and larger, there tends to be less and less possible variability in the mean of that cycle. This is indicated on the cycle histogram plot since the base of these type of plots tends to be triangular in nature, which means that as the stress gets larger, the mean stress has less of an effect on the fatigue life.

Probabilistic Nature of Fatigue As a final exercise in this example, let us investigate two different materials as we did in the first problem. From the main menu of FEFAT’s design optimization mode, select Material optimization. Change the material S-N curve from MANTEN_MSN to RQC100_MSN and then press or double click the Recalculate switch again. Note that RQC100_MSN, being a much higher strength steel, gives a much higher life prediction (357,000 repeats vs. 184,000 repeats) for no mean stress corrections. This means RQC100_MSN is a better material to use (or does it?). Just looking at the S-N curve might indicate this also.

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Press or double click the Original parameters button to put the material back to MANTEN_MSN and then press or double click the Change parameters switch and change the Design Criterion to 99. Press or double click the Recalculate switch. Note the life of approximately 85,400 repeats. Now change the material to RQC100_MSN as done earlier and press or double click the Recalculate switch. The life using the higher strength steel is now only about 30,900 repeats, less than that of the lower strength steel. This is due to the probabilistic nature of fatigue and the scatter associated with the SN curves themselves. By specifying 99 as the design criterion, we are asking MSC.Fatigue to calculate a life value based on a 99% certainty of survival. The larger the scatter in the original S-N data that makes up the curve, the less certain we will be of survival and the code takes this into account by reporting a more conservative answer. The default is a 50% probability of survival (or failure). Note: Scatter is associated with S-N curves and other damage curves due to the fact that, for example, if you take 10 identical test coupons and subject them to what you think are identical tests, you will get ten slightly different answers. The material parameters associated with S-N curves take this into consideration with the Standard Error of Log(N) (SE) determined by regression analysis of the raw data.

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3.5

Concluding Remarks This exercise introduced you to rainflow cycle counting, damage summation, mean stress effects, and the probabilistic nature of fatigue by using a randomly varying load on our simple keyhole model. Though this example still did not help us identify critical locations since we already knew where failure would occur, it did start to show the power of MSC.Fatigue by being able to handle complex time signals and to make compensation for parameters that may effect the fatigue life, something that would be a daunting task to do by hand. The next exercise will introduce the concept of a component S-N curve. Quit from Pre&Post or MSC.Patran when you are through with this exercise. Note: MSC.Fatigue does not take into account the frequency (speed at which cycles are experienced) or the sequence (when a particular cycle is experienced relative to other cycles) of cycles from a given signal. Rainflow cycle counting simply counts the number of cycles and determines their range and mean. Frequency and sequence can have an influence on the fatigue life but is a third or fourth order effect on life prediction in most cases. MSC.Fatigue does provide you with certain fatigue analysis utilities to determine if these influences are important after the initial analysis using the MSC.Fatigue module MTCD (for time correlated damage).

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MSC.Fatigue QuickStart Guide

CHAPTER

4

Component S-N Analysis

■ Problem Description ■ Geometry ■ Set Up the Fatigue Analysis ■ Run the Fatigue Analysis ■ Review the Results ■ Concluding Remarks

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4.1

Problem Description 1”

10” P

P

P

P

2”

A simple bracket as shown has a design life of 7 years (61,320 hours). Loading occurs at the end of the short section which has been welded and the component is constrained at both ends of the main bar. Because failure is known to occur at the weld, the finite element modeling at the loading point and the stresses found there can be ignored for the purposes of this exercise. The load applied in the model was 900 lbs total. In service, the component experiences loading of 3000 lbs in the direction of the finite element load and 7000 lbs in the reverse direction. This occurs once every 30 minutes. Only a 4% failure rate is allowed.

Objective • • • •

To introduce the concept of a component S-N curve. To learn how to enter materials data into the database manager. To determine if the component achieves its design life. To determine what level of loading can be achieve and what failure rate could be expected - a sensitivity study.

• To understand what files are created by an MSC.Fatigue analysis. Table 4-1 Chapter 4 Necessary Files File P3_HOME/mscfatigue_files/examples/bracket.op2

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CHAPTER 4 Component S-N Analysis

4.2

Geometry Invoke Pre&Post or MSC.Patran as you did in the previous examples. The geometry can be found in the file bracket.op2. The results are from MSC.Nastran. Copy the file to your working directory. Open a new database in a clean, empty work directory from the File | New menu. Give the name bracket to the database.

Import the Model and Results Press the Import toggle switch (Analysis in MSC.Patran) on the main form. When the form appears set the Action to Access Results, the Object to Read Output2, and the Method to Both (model and results) then press the Select Results File button and select the file bracket.op2 and press Apply. The model will then appear and you are ready to set up a fatigue analysis.

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View the Stress Results Before moving on to the fatigue analysis, press the Results application switch on the main form to view the stress results from the MSC.Nastran analysis. Select Stress Tensor, from the listbox and set the Quantity to Maximum Principal. Press the Apply button and note the areas of high stress mostly around the applied load. This however, is not of concern to us. What we are interested in is the stress at Node 514 of around 2,690 PSI. This will be explained in more detail as we set up the material information. To rotate the model, press the middle mouse button or for a two button mouse, press both at the same time. When you are done, press the Results switch again to close down the Results application form.

Node 514 Highlighted

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CHAPTER 4 Component S-N Analysis

4.3

Set Up the Fatigue Analysis To begin setup for a fatigue analysis, press the Analysis switch in Pre&Post (or from the Tools pulldown menu in MSC.Patran, select MSC.Fatigue and then Main Interface). This will bring up the MSC.Fatigue main form from which all parameters, loading and materials information, and analysis control are accessed.

Access from MSC.Fatigue Pre&Post

Once the form is open, set the General Setup Parameters as follows: 1. Analysis: S-N 2. Results Loc.: Node This simply means that the fatigue lives will be determined at the nodes of the model. 3. Nodal Ave.: Global Accept the default which simply means element nodal stresses will be averaged to the nodes. 4. F.E. Results: Stress S-N analyses require stresses; you do not have a choice. 5. Res. Units: PSI Model dimensions are inches and forces are in Pounds, therefore stress units are PSI. 6. Jobname: comp_sn 7. Title: Component S-N Analysis

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Solution Parameters Open the Solution Params... form. On this form leave all the defaults except: Certainty of Survival: 96 As we learned in the last exercise, the S-N data can have significant scatter associated with it. We are asking MSC.Fatigue to calculate a fatigue live with 96% certainty of survival based on the scatter in the S-N data. This corresponds to a 4% failure rate. Press the OK button to continue.

Material Information The component was tested under constant amplitude, fully-reversed conditions to produce S-N data. In the previous examples we have used S-N curves that are representative of the material and independent of geometry. They related local stress (σ) to life. Now we have a different situation where the actual component geometry itself as well as the material has been used in tests to create the S-N curve. This type of S-N curve is called a component S-N curve. These type of curves relate nominal stress (S) to life and are dependent on the geometry of the component. If you change the geometry, the curve will no longer be valid. The nominal stress is a location away from the actual failure location. This is usually because it is impossible to place a measurement device such as a strain gauge in the failure location. The stress for the SN curve was measured using strain gauges at a point one quarter of an inch from the weld on the main bar and 5 inches from each end of the bar. Node 514 of the model corresponds to this measurement point for the S-N curve. The point of measurement is sometimes referred to as the reference location.

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For this model we have an S-N curve that needs to be input to PFMAT, the materials database manager. Two methods of entering this data will be given. Table 4-2 S-N Data Set for Bracket Assembly Properties

SI

Imperial

S-N Properties: Stress Range Intercept, SRI1

10,710 MPa

1553 KSI

First fatigue strength exponent, b1

-0.33333

-0.33333

Fatigue transition point (cycles), NC1

1E7

1E7

Second fatigue strength exponent, b2

-0.2

-0.2

Standard error of Log (N), SE

0.2

0.2

R-Ratio of test, RRAT

-1

-1

Young’s Modulus, E

205,800 MPa

29, 850 KSI

UTS

700 MPa

101.5 KSI

Monotonic Properties:

Manual Entry of Materials Data Open the Material Info... form and press the Materials Database Manager button. This will invoke PFMAT. Once the program has started, select Create | data set 1. You will be asked for a password to modify the central database location. If you do not enter a password and simply press the carriage return or the OK button, a copy of the central materials database will be copied to your local directory where you can then proceed to enter your materials data. Note: PFMAT always tells you at the top of its main menu whether it is connected to the central database in the MSC.Fatigue installation area or a local database in the current directory, or even some other database that you may have created in another directory.

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Now a series of forms will open requesting data entry. On the first form, Names, enter: 1. Primary name: BRACKET_SN 2. Anything else you want - not required On the next form, Static Data, enter the generic (monotonic) information: 1. UTS: Ultimate Tensile Strength (MPa): 700 2. E: Elastic modulus (MPa): 205800 Only these two parameters are required to be entered. The next form (E-N data) is for strain data. Skip over this form by pressing the OK button. The next form is for S-N data. Select Component from the pulldown menu. For the rest of the data, enter the SI values as indicated in Table 4-2. Press the OK button when done. Fracture Mechanics Data is requested next. Just press the OK button to skip over this. Multiaxial data is requested next. Skip over this form also by pressing the OK button. The material will be entered into the database. Press or double click the Graphical Display switch to view the S-N curve.

Hint:

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We are entering the data here in SI units. All underlying fatigue calculations are done using SI units. However if you wish to enter and view materials data in Imperial units, set the preference using Preferences | Stress units | PSI. You can save this setting globally, or just locally in your working directory (or not at all) so that each time you invoke PFMAT it remembers to display values and plots in your units of preference.

CHAPTER 4 Component S-N Analysis

Note: S-N curves are characterized by a power law and thus appear as straight lines in log-log space. The equation is S=SRI1(N)b where SRI1 is the yintercept and b is the slope (after Basquin). It is interesting to note historically that, although invented in 1870 by August Woehler, the S-N curve was not actually displayed graphically until some 30 years later. And it was not until 10 years after that that the curves were characterized in equation form. Our curve actually has two slopes and a transition point. If the second slope were zero it would act as a fatigue limit. Exit from PFMAT when you are done using the File | Exit and the eXit switch. Batch Entry of Materials Data For use later in this exercise, we are going to input another S-N data set. To illustrate batch mode operation of PFMAT we are going to define the parameters of the second S-N set in a file. Go to an available window and using your favorite editor, create a file called bracket.mat. Table 4-3 Second S-N Data Set for Bracket Assembly S-N Properties

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SI

Imperial

Stress Range Intercept, SRI1 (MPa)

13950

2023ksi

Slope, b1

-0.29

-0.29

Transition life, NC1 (cycles)

2E7

2E7

Slope, b2

-0.16

-0.16

Standard error, SE

0.14

0.14

Stress ratio, RRAT

-1

-1

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Enter the following lines in this file: /OPT=CREATE /INDB=YES /PASS= /MATNO=2 /PRI=BRACKET_SN2 /UTS=700 /E1=205800 /SNT=C /SRI=13950 /B1=-0.29 /NC1=2E7 /B2=-0.16 /SE=0.14 /RRAT=-1 /OPT=EX

Then from the system prompt or a DOS window issue the following command: pfmat @bracket.mat

ASCII Materials File Reader The MAT file created above can also be entered in the S-N data set by using the ASCII Materials File Reader. This form can be accessed by going to the Tools pulldown menu and selecting MSC.Fatigue (for the MSC.Patran version) or Fatigue Utilities (for the Standalone version). From here, select Material Management and then ASCII Materials File Reader. On the form that comes up, enter the name bracket.mat into the MAT Filename databox and press the Apply button.

Note: The above mentioned MAT file can also be created from scratch by using the “Edit” button on the form shown above. Either of the above mentioned two methods will put the second data set into the database. Graphically compare bracket_sn and bracket_sn2 by running PFMAT interactively and using the Graphical display option. To run interactively you can

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CHAPTER 4 Component S-N Analysis

either just type pfmat at the system prompt or go back to Pre&Post or MSC.Patran and spawn it from the MSC.Fatigue Material Info... form. Make sure both bracket_sn and bracket_sn2 are loaded as data set 1 and 2 using Load | data set n. Hint:

If you do not have any S-N data, but only know E and UTS, you can have PFMAT generate generic material properties based on empirical formulas and the type of material. Simply enter E and UTS as if you were going to enter your own S-N data and the Material Type Number (see the MSC.Fatigue User’s Guide) and the S-N parameters will be generated automatically for you. (99=steel of unknown heat treatment) Of course you have to turn on the Generate all parameters from UTS toggle.

Specify the Material for the Analysis On the Material Info... form enter the following in the spreadsheet: 1. Material: BRACKET_SN Select the cell under the Material column to activate it and select the S-N curve from the listbox that appears below the spreadsheet. The next cell will become active. 2. Finish: No Finish Select No Finish from the pulldown menu that appears below the spreadsheet. Finish and treatment are not allowed in a component S-N analysis (they are built into the curve). They will be ignored if you set them. The next cell will become active once you select the finish. 3. Treatment: No Treatment Select No Treatment. The next cell becomes active. 4. Region: default_group Select default_group which contains the nodes and elements from the entire model.

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Close the Material Info... form when you are done by pressing OK.

Loading Information To create the time history which represents the actual loading conditions of the bracket, use PTIME and the X-Y points option representing y-values only. The time history will have a maximum of 3000 lbs and a minimum of –7000 lbs. No other information has been given so you can assume that there are no peaks and valleys between these points and that only these two points are required. You will enter the values 0, 3000, –7000, and 0 to create this loading. The 1/2 hour interval can be modeled using the fatigue equivalent units. This is a term relating to the real value of one repeat of the time history. In this case, you can use 30 minutes, 1/2 hours, 1/48 days, etc. The answer will be the same of course, but you can choose the best parameter for reporting the life of your product. Open the Loading Info... form and press the Time History Manager button. Define the Load When PTIME comes up, select Enter X-Y points as the method of input. Note: If you have been working sequentially through this document, then you will already have some entries in the PTIME database. The version of the form that is displayed will be different than the one shown here. On this form, select Add an entry and then select the option X-y time series, which is the equivalent of selecting Enter X-Y points on the shown form.

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CHAPTER 4 Component S-N Analysis

A form will appear that will ask for a name, description and other information. Enter the following leaving defaults for those not mentioned: 1. Filename: BRACKET_LOAD 2. Description 1: Bracket Loading 3. Load Type: Force 4. Units: lbs force 5. Number of fatigue equivalent units: 0.5 6. Fatigue Equivalent Units: Hours We are defining a single occurrence of this signal as representing 1/2 hour. Press the OK button to go on. Next you will be prompted to enter the Y points. Enter the following numbers with a carriage return after each: 0, 3000, -7000, 0. End by putting in a blank entry and then press the End button. Plot the Time History PTIME returns to its main menu where you can select Plot an entry to make sure it took correctly. Accept the default file, BRACKET_LOAD. Select File | Exit to close the plot and press or double click the eXit switch in PTIME. Associate the FE Load to its Time Variation Now back on the Loading Info... form you must associate the time variation of the load that you just created to the FE load case. Go to the spreadsheet as was done in the previous example. Select the first cell with the mouse to activate it. 1. Load Case ID: 1.1-3.1-1This is the internal database ID. You select the FE results from the listboxes below. You must select a Result Case, a Stress result, and a layer. Then you press the Fill Cell button to enter it in the spreadsheet cell. The listboxes may appear empty at first. To fill them select the Get/Filter Results... button and turn ON the Select All Result Cases toggle and press Apply. Note: The load case ID may be different than that shown here.

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2. Time History: BRACKET_LOAD The middle cell should become active after selecting the FE result. Another spreadsheet appears at the bottom of the form from which you select the time history file. Click on the BRACKET_LOAD row anywhere with the mouse. This will fill the cell with the time history file name.

3. Load Magnitude: 900 The next cell becomes active and a databox appears below the spreadsheet. Change this entry to 900. You must press a carriage return (Return or Enter) to accept the value in the databox below the spreadsheet. A common mistake is to forget to press the carriage return to accept the value. Remember we are normalizing the FE stresses by dividing by the total applied load magnitude of 900 lbs from the FE analysis to simulate a stress distribution due to a unit load. The time variation represents the actual load magnitudes. The time variation of the loading is now associated to the static FE results. Press the OK button to close the Loading Info... form.

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CHAPTER 4 Component S-N Analysis

4.4

Run the Fatigue Analysis You are ready to run the fatigue analysis. Open the Job Control... form. Set the Action to Full Analysis and press the Apply button. The database will close momentarily as the results information is extracted. When the database reopens, the job will have been submitted. You can then set the Action to Monitor Job and press the Apply button from time to time to view the progress. When the message Fatigue analysis completed successfully

appears, the analysis is complete. Close down the Job Control... form when done.

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4.5

Review the Results Open the Results... form on the main MSC.Fatigue setup form (not to be confused with the Results application switch on the main Pre&Post or MSC.Patran form). With the Action set to Read Results press Apply. The fatigue analysis results have been read into the database.

View the Life Contour Plot Just as you viewed the stresses earlier, you can view the life plot. Press the Results application switch on the main from and select the Total Life result case and Log of Life (Hours) as the Fringe Result and press Apply. Press the Results switch again to close the Results application. Now, the point of putting up this life contour plot is to make a point. The plot is of absolutely no value and is meaningless. The only node on this structure with the correct fatigue life prediction is Node 514, the reference point of the component S-N curve. By allowing all the nodes of the model in the analysis, MSC.Fatigue treats them all as reference

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CHAPTER 4 Component S-N Analysis

nodes but only Node 514 is of interest to us. This is only the case when using component S-N curves. Contour plots from material S-N curves and the crack initiation method are perfectly valid and meaningful. Note: Since only Node 514 is valid in this analysis, it would have been better to have created a group (under Group | Create) that contained only Node 514 and then have assigned it as the region of analysis in the Material Info... form as opposed to using default_group.

Tabular Listing Now let us find out what the actual fatigue life is at Node 514. On the MSC.Fatigue Results... form, change the Action to List Results and press Apply. This will start the module PFPOST which tabularly lists the fatigue analysis results. Accepting the jobname and the default filtering values by pressing OK a couple of times will get you to the main menu. Press or double click the User specified nodes switch, enter 514 as the node number. Note the life value of approximately 10 4.115 =1.303E4 repeats (=6,515 hours) hours. This is certainly less than the design life of 7 years (61,320 hours). Press Cancel to quit the listing and press or double click eXit to leave PFPOST.

Design Optimization The objectives of this example have been partially met. The life of the component is below that of the design life for a 96% confidence level. You can enter the design optimization portion of MSC.Fatigue to answer the other objectives. This can be done by picking Optimize from the MSC.Fatigue Results... form. This time however, enter Node 514 as the node to optimize (or select it graphically from the screen).

Once in FEFAT’s design optimization mode, you can reanalyze the component. Enter the design life of 61,320 hours. You should obtain the same life estimate of around 6,500 hours. Press End to continue. Note: A file called pfatigue.ents is created when you select nodes or elements from the graphical screen or type them into the Results...| Optimize form. Node 514 is contained in this file in this case. You can also simply type 514 in the Node/Element field also in FEFAT.

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You will be placed into the FEFAT design optimization main menu. Select Parameter optimization | Scale factor to back calculate a scale factor that will be needed to achieve the appropriate design life of 61,320 hours and then press or double click Recalculate. This should give you a scale factor of about 0.5 which tells you that to achieve your design criteria you need a 50% reduction in load. This may be unacceptable. You can also set the Design criterion under Parameter optimization to determine the certainty of survival after 7 years. Remember to press the Recalculate switch. Note that it is less than one percent. So premature failure is certain. You have submitted a report to your manager which has caused panic and have been asked to re-analyze the component after using a modified welding technique, which is more expensive. After re-testing, a new S-N data set has been generated. This is BRACKET_SN2 which was imported earlier. Try a new analysis using this modified S-N data set to see if the life is satisfactory. Reset the analysis from the main menu of FEFAT by selecting the Original parameters switch. Next go to Material optimization and change the S-N curve to BRACKET_SN2 and press or double click Recalculate. You should find that the new life is around 97,000 hours or approximately 11 years. By back calculating a scale factor again in FEFAT, you will get around 1.1, which means your component should be able to survive a 10% overload and still maintain the design criteria. Also, the failure rate after seven years should be less than 0.1%. This can all be seen by repeating the steps done with the new S-N curve. Sensitivity Analysis As one last exercise in this example, select Sensitivity analysis | Scale factors. Enter the following for scale factors: (.5, 1.5, .1). This includes the parentheses. Press or double click the Recalculate switch. A sensitivity analysis will proceed and the results displayed tabularly. The scale factor input signifies (to, from, increment) a 50% reduction to a 50% overload by increments of 10%. (You can also enter a series of values separated by commas or spaces.) It is, of course, more interesting to view the results graphically. Select results Display | Sensitivity plot. The last sensitivity analysis results will be

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CHAPTER 4 Component S-N Analysis

plotted. You have specified to scale the loading (or the stresses) or you can think of the scale factors as stress concentration factors (Kt). Now you can see how sensitive the component is to loading. The same thing can be done for certainty of survival.

Hint:

When you do a sensitivity plot in FEFAT, it creates a couple of files, one XY (.xyd) plot file and a template (.tem) file that can be read into Pre&Post’s or MSC.Patran’s XY plotting application. From the MSC.Fatigue Results... form, set the Action to Plot Sensitivity. There you will see all sensitivity plots that have been created by FEFAT. You can simply select one and it will plot after you press the Apply button.

When you are done, close the plot (File | Exit) and exit from FEFAT.

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4.6

Concluding Remarks The component S-N method is the most macro view of the world of life prediction since all the failure mechanisms are built right into the component S-N curve: plasticity, geometry effects, residual stresses, surface conditions, etc. When the failure mechanisms are unknown or not well understood this method must be used. For this reason it is a completely general purpose method and lends itself well to most applications where other methods of life prediction fail. Non-ferrous materials such as plastics, ceramics, rubber, and composite structures as well as welds can use this method, whereas the other two main methods of life prediction (crack initiation and crack growth) are mainly restricted to metals or materials that behave like metals under cyclic loading conditions.

Batch Operations In this example you ran one of the MSC.Fatigue modules in batch mode. Most MSC.Fatigue modules can be run in batch mode either by including the batch commands in a file and then issuing the command using the @ sign to direct the module to read the commands from the file (pfmat @filename). Or the commands can be included on the same line as the command: fefat /opt=p/inp=filename/out=filename/ov=y

Batch operation can be quite convenient if you have to do a lot of repetitive tasks. See the MSC.Fatigue User’s Guide for batch operation descriptions.

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MSC.Fatigue QuickStart Guide

CHAPTER

5

A Simple e-N Analysis

■ Problem Description ■ Geometry ■ Set Up the Fatigue Analysis ■ Run the Fatigue Analysis ■ Review the Results ■ Concluding Remarks

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5.1

Problem Description A model, as shown to the side, aptly named the spider model because of its unique shape, is fixed at the shafts of its three legs. The center shaft is subject to a fully reversed 15 KSI amplitude pressure loading on its underside that oscillates in a sinusoidal fashion. A linear static finite element analysis was performed using MSC.Nastran with this load magnitude of 15 KSI. Everything that you have learned thus far using 15 KSI Pressure MSC.Fatigue and the Total Life method is now also applicable to the next method of fatigue life prediction. We will build on this knowledge to introduce and explain the Crack Initiation method, sometimes known as the local strain or strain-life method. As the name implies, the failure criterion now is life to initiate a crack. Once an engineering crack appears, failure is said to have occurred.

Objective • • • • •

To introduce the Crack Initiation method To understand cyclic hardening/softening To learn how cyclic stress-strain and strain-life curves are created To understand how plasticity is accounted for To relate stress-life to strain-life prediction methods Table 5-1 Chapter 5 Necessary Files File

P3_HOME/mscfatigue_files/examples/spiderCI.op2

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5.2

Geometry The geometry of the model and the FE results of the linear static analysis can be found in the file spiderCI.op2. By now you should know how to invoke Pre&Post or MSC.Patran. Do so now in a clean working directory.

Import the Model Open a new database from File | New and call it spider. The model was run through a MSC.Nastran analysis so keep the Analysis Preference set to MSC.Nastran when asked. In Pre&Post, press the Import toggle switch on the main form (not to be confused with the File | Import pulldown) or in MSC.Patran, press the Analysis toggle. When the form appears set the Action to Access Results, the Object to Read Output2, and the Method to Both (model and results); then, press the Select Results File button and select the file spiderCI.op2 and press Apply. The model will then appear and you are ready to set up a fatigue analysis.

View the Stress Results Before moving on to the fatigue analysis however, first press the Results application switch on the main form to view the stress results from the MSC.Nastran analysis. Select Stress Tensor from the listbox and set the Quantity to von Mises. Note the areas of high stress. You can rotate the model using the middle mouse button and then dragging for a better view. The areas of interest are going to be the nodes with the highest stresses. These are nodes 981, 2314, and 3650 on the top, inside portion of the center shaft between the three legs.

Node 2314 Node 3650

Node 981

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5.3

Set Up the Fatigue Analysis By now you should know how to access the MSC.Fatigue setup form. Once the form is open, set the General Setup Parameters as follows: 1. Analysis: Initiation 2. Results Loc.: Node This simply means that the fatigue lives will be determined at the nodes of the model. With a solid model this is always preferred since cracks always initiate at the surface (unless there is an internal flaw). If set to Element, the fatigue lives would be calculated at the element centroids. 3. Nodal Ave.: Global Accept the default which simply means element nodal stresses will be averaged for nodes with more than one element contribution. 4. F.E. Results: Stress You now have the choice of using either stresses or strains. Either one should give you equivalent answers. Stresses are converted to strains in this method. 5. Res. Units: PSI 6. Jobname: spider_ci 7. Title: Crack Initiation Analysis of Spider Model

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Solution Parameters Open the Solution Params... form. On this form, set only these parameters: 1. Analysis Method: None This is analogous to a mean stress correction method as was done in the S-N method. Selecting None is equivalent to no mean stress correction. 2. Plasticity Correction: Neuber We will correct for plasticity using the Neuber method. This is explained in the next section. 3. Stress/Strain Combination: Max. Abs. Principal This is the stress parameter that will be used in the fatigue analysis identical to what we have used in all previous examples thus far. Press the OK button to continue.

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Material Information This is where the major differences lie between what you have learned thus far with the Total Life method and the Crack Initiation method. Press the Material Info... button on the main MSC.Fatigue form. Create a Group Before selecting the material we wish to use, first create a group which contains all the nodes and elements of the finite element model except the nodes that have no stress results associated with them. Select Group | Create from the main menu bar of Pre&Post or MSC.Patran. Call it spider_only, change the Group Contents to Add all FEM. Press the Apply button. Now remove unwanted nodes. Change the Action to Modify. Press the Change Target Group and select our new group, spider_only. In the Member List to Add/Remove databox, type Node 10000:10006. Press the Remove button, then close the form. These nodes are associated to an MPC and have no FE results associated to them. They are removed from the analysis to avoid confusion. Select a Material First let us set up this form and then we will discuss the materialinformation involved in a Crack Initiation analysis. We will specify a material, a surface finish and treatment and a region on the model to which this combination will apply just as we have done in previous exercises. Main Index

CHAPTER 5 A Simple e-N Analysis

1. Material: BS4360-50D Only materials with strain data appear in the listbox. 2. Surface Finish: Polished Select Polished for now as most cyclic material data is created using polished test specimens, therefore we wish to use the data “as-is.” 3. Surface Treatment: No Treatment 4. Region: spider_only Select the group we just created in the previous step to calculate lives for the entire model less those that have no results associated with them. Cyclic Stress-Strain Curve Now press the Materials Database Manager button to launch PFMAT. Let us take a look at a couple of materials we have used before. Load the materials by pressing the Load | data set 1 switch and selecting MANTEN from the list. Do the same for Load | data set 2 and select RQC100. Although we do not use these materials in this analysis, they serve as good examples. Press or double click Graphical Display | Cyclic stress-strain curve plot to view the cyclic stress-strain curves for these two materials. This plot shows how these two materials behave under cyclic loading conditions. It also shows how they behave relative to one another. RQC100 is obviously a much higher strength steel with its yield point well above that of MANTEN. Three parameters (E, K’, n’) need to be defined in order to characterize these plots according to the following equation that relates stress amplitude to strain amplitude: σa σ a 1 ⁄ n' ε a = ------ +  ------ E  K' 

The first part of the right hand side can be recognized as the elastic stress-strain equation (Hooke’s law) and the second part defines the plastic strain. This equation is identical to the Ramberg-Osgood equation that relates total strain to the elastic strain plus plastic strain in the monotonic sense. The only differences are the primes (’) on K’ and n’ to signify that it represents a cyclic condition as opposed to a monotonic condition.

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Cycle Hardening, Cycle Softening Now an interesting thing to do is to plot the cyclic and the monotonic stress-strain curves on top of each other for each of the two materials we have loaded. Select File | New Plot from the current plot pulldown menu. Now select cyclic Monotonic stressstrain curves plot and press the OK button. You will be asked which data set to plot. Select data set 1, MANTEN and press the OK button. Do the same operation for data set 2. Note the following from these plots:

1. It appears that RQC100’s cyclic yield point is below its monotonic yield value. This implies it is weaker under cyclic conditions. This is known as cyclic softening or strain softening. 2. MANTEN’s cyclic yield point is above its monotonic yield point, implying that it is stronger under cyclic loading. This is known as cyclic hardening or strain hardening.

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CHAPTER 5 A Simple e-N Analysis

When a material softens under cyclic material properties, this can be very bad from a durability standpoint. This is why many structures or components fail prematurely even though, supposedly, they have been designed to remain below yield. The problem in these cases is that the actual yield point is much lower when subject to cyclic loading. Cyclic Hardening

strain

4 Measured Response

4 σ

1

5

3 t

Hysteresis Loops

2 2

5

1 3 5

stress

3

strain

1

ε

t

4 Control Condition

Cyclic Softening

5 3 1

1

5 t

2

σ

5

stress

3

1

3

ε

t 4

2

4 Control Condition

2

4 Measured Response

2

Hysteresis Loops

Note: Because of this hardening or softening phenomenon, it is highly suggested that if you do non-linear (load step) FE analysis, that you use the cyclic stress-strain curve(s) and not the monotonic ones if fatigue and durability is of concern to you. Hint:

You can put the elastic line on the stress-strain curves by selecting Plot_Type | Elastic Line. To remove the line select Plot_Type | Remove Lines.

Cyclic Material Tests How are these cyclic stress-strain curves created? The monotonic stress-strain curve is created by simply placing a test coupon in a servo-hydraulic test machine and slowly increasing the load until the component breaks. The elastic modulus can be determined from this test as well as the yield and ultimate tensile strengths. A cyclic stress-strain curve is created through a series of tests where the strain level is precisely controlled. The load is increased until the specified strain level is reached and then the load is reversed. These tests are typically performed using fully reversed loading. Each test is done at a different strain level. Initially each test will exhibit a softening or hardening effect as can be seen if the hysteresis loops are plotted for a given test (see diagram above). Eventually the material will stabilize and stop hardening or softening. This stable hysteresis loop is then extracted. The maximum stress/strain level from the stable hysteresis loop of each test is then cross-plotted onto its own stress-strain space which then constitutes the cyclic stress-strain curve.

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σ3

Stable Hysteresis Loops σ3 σ2

σ1 ε1 Test 1

ε2 Test 2

σ2 ε3

Test 3

σ1 ε1 ε2 ε3 Cyclic σ-ε Curve

Strain-Life Curve From each of these strain-controlled tests also comes another piece of information: the number of cycles to failure. This information can be plotted onto its own curve called the strain-life curve. Select File | New Plot and then select Strain-life plot. You can compare the two strain-life plots for MANTEN and RCQ100. Note that they cross each other and therefore exhibit different life behavior depending on the stain level experienced. So it is impossible to say from the plot which would perform better.

Note: The failure criterion (that is, when a crack has actually initiated) is determined by ASTM standard E606. It is not a specified length of the crack, but instead a percentage drop in load as measured by the test device. When a crack initiates the component can no longer sustain the same level of stress for the same strain rate. If you actually looked at a test specimen after reaching the failure point, there would appear to be an engineering sized crack of, say, 1 to 2 mm in length.

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CHAPTER 5 A Simple e-N Analysis

Now unload both materials. First, you will need to exit your plot. This is done by selecting File | Exit. Now you can unload the materials by doing an unload | data set 1 and then Unload | data set 2. Now let us look at the strain-life plot of BS4360-50D. Select Load | data set 1; select BS4360-50D. Now go and select Graphical Display | Strain life plot. This curve can be fully characterized by knowing four material parameters as shown in the equation of the strain-life plot (σf’,b,εf’,c): σ' ∆ε b c ------ = -----f- ( 2N f ) + ε f' ( 2N f ) 2 E (Elastic)

(Plastic)

Like the stress-strain curves, it also is broken into an elastic component and a plastic component which can also be plotted separately (Plot_Type | EP Lines). The summation of the two lines makes the total strain-life curve. The following notes are made about this plot: 1. The elastic and plastic lines cross each other at some point which is called the transition point. 2. The transition point defines the difference between high cycle fatigue (HCF) versus low cycles fatigue (LCF). 3. To the right of the transition point is considered HCF because elastic events dominate plastic events. 4. To the left of the transition point is considered LCF because plastic events dominate elastic events. The software makes every attempt to inform you (in FEFAT’s design optimization mode) what amounts of HCF, LCF, or transition life the component is experiencing. Why is this important? The type of fatigue being experienced will dictate the remedy or solution. For example, in HCF you might try a higher strength steel, or change the surface finish/treatment or mean stress (residual) to better the fatigue life. If LCF is being experienced these remedies would have little or no effect and in some cases, perhaps even worsen the situation. The solution there is to find a material that is more resistant to plastic deformation, such as a more ductile material.

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Note: Just as with the S-N method, an ε-N curve is also based on the principle of similitude. This means that if we can reproduce the same local strain as that experienced in, say, the plate with a hole shown to the right, in a test laboratory specimen made of the same material, then we can expect the life of the two to be about the same, when subjected to the same strain levels. Stress-Life vs. Strain-Life With the strain-life plot on the screen and the elastic-plastic lines posted, take a good look at the elastic portion of the strain-life equation. If you ignore the plastic component and take E to the other side of the equation you get a stress equaling some constant times the number of cycles to failure raised to the power b. This is the exact formula for the stress-life curve. So the elastic strain-life curve is, in fact, a material SN curve (with crack initiation as the failure criterion). The significance is that the S-N method is nothing more that a subset of the Crack Initiation method ignoring plastic influences. Note: The crack initiation method, taking into account elastic and plastic contributions to fatigue life, is more widely applicable to a greater range of problems (HCF and LCF) whereas the Total Life method breaks down below the transition point (usually around 105 cycles) because plasticity dominates. This is why S-N curves are only good for HCF. When finished, select File | Exit to close any plot and eXit to quit PFMAT. Press the OK button to close the Material Info... form.

Loading Information Open the Loading Info... form. Then press the Time History Manager button to launch PTIME. The load will be defined as a constant amplitude, fully reversed loading. This will have the effect of oscillating the 15 KSI load from +15 KSI to -15 KSI.

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CHAPTER 5 A Simple e-N Analysis

Define a Sinusoidal Unit Load - Fully Reversed As we have done in previous exercises, when PTIME comes up, select Copy from central as the method of input. Note: If you have been working sequentially through this document, then you will already have some entries in the PTIME database. The version of the form that is displayed will be different than the one shown here. On this form, select Add an entry and then select the option Copy from central, which is the equivalent of selecting Copy from central on the shown form. Use the List button to select SINE01. This will copy a unit sinusoidal signal to your local directory. Now select Change an entry | edit Details. Enter SINE01 as Target Filename and allow overwrite when asked and enter the following, leaving defaults for those fields not mentioned: 1. Description 1: Constant Ampl., Fully Reversed Sinusoidal Unit Load 2. Description 2: whatever you want 3. Load type: Pressure 4. Units: PSI 5. Fatigue equivalent units: Cycles We are defining a single occurrence of this fully reversed, constant amplitude signal as one cycle of the loading. Press the OK button to go on.

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Plot the Time History PTIME returns to its main menu where you can select Plot an entry. Accept the default file, SINE01. Select File | Exit to close the plot and press or double click the eXit switch in PTIME.

Associate the FE Load to its Time Variation Now back on the Loading Info... associate the time variation of the load that you just created to the FE load case just as you have done in previous exercises. Fill out the spreadsheet in the center of the form as follows with all other parameters using their default settings. 1. Load Case ID: 1.1-4.1-1Use the Get/Filter Results... button to see the available results in the database. Select the only Result Case from the first listbox and Stress Tensor from the second listbox and then press the Fill Cell button. This will fill the cell with the internal IDs of the selected load case and its stress results. Remember, the load case ID shown here may not correspond to the ID you see. 2. Time History: SINE01 When this cell become active, select the sine wave you just created.

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CHAPTER 5 A Simple e-N Analysis

3. Load Magnitude: 1.0 Remember a specification of unity here signifies that the stresses from the FE analysis will be used “as-is” in the fatigue analysis and the time variation loading that we defined will be used to scale the stresses up or down as needed. You must press a carriage return to accept the value in the databox below the spreadsheet. The time variation of the loading is now associated to the static FE results. Press the OK button to close the Loading Info... form.

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5.4

Run the Fatigue Analysis You are ready to run the fatigue analysis. Open the Job Control... form, set the Action to Full Analysis and press the Apply button. The database will close momentarily as the results information is extracted. When the database reopens, the job will have been submitted. You can then set the Action to Monitor Job and press the Apply button from time to time to view the progress. When the message Fatigue analysis completed successfully

appears, the analysis is complete. Close down the Job Control... form when done.

Rainflow Cycle Counting A

σ

B C

D

E F

time

When the analysis starts it first converts stresses to strains if stresses have been supplied. The rainflow counting procedure then takes place as discussed in earlier chapters. This results in a matrix of strain cycles with their respective ranges and means. The Crack Initiation method helps to, perhaps, better understand rainflow cycle counting.

G H

D

B F

σ

C

H The reason rainflow cycle counting works so well E A is because it actually counts the number of stressε G strain cycles (hysteresis loops) in a time varying signal. So consider a signal stood on its end. As it is loaded from point A to B and unloaded from point B to point C, this converts into the section A-B-C in stress-strain space. On loading from point C to D, in stress-strain space, it actually remembers it was on the original path from A to D and the interruption B-C-B is counted as one cycle. E-F-E also counts as a cycle as does G-HG. All cycles fall inside of one large, outer cycle (A-D-A) representing the maximum and minimum of the signal. Cycles with some noticeable area inside within this large, outer loop cause damage while those that are straight lines are purely elastic and cause no damage.

Elastic-Plastic Correction Now before damage can be determined and summed for each cycle certain corrections need to take place, the main correction being the conversion of purely elastic stresses and strains to elastic-plastic stresses and strains. Other corrections will be dealt with in later chapters.

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CHAPTER 5 A Simple e-N Analysis

Plasticity is accounted for in the Crack Initiation method by the Neuber method. The elastic stresses and strains are looked up on the elastic line and then corrected to fall onto the cyclic stress strain curve to determine the elastic-plastic stresses and strains. This elastic-plastic strain is used to look up damage on the strain-life damage curve. Neuber’s elastic-plastic correction (sometimes called a notch correction) is based on the simple principle that the product of the elastic stress and strain should be equal to the product of the elastic-plastic stress and strain from the cyclic stress-strain curve. Then through an iterative method, the elastic-plastic stress and strain can be determined. This is illustrated below.

∆σ

∆ε = σ +

Ε∆εe

2

2E

1/n’

[ ∆σ ] 2K’

∆σ∆ε = E∆ε ∆εe2

σ, ε ∆εe

Note: If we want to do stress-strain tracking so that we can calculate the maximum or mean stress of each cycle we need to know what the shape of each arm of a hysteresis loop is. We get this by using Masing's hypothesis which says that the hysteresis curve is the same shape as the cyclic stress strain curve, but doubled up in both directions, hence the factors of two in the equation for the cyclic stress strain curve above.

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5.5

Review the Results Before we actually look at the results of this analysis, let us try and predict approximately what the life prediction will be. This will help solidify some of the concepts introduced in this chapter. We can do this because the loading is simple, constant amplitude, and has zero mean. We need the following information first: 1. Node with highest stress: Node 2314 2. Maximum Principal Stress: 58,732 PSI 3. Young’s Modulus: 2.776e7 PSI 4. Strain = Stress/E: 2.12e-3 Hint:

An easy way to recover the Maximum Principal Stress is to use the Report function in the Results application. Press Results application switch on main Pre&Post or MSC.Patran form. Set the Action/Object to Create/Report. Select Stress Tensor | Max Principal. Change the mode of the form to Target Entities (the second button icon) and change the Target Entity to Nodes and type in Node 2314. Press Apply. The report is sent to the invoking UNIX or DOS window.

Now before doing anything else, look this strain level up on the strain-life curve: 55,900 reversals = 29,110 cycles. To do this yourself, go to the Material Info... form and invoke the database manager and graphically plot the strain-life plot for BS4360-50D. Using the left mouse button, click on the curve to have the coordinate locations reported to you in the lower left corner of the graphics screen. (On UNIX the coordinates are reported above the graphics on the plot command line.) This will of course be an approximation. Now correct for plasticity. The value we just read off the curve was using the elastic strain only. To find out what the elastic-plastic strain is we need to use the cyclic stress-strain curve for BS4360-50D. We need to solve this equation for σ and ε, knowing σe and εe: σeεe = σε = 124.5. This has to be done using trial and error. Graphically display the cyclic stress-strain curve. Then use the mouse as you did on the strain-life curve to find a stress and a strain that lies on the stress-strain curve that has the product of approximately 124.5. This again will be an approximation. You may end up with slightly different answers than reported here if you perform this exercise yourself: ε = 2.835e-3, σ = 45,290. Quit from PFMAT when you are done if you followed this exercise. Hint:

Main Index

It might help to zoom in on the area of interest of the curve for a more accurate coordinate reading. Either press the right mouse button in the bottom left zoom corner and again in the top right zoom corner or use the View | Window X and Window Y options and type coordinate values in.

CHAPTER 5 A Simple e-N Analysis

Now look this new value of strain up on the strain-life as you did before. So our guess is that the life prediction will be around 22,2880 reversal = 11,440 cycles. Note: The S-N curve usually is reported as stress range (∆S) versus cycles to failure (N). The ε-N curve is usually reported as strain amplitude (εa) versus reversals to failure (2N). Be aware of these facts since they could throw your calculations off by a factor of two or more if you think you are using range instead of amplitude or reversals instead of cycles.

View the Life Contour Plot Open the Results... form on the main MSC.Fatigue setup form (not to be confused with the Results application switch on the main Pre&Post or MSC.Patran form). With the Action set to Read Results, press Apply. The fatigue analysis results have been read into the database. Now go to the Results application in Pre&Post or MSC.Patran and plot the Log of Life (Cycles). Set the Action/Object to Create/Quick Plot. Select the Crack Initiation result case and Log of Life (Cycles). Press the Apply button. Note that the smallest life reported is at Node 2314 of approximately 10 4.124 = 13,346 Cycles, very close to our hand calculation of 11,440 cycles.

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5.6

Concluding Remarks This exercise has served to introduce the Crack Initiation method which uses local strain and is mostly accredited to Manson and Coffin; the material parameter, c, is named after Coffin. The cyclic stress-strain curve and the strain-life curve have been introduced as well as the Neuber notch correction method.

Other Notch Corrections Other elastic-plastic correction methods are available in MSC.Fatigue which are valuable to use for very low cycle fatigue where the Neuber method tends to break down and not be as accurate. To use the other methods (Seeger-Beste or MertensDittman) you need to define a parameter, αp. These methods, as αp tends to infinity, revert to the Neuber method. This parameter, αp, is known as a shape factor or a limit load ratio (Lu/Ly). Ly is the yield limit and Lu is the limit A load where you assume elastic-perfectly plastic B behavior yield stress (+ or -) E D across the whole section. To C determine Lu it becomes a A: Neuber Eεε2 formulation B: Neuber εσ formulation simple integration (if you C: Neuber εσ and corrected curve have a simple geometry). If D: Mertens-Dittman and corrected curve E: Seeger-Beste and corrected curve you also have a small notch, the Kt of the notch will reduce the yield load but not the limit load (much) so the shape factor goes up. These ratios generally fall somewhere between 1.5 and 3. See the MSC.Fatigue User’s Guide for a more detailed definition of αp and its value for some standard shapes. The diagram above compares an elastic-plastic FE analysis to the Neuber and other notch correction methods.

Stresses vs. Strains In this example we used stresses from our FE model. We could have just as easily selected the strains to use instead. We would expect to get exactly the same answers, however there are a few things to be aware of. 1. Young’s Modulus must be the same as that used in the FE analysis and that defined in the material BS4360-50D. Otherwise the strains reported in the FE model will be different than the ones calculated by MSC.Fatigue when converting stresses to strains.

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CHAPTER 5 A Simple e-N Analysis

2. The number of rainflow bins can influence the accuracy between using stress vs. strains. Try this as an exercise to see the influence of the number of bins on the fatigue life prediction. From the Results... form select Re-Analyze and enter Node 981 2314 3650. These are the nodes with the highest stresses. This will run FEFAT for you. When FEFAT appears accept all the defaults except change the Matrix size to 64. Then do it for 128 bins. Note that the fatigue life predictions increase to over 14,000 cycles. Now go back to the original job setup and change the General Setup Parameter, FE Results: to Strains, and go to the Loading Info... form and select the Strain Tensor in the Load Case ID column. Re-run the analysis and do the same Re-Analyze operation as you did when using the stress FE results. Note that for 32 bins, the same exact results are determined for all three nodes. Even for a higher matrix size, the strain FE results are less conservative than when using the FE stresses. This is because the resolution of the bins is better when using stresses. 3. You should be very careful using FE strains from plate models. Because many FE codes do not calculate or do not include the out-of-plane strain (εz), which is needed to determine the proper strain combination parameter (max. abs. principal, signed von Mises, etc.), it is safer to use the stresses from the FE analysis. 4. One final thing to be aware of using FE strains: the strains that are usually stored in the database when imported from a typical analysis code such as MSC.Nastran are stored as strain tensors, not as engineering strain. MSC.Fatigue multiplies the three shear strain components by two to convert them to engineering strain before using them in a fatigue analysis. This does not happen when external result files are used.

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MSC.Fatigue QuickStart Guide

CHAPTER

6

Residual Stress

■ Problem Description ■ Geometry ■ Set Up First Fatigue Analysis ■ Set Up Second Fatigue Analysis ■ Investigate Mean Stress ■ Investigate Surface Finish/Treatment ■ Concluding Remarks

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6.1

Problem Description In this example problem we investigate how residual stress can be incorporated into a fatigue analysis. The techniques used here are applicable to any type of fatigue analysis but for illustration purposes we use the Crack Initiation method. An injection mold, already in service, experiences a 12 KSI pressure load when it is filled. The mold experiences premature failure in the fillet area. To investigate ways to improve the fatigue life an overload of 20 KSI is applied to the mold to induce a compressive residual stress. Due to symmetry, only one quarter of the actual injection mold is modeled using finite elements. The design life of the mold is to last a half a million injections (Fills).

Objective • To illustrate how residual stress can be incorporated into a fatigue analysis • To investigate mean stress effects in Crack Initiation • To investigate surface finish and treatment Table 6-1 Chapter 6 Necessary Files File P3_HOME/mscfatigue_files/examples/mold_linear.op2 P3_HOME/mscfatigue_files/examples/mold_nonlin.op2

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CHAPTER 6 Residual Stress

6.2

Geometry The geometry of the model and the FE results of the linear static analysis and the residual stress can be found in the files mold_linear.op2 and mold_nonlin.op2. Two FE analyses were performed on this geometry. The first was to simulate a normal fill (the operating load) of 12.5 KSI. The second was to simulate the overload that imposed the residual stress (20 KSI). The load was imposed and then removed using a nonlinear load step analysis in MSC.Nastran. To begin, invoke Pre&Post or MSC.Patran in a clean working directory.

Import the Model Open a new database from File | New and call it mold. The model was run through a MSC.Nastran analysis so keep the Analysis Preference set to MSC.Nastran when asked. Press the Import toggle switch in Pre&Post (Analysis in MSC.Patran) on the main form. When the form appears, set the Action to Access Results, the Object to Read Output2, and the Method to Both (model and results); then, press the Select Results File button, select the file mold_linear.op2, and press the Apply button. The model will then appear and you are ready to set up a fatigue analysis. Also read in the non-linear result with the Action set to Access Results, the Object set to Read Output2, and the Method set to Results Entities, and then select the file mold_nonlin.op2.

View the Stress Results Before moving on to the fatigue analysis however, view the stress results from the MSC.Nastran analysis if you wish by pressing the Results application switch on the main form. There are two Result Cases of interest: LS_PRESSURE_12.5KPSI,Static Subcase and LS_PRESSURE_20KPSI_REMOVE, PW Linear : 200.% of Load. The first is simply the static operating load and the second is the result of removing the 20 KSI overload showing the remaining residual stress. Select Stress Tensor from the

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114

listbox and set the Quantity to von Mises. Note the area of high stress is in the fillet area as expected. You can rotate the model using the middle mouse button and then dragging, for a better view.

Node 2314 Node 3650

Stress Distribution due to Operating Load

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Node 981

Residual Stress after Overload

CHAPTER 6 Residual Stress

6.3

Set Up First Fatigue Analysis By now you should know how to access the MSC.Fatigue setup form and have a fairly good idea how to set up a basic fatigue analysis. Open the form and set the General Setup Parameters as follows to run a fatigue analysis on just the operating load case: 1. Analysis: Initiation 2. Results Loc.: Node 3. Nodal Ave.: Global 4. F.E. Results: Stress 5. Res. Units: PSI 6. Jobname: mold 7. Title: Crack Initiation Analysis of Injection Mold

Solution Parameters Open the Solution Params... form. Nothing needs to be changed here. Simply accept all the defaults.

Material Information Open the Material Info... form. Set the following on this form as done in previous exercises: 1. Material: SAE4340350A_QT 2. Surface Finish: Polished 3. Surface Treatment: No Treatment 4. Region: default_group

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Loading Information Open the Loading Info... form. Then press the Time History Manager button to launch PTIME and define a unit load with R= ∞ . Use X-Y time series with three points (0, 1, 0) to define this simple load simulating a fill of the injection mold form zero load to the maximum and back to zero. Note: As mentioned in the previous chapters, if you have been working sequentially through this document, then you will need to select Add an entry before you can enter the X-Y point data. Call it FILL_LOAD. Give it the following details when asked: 1. Description 1: Constant Amplitude, R=infinity, Unit Load 2. Description 2: whatever you want 3. Load type: Pressure 4. Units: PSI 5. Fatigue equivalent units: Fills Next fill out the Loading Info... form with the following input:

1. Load Case ID: 2.1-2.1-1This corresponds to the LS_PRESSURE_12.5KPSI result case which is the operating load only without any residual stress. Remember, the load case IDs may not match those shown here. 2. Time History: FILL_LOAD 3. Load Magnitude: 1.0

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CHAPTER 6 Residual Stress

Run the Fatigue Analysis You are ready to run the fatigue analysis. Open the Job Control... form, set the Action to Full Analysis and press the Apply button.

Review the Results Open the Results... form on the main MSC.Fatigue setup form and either read the results into the database and create a contour plot or use the tabular listing Life Contour Plot facility to find the node with the lowest life as has been done in previous analyses. You should find that the smallest life reported is approximately 67,000 Fills, which is far off from the design goal of 500,000 Fills.

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6.4

Set Up Second Fatigue Analysis Now we will set up the second fatigue analysis where we will include the residual stress to see how this effects the life of the mold. The Solution Params... and the Material Info... form setups will remain identical. Only the Loading Info... form requires a change. First change the Jobname to residual and change the Title to something such as Mold with Residual Stress and then open the Loading Info... form.

Include the Residual Stress To include the residual stress in the analysis is a simple task. On the Loading Info... form set the Number of Static Load Cases: to 2. (You must press the Return or Enter key to effect a change.) Two rows will appear in the spreadsheet. The first row should still be set from the first analysis with the load case that represents the operating stress. Set up the second row as follows: 1. Load Case ID: 4.7-2.1-1In the second row, select the Load Case ID column cell and select the result case which contains the residual stress: LS_PRESSURE_20KPSI_REMOVE, PW Linear: 200.% of Load. Press the Fill Cell button after selecting the Stress Tensor (not the Nonlinear Stresses). Again, the load case IDs may not match those shown here. 2. Time History: STATIC When the next cell becomes active you will see that the bottom spreadsheet containing the names of externally defined load histories now has two entries. The original FILL_LOAD is accompanied by a new one called STATIC and its Type is Offset. Whenever more than one static load case is used, you have the option of setting any one of them to be defined as a simple static offset that does not have any time variation associated with it. This STATIC load history shows up automatically. You do not have to create it. 3. Load Magnitude: N/A This is automatically set to N/A since it is “not applicable.” Note: You must have at least one load case associated to a time varying load.

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CHAPTER 6 Residual Stress

Run the Fatigue Analysis You are ready to run the fatigue analysis. Open the Job Control... form and set the Action to Full Analysis and press the Apply button. What happens during the analysis when a STATIC load case is specified is that the FE stresses from the operational load case are scaled by the magnitude of the load history at any given time step and the FE stresses from the STATIC (residual) load case are then used to offset the stress.

Review the Results Open the Results... form on the main MSC.Fatigue setup form and either read the results into the database and create a contour plot or use the tabular listing facility to find the node with the lowest life. You should find that the smallest life reported is approximately 225,000 Fills. We have significantly bettered the life of the mold and but appear not to have achieved the design goal of 500,000 Fills.

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6.5

Investigate Mean Stress As with the S-N method, there are ways with the Crack Initiation method to account for mean stress also. The material properties (cyclic stress-strain and strain-life curves) are derived with zero mean stress (R = minus 1). The signal used in this exercise has tensile mean stress and R= ∞ . Two methods are available for mean stress correction: Smith-Watson-Topper (SWT) and Morrow. SWT is the default and was used in these analyses. It is not necessary to go back and redefine anything in the original jobs to investigate the effect of mean stress correction. Open the Results... form if it is not already open and set the Action to Optimize and press Apply. When FEFAT comes up in its Design Optimization mode, select Worst Case node, enter a Design Life of 500,000, and press OK. You should see the same fatigue life at the worst case node of about 255,000 fills or 67,000 fills for the first job. Press End to move to the main menu. You can do this with either job (mold or residual). Type in the name of the analysis you want to investigate in the Jobname databox on the main MSC.Fatigue job setup form. Any options you select will retrieve the jobname and use it.

Hint:

Now select Sensitivity analysis | Mean stress correction (all). Then press or double click the Recalculate switch. Note the life values calculated for each:

Table 6-2 Mean Stress Effects Mean Stress Correction

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mold (no residual stress)

mold (with residual stress)

Smith-Watson-Topper

67,000 Fills

225,000 Fills

Morrow

132,000 Fills

323,000 Fills

Strain-Life (none)

546,000 Fills

546,000 Fills

CHAPTER 6 Residual Stress

The following observations are made: 1. Note that with no mean stress correction, the life prediction is identical. This is expected since all a residual stress is an offset. The only difference between the two analyses is that they have different mean stresses. The actual strain range between the two is identical. If mean stress is not taken into account, the two will give identical answers. 2. SWT gives the most conservative answer for predominately tensile signals. SWT tends not to account too well for compressive mean stress. For this reason Morrow gives more conservative answers for compressive signals. 3. Had we not considered mean stress in this example we might have been mislead to think that we had met our design life of 500,000 Fills. 4. Changing the mean stress tends to only have effects in the high cycle fatigue (HCF) region. The effect of mean stress gets washed out with low cycle fatigue (LCF) problems due to the higher plasticity. This can be seen in the Morrow equation for mean stress where the mean stress is accounted for only on the elastic side of the equation. The plot above also illustrates this comparing a strain-life plot with and without Morrow mean stress correction (note only the HCF side is effected). σ f' – σ m ∆ε b c ------ = ------------------- ( 2N f ) + ε f' ( 2N f ) 2 E

Morrow Mean Stress

SWT mean stress correction has the effect of shifting the entire curve and plotting a new parameter on the right hand side of the equation by multiplying by the maximum stress. 2

σ f' 2b b+c ∆ε ------ σ m ax = --------- ( 2N f ) + σ f'ε f' ( 2N f ) 2 E

SWT Mean Stress

To illustrate this last point using FEFAT, do a sensitivity plot from each analysis by increasing the loading. You will see that at higher load levels the answers tend to converge between the two analyses, negating the effect of the residual stress. Follow these instructions assuming you are at the Design Optimization main menu of FEFAT still: Main Index

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122

1. Select Original parameters. This resets the analysis to all original settings. 2. Select Sensitivity analysis | Scale Factor. Enter (1,3,0.2) including the parentheses to calculate all factors between one and three by increments of 0.2. 3. Select Recalculate. This will calculate lives based on SWT. 4. Select Change Parameters. Change the Mean Stress Correction to Morrow. Leave all other settings as is. 5. Select Recalculate. This will calculates lives based on Morrow. 6. Select new Jobname and re-do these steps with the other analysis job if you wish.

Table 6-3 Mean Stress Effects Scale Factor

mold (no residual stress) SWT

Main Index

mold (with residual stress)

Morrow

SWT

Morrow

1.0

67,000

132,000

225,000

323,000

1.2

23,700

37,700

51,500

65,500

1.4

10,100

15,300

18,172

21,500

1.6

5,400

7,600

8,400

9,600

1.8

3,200

4,400

4,700

5,200

2.0

2,100

2,800

2,900

3,200

2.2

1,500

1,900

1,950

2,100

2.4

1,100

1,400

1,400

1,500

2.6

820

1,050

1,040

1,100

2.8

640

814

801

860

3.0

515

650

635

680

CHAPTER 6 Residual Stress

6.6

Investigate Surface Finish/Treatment MSC.Fatigue can compensate for different surface treatments and finishes as you have noticed when setting up analysis jobs. Up to this point we have always set the finish and treatment to either none or a polished finish (which are the same) signifying that we wish to use the material properties “as-is” with no corrections. Do the following to investigate the effect of surface finish and treatment assuming you are still at the Design Optimization main menu of FEFAT: 1. Select Original parameters. This resets the analysis to all original settings. 2. Select Sensitivity analysis | surface Finishes (all). 3. Select Recalculate. This calculates lives based on SWT for all surface finishes. 4. Select Original parameters. 5. Select Change Parameters. Change the Surface Condition to Poor Machined. Leave all other settings as is. 6. Select Sensitivity analysis | surface Treatment (all). 7. Select Recalculate. This calculates lives based on SWT with Poor Machined finish for all surface treatments. Select new Jobname and re-do these steps with the other analysis job if you wish. To meet the design life of this injection mold we could have left it machined with a poor finish and nitrided it and not imposed a residual stress at all:

Table 6-4 Mean Stress Effects Surface Condition

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mold (no residual stress)

mold (with residual stress)

Polished

67,000 Fills

225,000 Fills

Ground

44,000 Fills

122,000 Fills

Good Machined

27,000 Fills

60,900 Fills

Average Machined

20,000 Fills

41,200 Fills

Poor Machined

15,500 Fills

29,300 Fills

Nitrided

3,170,000 Fills

946,000 Fills

Cold Rolled

738,000 Fills

287,000 Fills

Shot Peened

130,000 Fills

72,400 Fills

123

124

Surface finish and treatment corrections are imposed by changing the material properties. This is accomplished by changing the slope of the S-N curve or for strainlife curves, the slope of the elastic line at the endurance limit. A scale factor for each finish or treatment is stored in the materials database. These factors are based on the UTS of the material and derived from empirical data. Surface treatments and finishes tend again to only effect HCF jobs. To illustrate, you can perform an exercise similar to that done in the previous section where the load was increased by doing a sensitivity analysis on the scale factor. Except this time do it for different surface finishes or treatments. Note that the answers tend to converge between the various surface finish/treatments at higher load levels. The curve to the right shows two strain-life curves, one with polished and one with some other finish, where only the HCF end is effected.

Note: Shot Peening is a mechanism used to impose a compressive residual stress into the surface, thus changing the mean stress. However, it is compensated for by surface finish/treatment techniques.

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CHAPTER 6 Residual Stress

6.7

Concluding Remarks Imposing a residual stress is simply a mechanism of changing the mean stress. Residual stress can be the result of a manufacturing process or an overload as in the case of this example. Prestress and offset stresses due to other effects such as gravity or centrifugal forces can be accounted for in the same manner. When these offset stresses vary over the model, generally an additional FE load case must define them. If the offset is constant, other methods of accounting for residual stress are possible in MSC.Fatigue:

Accounting for Constant Residual Stress 1. Material Info... form. On this form, if you scroll the spreadsheet over you will see a column called Offset. Here you can impose a constant residual offset that will be applied to only the area of your model defined in Region (Group) with the specified combination of material, finish and treatment.

Scroll spreadsheet.

2. Loading Info... form. Aside from selecting an entire FE load case as a offset which varies from node to node, you can also set a constant offset for any specified FE load case. If you scroll the spreadsheet over on this form you will also see a column to specify an Offset.

Scroll spreadsheet.

3. FEFAT: Residual stress can be entered in the form of a loading or stress offset when performing analysis interactively using FEFAT. In the Design Optimization mode, Sensitivity analysis on residual stress is possible as well as in the Parameter optimization menu to back calculate a residual stress to meet the design life. Main Index

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MSC.Fatigue QuickStart GuideG

CHAPTER

7

Introduction to Crack Growth

■ Problem Description ■ Geometry ■ Set Up the Fracture Analysis ■ Run the Fracture Analysis ■ Review the Results ■ Concluding Remarks

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7.1

Problem Description We now revert back to our first problem, the keyhole model to introduce the concept of linear elastic fracture mechanics (LEFM) or Crack Growth. Subject to the same loading as before and to the complex transmission loading sequence (SAETRN), we are interested in determining what remnant of life is left in this component after crack initiation and how long until the crack will become a catastrophic failure. A detailed discussion of fracture mechanics and its governing principles is beyond the scope of this tutorial guide and the user is referred to the for a more detailed description. However, the concepts are explained here in as simple terms as possible to introduce the method.

Objective • To introduce the LEFM life prediction method, commonly referred to as “Crack Growth.”

Table 7-1 Chapter 7 Necessary Files File P3_HOME/mscfatigue_files/examples/simpleSN.op2

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CHAPTER 7 Introduction to Crack Growth

7.2

Geometry Copy the file simpleSN.op2 to a clean working directory to begin. A linear static finite element analysis has been performed already with a load magnitude of 10,000 Newtons. To begin, read this model and results information into a new database using MSC.Fatigue Pre&Post or use MSC.Patran. Open a new database from File | New and call it keyhole. The model was run through a MSC.Nastran analysis so keep the Analysis Preference set to MSC.Nastran when asked.

Import the Model Press the Import toggle switch (Analysis in MSC.Patran) on the main form. When the form appears, set the Action to Access Results, the Object to Read Output2, and the Method to Both (model and results); then, press the Select Results File button and select the file simpleSN.op2. The model will then appear and you are ready to set up a fatigue analysis.

Define a Compliance Function For all fatigue and fracture analyses thus far, we have been defining the three major inputs: geometry, materials, and loading. This is no different for a Crack Growth analysis except that geometry definition takes on a different form than what we have dealt with to this point. For the Total Life and Crack Initiation methods we have been tightly linked to the FE analysis and the stress/strain distribution. This link to FE analysis is much weaker for the Crack Growth method in that the only information necessary is the remote stress used in the Paris Equation, defined as cyclic range of stress intensity, ∆K, da m ------- = C ( ∆K ) dN

where

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∆K

= Cyclic range of stress intensity

da ------dN

= Rate of cracked growth

C

= Paris Law coefficient

m

= Material constant

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130

and the definition of stress intensity, defined as driving force, K, the applied stress, σ, the crack size, a K = Yσ ( πa )

where K

= The driving force

σ

= The applied stress

a

= The crack size

Y

= The compliance function

Y is known as the compliance function and describes the geometry in which the crack exists.

The Fracture Mechanics Triangle The driving force behind a crack that causes it to Stress Intensity propagate is not stress or strain but the stress intensity factor, K. (This is not to be confused with stress concentration Kt.) The stress intensity factor accounts for both the stress and the crack size and is a way of Fracture describing the stress field around a crack tip Mechanics Triangle independent of the overall geometry. The relationship between stress intensity, stress, and crack length is known as the fracture mechanics triangle. If you know Crack Size two of the corners you can derive the other.

Stress

Compliance Function Library MSC.Fatigue contains a library of standard crack geometries from which you can derive a compliance function. Open the main MSC.Fatigue setup form and set the Analysis to Growth. Then open the Solution Params... form. On the top of the form you will see a button called Compliance Generator. Press this button. An MSC.Fatigue module called PKSOL will initiate that will give you access to the compliance function library.

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CHAPTER 7 Introduction to Crack Growth

The first thing that you are asked for is the units in which to define the geometry. Select 1. Millimeters. Then select option four, 4. Generate a Y function table. Call it keyhole when asked. A file called keyhole.ksn will be created containing the compliance function lookup table.

127 (5.0)

68.6 (2.7)

25.4 (1.0)

P

Notch Depth 152 (6.0)

6.35 (0.25)

mm (inches)

76.2 (3.0)

Two more menus will be presented to you to select the library entry to use. Select option 1. Standard specimens and then option 8. Compact tension specimen (CTS). You will then be presented with a graphic representation of the geometry to which you will specify dimensions. The dimension of our keyhole model are as shown here.

9.525 (0.375) Dia. Press the Define button on the top of the form. At 3.175 (0.125) Notch Root Radius this point you will be asked for the dimensions of the geometry, B (thickness) and W (width). Enter P 9.525 (0.375) Thick 9.525 and 94 mm. Press the carriage return each time and a final carriage return when asked if any changes are necessary. To generate the compliance function, select the Calculate item on top of the form.

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At this point you are presented with one final menu selection which allows you to either plot, tabulate or create another compliance function. Select Plot Y function against crack ratio.

This plot gives you a good feel as to how the crack will grow as it gets larger. In this case, as the crack ratio (a/W) increases, the crack growth rate will accelerate whereas, at first, the growth rate will be much slower. When you are done select File | Exit to close the plot and then quit from PKSOL by selecting eXit. Hint:

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The compliance library contains around 35 different crack geometries. You can view the different geometries once you enter PKSOL by selecting option 5. Display solution library from the main PKSOL menu. A graphical display of available geometries for the selected option is plotted.

CHAPTER 7 Introduction to Crack Growth

7.3

Set Up the Fracture Analysis The MSC.Fatigue setup form should be visible; set the General Setup Parameters as follows: 1. Analysis: Growth 2. Results Loc.: Node 3. Nodal Ave.: Global 4. F.E. Results: Stress Crack Growth analyses require stresses; you do not have a choice. 5. Res. Units: MPa Model dimensions are millimeters and forces are in Newtons, therefore stress units are MPa. 6. Jobname: simple_cg 7. Title: Simple Crack Growth Analysis

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Solution Parameters Open the Solution Params... form. On this form, set these parameters: 1. Select a Compliance Function: KEYHOLE The name of the compliance function that you created earlier should appear in this listbox. Select it. 2. Stress Combination: Max. Abs. Principal This is the stress parameter that will be used in the fatigue analysis. The stress tensor from the FE analysis results will be extracted at each node, the maximum absolute principal calculated and then averaged over all nodes defined in the Region specified on the Material Info... form. It is this stress that will be used to determine the stress intensity range for each cycle. 3. Crack Length Units: Inches Define the units in which all the below parameters will be defined. 4. Initial Crack Length: 0.1 This is the initial crack length which can be anything but zero. If zero is entered this acts as a flag to tell MSC.Fatigue to use minimum crack sizing rules to determine the minimum crack size for valid fracture mechanics analysis. Here we have entered a typical engineering crack size that might be detectable after crack initiation. 5. Final Crack Length: 2.13 We do not know what this will be but it cannot be any larger than from the end of the notch to the other side of the keyhole specimen. It is very likely that the crack will not actually grow to this size before catastrophic failure. 6. Notch Depth: 2.3 This is the physical depth of the notch (keyhole) in this compact tension specimen. By entering a non-zero value here, MSC.Fatigue will compensate for notch influences. 7. Notch Radius: 0.375 This is the radius of notch. The default is zero.

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CHAPTER 7 Introduction to Crack Growth

8. Sharp Crack Radius: 0.0 This is a sharp crack radius. The default is zero. If zero is entered here and/or for the Notch Radius, a minimum default is used since they cannot actually be zero. Note: We have mixed units in this example using Inches in the above discussion and Millimeters when the compliance function was defined. This is OK and there is no mismatch here because the compliance function is a nondimensional number and the initial and final crack lengths and the notch dimensions are converted to SI units internally. Press the OK button to continue.

Material Information Press the Material Info... button on the main MSC.Fatigue form.

Make a Group Before proceeding, use the Group facility to create a group of nodes that is representative of the far field stress. The stress from these nodes will be averaged and used in the determination of the stress intensity as earlier described in the equation K = Yσ πa. This is where the departure from the other two methods can be somewhat confusing. The region that is defined on the Material Info... form should not contain the nodes from the entire model or an interested portion thereof, but must contain the area of the model that is representative of the stress in this stress intensity equation. What this stress should be is best described as the stress that would be there if there were no crack (or notch) in the structure. So in this case the stress would be load divided by area (P/A) where the area is the entire cross section without the notch. Open the Group | Create form from the main pull down menus of Pre&Post or MSC.Patran. Give the new group a name such as far_field. Select all the nodes from Element 166. This can be done by activating the Entity Selection databox and graphically selecting them (use the shift key for multiple selections) or simply type the node numbers in as Node 211:213 594:595 606:608. Press the Apply button to create the group and the Cancel button to close the form. Note: Although the above statements are true, the nodes selected here to represent the far field stress have been chosen somewhat arbitrarily to force the crack to grow rapidly for illustration purposes only.

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Select a Material and Environment On the Material Info... form the spreadsheet layout is slightly different than that for other analysis types. It now asks for a material as before, and also an environment. No surface finish or treatments are applicable. Also only one combination of material, environment, and region can be specified. It no longer makes sense to define multiple materials. 1. Material: MANTEN Select the first cell with the cursor. A listbox appears at the bottom of the form from which you select a material. Only datasets with LEFM data appear in the listbox. Select MANTEN. 2. Environment: Air You only have one choice. 3. Region: far_field Select the group you just created defining the area of far-field stress.

View the da/dN Curve It is of interest to view the actual material information that will be used to look up damage and calculate crack growth rate. Press the Materials Database Manager button. This will launch PFMAT, the materials database manager. First load the material by pressing the Load switch and selecting the data set 1 option. Now select

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CHAPTER 7 Introduction to Crack Growth

MANTEN from the list. You can then select the Graphical Display | Apparent delta k plot switch to view the da/dN curve. You will be asked to enter an R-ratio. Enter 0.5 and press the OK button. The curve will be displayed. Note: You can enter a couple of R-ratios if you wish. Mean stress is accounted for in Crack Growth analysis by using the curve corresponding to a given stress cycles R-ratio

Material response is modeled by measuring crack growth rates versus stress intensity (∆K) in constant amplitude tests. From these tests are derived the da/dN curve and the threshold characteristics and fracture toughness of the material. da/dN

Fast Fracture Effects Paris Equation Region Threshold Effects

∆K

In fatigue we are concerned with stable crack growth occurring below a catastrophic level. When you plot crack growth rates against ∆K on log scales, you get sigmoidal shaped curves like these which have three distinct regions. There is a linear region in the middle of these curves which is described by the Paris Equation. At the bottom end of the curves there is a threshold below which no crack growth occurs (very similar to a fatigue limit). This is caused by crack closure and the interaction of the crack with the micro-structure. If the mean stress is raised the threshold decreases because the cracks are held open for more of the time. At the other end of the curve, crack growth rates increase as the maximum stress of each cycle gets close to the fracture toughness of the material. The curve you just plotted in PFMAT is called the Apparent ∆K curve describing the apparent driving force acting on the crack. However there are many effects that this equation does not take into account, such as crack closure, corrosive environments, the influences of a notch, and static fracture mode contributions to name a few. MSC.Fatigue models these by using an Effective ∆K curve which has the effect of linearizing the entire Apparent ∆K curve through all three of its distinct regions. It is

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this Effective ∆K that is the actual (effective) driving force that is then used in the Paris Equation to determine crack growth. Select File | New Plot | Effective delta k plot to view this da/dN curve. The material information is complete. Select File | Exit to close the plot and eXit to quit PFMAT. Press the OK button to close the Material Info... form. Note: A da/dN curve is based on the principle of similitude just as with the previous two methods discussed thus far. This simply means that if we can reproduce the same driving force as that experienced in the real structure, in a test laboratory specimen made of the same material, then we can expect the crack propagation rate to be about the same, when subjected to the same driving force

Loading Information We are going to use the same loading as was used in Rainflow Cycle Counting (Ch. 3) but with a different scale factor to accelerate the crack growth for illustration purposes. So open the Loading Info... form and then press the Time History Manager button:

Copy SAETRN from the Central Database When PTIME comes up, select Copy from central as the method of input. A form will appear that will ask for a name. Use the List button to select SAETRN from the central database. Note: If you have been working sequentially through this document, then there is a good chance that the entity SAETRN already exists in your database. To check this, select List all entries. If the entity SAETRN is listed, you will need to delete it before continuing. Go back to the PTIME main menu, select Delete entries, then go and select the database entry SAETRN and press OK. You will be asked to verify that you want to delete this entry. Now that the entry has been deleted, we can get a fresh copy from the central database. Select Add an entry..., then select Copy from central. Use the List button to select SAETRN from the central database.

Scale the Time History Load From the PTIME main menu, select Change an entry... and then Polynomial transform. Allow overwrite when asked. Scale up the time history to represent the actual loading applied to the component. Enter scale factor of 40 in the second databox. Press OK when done. Enter the following details when asked: Main Index

CHAPTER 7 Introduction to Crack Growth

1. Description 1: Leave as is 2. Description 2: Blank this out 3. Load type: Force 4. Units: Newtons 5. Number of fatigue equivalent units: 1 6. Fatigue equivalent units: Repeats Press OK to continue. Plot the time history if you wish and then select File | Exit to close the plot and press the eXit switch in PTIME.

Associate the FE Load to its Time Variation On the Loading Info... form, associate the time variation of the load that you just created to the FE load case as you have done in all previous exercises. 1. Load Case ID: 1.1-3.1-2Make sure to select Default, Static Subcase and Stress Tensor at layer Z1. 2. Time History: SAETRN 3. Load Magnitude: 10,000 The Load Magnitude of 10,000 Newtons is the same as that used in the previous S-N analysis and is used to normalize the stresses such that they are due to a unit load. 4. Results Transformation: No Transformation In the upper right hand corner of the Loading Info... form, set this to No Transformation. FE results will not be transformed to the basic coordinate system before averaging. Generally you will want to transform results to the basic coordinate system when they are element nodal (integration points), such that proper averaging can be done. There is no need to transform element centroidal or nodal results, since no averaging takes place for these types. The time variation of the loading is now associated to the static FE results. Press the OK button to close the Loading Info... form. Remember, the load case ID may not match that shown here but should be the only one available in the database.

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140

7.4

Run the Fracture Analysis You are ready to run the fracture analysis. Open the Job Control... form, set the Action to Full Analysis and press the Apply button. The database will close momentarily as the results information is extracted. When the database reopens, the job will have been submitted. You can then set the Action to Monitor Job and press the Apply button from time to time to view the progress of the crack growth. When the message Crack growth calculation completed successfully

appears, the analysis is complete. Close down the Job Control... form when done.

Cycle by Cycle Growth The only accurate way of predicting crack growth is by adopting the cycle-by-cycle approach. Normal rainflow cycle procedures are used in Crack Growth analysis as with the other two methods, however the cycles are re-ordered into their original sequence to retain the sequence effects. For each cycle the crack extension, da, is calculated and added to the current crack size and this process continues until a failure condition is reached. The driving force for propagation due to each cycle is the range of stress intensity, ∆K. For each cycle the apparent or applied driving force is calculated from the stress range, current crack size, and geometry (compliance function) of the component. It is then modified due to various considerations of crack closure, history or overload effects, notch and environmental influences, and static fracture mode contributions. The modified (effective) ∆K is then used to determine crack extension for any given cycle using material parameters and the Paris Equation. Note: It should be clearly understood that even though the growth of microscopic cracks is governed by linear elastic fracture mechanics, the microscopic crack extension process by fatigue still necessitates local plasticity. At the tip of a fatigue crack there is a plastic zone. The crack tip plastic zone is assumed to be negligibly small in relation to crack size, especially for high strength materials, but essential to the fatigue crack growth process.

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CHAPTER 7 Introduction to Crack Growth

The Fatigue Crack Propagation Rectangle As discussed earlier, the Fracture Mechanics Triangle relates stress intensity, stress, and crack length. When speaking in terms of crack growth and overall life, a rectangular rather than a triangular representation is used. In Crack Growth there is a relationship between stress range and life just as with the Total Life (S-N) method except it is extended to include the initial and final crack lengths (and all crack sizes in-between these two limits). So in a similar way to solving the triangle, the fatigue crack propagation rectangle can be solved by knowing any three of the four corners to derive the fourth.

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7.5

Review the Results Open the MSC.Fatigue Results... form. No color contour plotting is available with Crack Growth.

Tabular Listing On the MSC.Fatigue Results... form, change the Action to List Results and press Apply. This will start the module PCPOST which tabularly lists the fracture analysis results and also plots the final situation. The main menu of PCPOST includes a host of items, the most useful perhaps, being the Results summary page. Selecting this will reveal that the crack grew to a bit over 10 mm before fracture and took over 400 repeats of the SAETRN time history. The mode of failure is also revealed (stress intensity exceeding the fracture toughness of the material). Press the End button to continue. Also plot the final a-N curve. This plot will be described in more detail shortly. This reveals how the crack grew over time. Select Return to return to the main menu of PCPOST.

Interpolate Crack Sizes One of the interesting things you can do with PCPOST is interpolate the life based on different crack sizes. Select Interpolate life. Now you can enter a different initial crack length or a different final crack length or both. It will then, on-the-fly, report back to you the interpolated life. It is very possible that the actual initial crack is much larger than previously thought. With this tool you can quickly assess any deleterious effect this may have on product life. Note that an extra millimeter initial crack length (3.54 mm) will half the life. Press eXit to leave PCPOST.

Interactive Operation Back on the MSC.Fatigue Results... form, change the Action to Optimize and press Apply. This will invoke the Crack Growth analyzer PCRACK.

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CHAPTER 7 Introduction to Crack Growth

By running PCRACK interactively we can re-run the entire Crack Growth analysis and make any changes necessary. You will be presented with a number of setup screens before the job is started. 1. Loading Definition: The first of these is where you can alter the scaling factor or impose a constant residual offset. Accept all the defaults by pressing the OK button. 2. Output Parameters: The second screen is for graphical updates and general output parameters. Change the Results File Output Interval and the Screen Update Interval to 0.25 Repeats. This is done so that the updates do not occur at the end of the signal but somewhere in the middle of the signal where the crack growth rate and stress intensity are more certainly non-zero. This interval is simply a snap-shot at a particular time or location in the signal; so we will get four snap-shots per repeat of the signal. Press OK to continue. 3. Local Geometry Definition: On this page you select the compliance function. Press OK. The form then updates to allow you to modify initial and final crack length specifications and notch dimensions. Accept the defaults and press OK. 4. Material and Environment Selection: The last page allows for selection of material and environment. Accept the defaults and press OK. The Crack Growth analysis will initiate and you will be presented with a graphical screen that updates as the crack grows. You will see the plot update as well as the numbers on the top of the plot and the modifying effects to the right of the plot. The plot features crack size versus life in cycles. The following explanations are given from left to right and top to bottom: 1. Repeats: This is self explanatory. This reports the number of repeats of the time history that the component has survived. 2. Size: This reports the length of the crack at the given snap-shot in time. 3. DLKAPP: This is the apparent stress intensity (∆K) or the apparent crack driving force without accounting for any modifying effects. 4. DLKEFF: This is the effective stress intensity (∆K) or the actual crack driving force which is based on the apparent ∆K with modifying effects. All modifying effects are listed to the right of the plot. If a modifying effect is highlighted, it was being experienced at that particular snap-shot in time.

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144

5. da/dN: This is the current crack growth rate at the reported time. 6. CLOSURE: This is an effect used to modify the apparent ∆Κ. When this modifying effect is lit, the component is currently experiencing crack closure which necessarily slows down the crack growth. 7. HISTORY: This modifying effect to the apparent ∆Κ is caused by the sequence of cycles. A large cycle followed by many smaller cycles can actually cause a slow down in the crack growth rate due to an extension of the plastic zone around the crack tip. This is called crack retardation. It than takes some time for the driving force to become sufficiently large to overcome that plastic zone and continue the crack propagation. 8. NOTCH: In our example we modeled an additional notch into our compact tension specimen. The influence of a notch is also accounted for as a modifying effect to ∆K apparent. 9. ENVIRON: If we had selected a material and used an alternate environment (a function of the material properties), this modifying effect would be lit. 10. STAT FRAC: Static fracture modes are accounted for also as modifying effects. These occur when the driving force approaches the fracture toughness of the material causing the crack to grow rather rapidly. All of these effects are discussed in more detail in the MSC.Fatigue User’s Guide. When the analysis is done you are presented with a page describing the final situation just as was done when using the result listing facility, PCPOST. Press the End button when you are satisfied that the same answers are given as the original analysis. The PCRACK main Post Analysis Menu will appear. This is very similar to the Design Optimization main menu of FEFAT for Crack Initiation and Total Life jobs.

Optimization From this Post Analysis Menu you can do numerous things such as view the final situation graphically or tabularly, interpolate on life as has been discussed already, or change any of the original parameters without re-doing the entire job setup. As an exercise let us change the material from MANTEN to RQC100 as was done with the original Total Life analysis to see the effects on the propagation life of the component. Select Edit analysis parameters | Select material and environment. When this form appears, change the material to RQC100 and press OK. Press or double click the Recalculate switch on the main menu. You will be asked to allow overwrite of the output file. Select the Yes button. The job will restart using the new material.

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CHAPTER 7 Introduction to Crack Growth

Note that with RQC100, which is a higher strength steel than MANTEN, the propagation life is a little bit longer (closer to 500 Repeats) but the final crack size is a couple of millimeters shorter (~8 mm) before ultimate failure. This would indicate that it is a more brittle material and less resistant to plastic deformation. Press the End button to go back to the main menu and then select final a-N graph. Note the final situation plot has a much flatter, constant slope confirming our suspicion. Select File | Exit and then press or double click the eXit button when done to quit from PCRACK.

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7.6

Concluding Remarks The branch of engineering science concerned with linear elastic fracture mechanics (LEFM) is by no means a new one. The earliest work in the UK dates back to Inglis (1913) but the major developments took place following the research of Griffith and Rae in 1920, and Irwin in the USA in 1956; and LEFM has since flourished. There are three modes of crack growth. Mode I - opening, Mode II - sliding, Mode III - tearing. Of the three modes, mode 1 is by far the most common. It is quite difficult to make cracks grow in modes 2 or 3. For these reasons MSC.Fatigue only supports Mode I.

Analysis without an FE Model It is not actually necessary to use a FE model to run a Crack Growth analysis. You may find this more convenient. Invoking PCRACK from a system prompt by typing the symbol, pcrack, will place you in its main menu mode where you can preprocess (rainflow and cycle re-order), run a full fracture analysis, output or display a time history or enter its utility menus. Within the Utilities, there is an option where you can create a simple input file (.fes file). You will be prompted for all the pertinent information including the far field stress.

Hint:

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A stress tensor is expected but you can simply put in one value for the Xcomponent and zero for the rest and specify the X-component only to be used in the analysis.

CHAPTER 7 Introduction to Crack Growth

MSC.Fatigue Files MSC.Fatigue Crack Growth analysis creates the same files as the other methods with the following two exceptions:

Table 7-2 MSC.Fatigue Files File jobname.tcy (simple_cg.tcy)

jobname.crg (simple_cg.crg)

Hint:

Description This binary file is the equivalent of the jobname.fpp preprocessing file created after rainflow cycle counting for the other two methods. The jobname.tcy file is also the result of the rainflow cycle count but after time cycle re-ordering created by PCRACK’s preprocessing phase. It serves as the input to the actual Crack Growth analysis. The Action, Partial Analysis on the Job Control... form will create all files up to this point and then stop. This is the results file of a fracture analysis created by PCRACK when a Full Analysis is requested. It is a binary file and can be processed by the result listing facility, PCPOST, only. It cannot be read back into Pre&Post or MSC.Patran to create life contour plots as with the jobname.fef file created by the other methods.

If you experience difficulty with a Crack Growth job, check the following files for clues: batlog.lst, jobname.sta, jobname.msg, pfatigue.prt. It is also helpful to interactively run the programs from the system prompt by issuing the proper commands: pksol, pcrack, pcpost.

Exit from Pre&Post or MSC.Patran when finished with this exercise.

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MSC.Fatigue QuickStart Guide

CHAPTER

8

Design Philosophies

■ Problem Description ■ S-N Analysis of Lug Weld ■ e-N Analysis of Lug ■ LEFM Analysis of Lug ■ Concluding Remarks

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8.1

Problem Description The component under design in this example is a lug assembly which is welded onto a base plate and is loaded in the opposite direction by a cyclic load acting at the top of the lug. The finite element analysis was carried out to simulate the load applied to the assembly during normal operation. (A sinusoidal distributed load variation was applied around the hole.) The design life objective is 100 years of service.

30mm 30mm

15mm 140mm

σx >> τxy

Now that all three major fatigue life prediction methods have been introduced, this exercise will use all three methods of fatigue analysis to analyze various parts of the lug assembly. Namely, you will use the Total Life approach to determine the useful life of the welded connection. For the non-welded part, you will use the Crack Initiation approach to investigate crack formation due to stress concentrations at the hole followed by Crack Growth. This will also illustrate the different fatigue life design philosophies. Since you should be quite familiar with the job setup procedure by now, only brief explanations are provided in this exercise. To begin, open a new database and call it lug_weld. Import the MSC.Nastran model and results using the file lug_weld.op2 as has been done in previous exercises.

load direction

Maximum Principal Stress Plot

Objectives • • • • • Main Index

To use the weld classifier to determine the type of weld To determine the useful life of the weld using the Total Life method To run a factor-of-safety analysis on the weld To determine the crack initiation fatigue life of the lug itself To investigate sensitivity of initiation life to alternative surface finishes

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• To determine if a crack will grow • To determine at what interval an inspection is necessary Table 8-1 Chapter 8 Necessary Files File P3_HOME/mscfatigue_files/examples/lug_weld.op2

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8.2

S-N Analysis of Lug Weld Due to symmetry, only one half of the lug assembly was modeled. Open the MSC.Fatigue setup form; set the General Setup Parameters as follows: 1. Analysis: S-N 2. Results Loc.: Node 3. Nodal Ave.: Global 4. F.E. Results: Stress 5. Res. Units: MPa 6. Jobname: lug_weld 7. Title: S-N Analysis of Lug Weld

Solution Parameters Open the Solutions Params... form and set the widgets as follows leaving the defaults if not mentioned: 1. Mean Stress Correction: None 2. Design Criterion: 96 Set the design criterion at 96% certainty of survival. 3. Run Factor of Safety Analysis: ON Turn this toggle ON. More will be explained about this analysis later. 4. Options: Life Based 5. Enter a Design Life: 100 This will correspond to 100 years of operation as will be designated when the service loading is defined.

Material Information Since the assembly has a welded connection, it is appropriate to assess the life of this feature using the Total Life approach since the weld material properties are unknown.

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CHAPTER 8 Design Philosophies

Enter the materials database manager PFMAT either from the MSC.Fatigue forms or directly from the system prompt by typing pfmat. One of the selections in PFMAT is a Weld Classifier. See if you can determine which type of weld you are dealing with by answering the questions from the weld classifier. Hint:

The assembly has a welded detail on the surface of a member with potential cracks initiating at short weld attachments. The weld toe is more than 10 mm from the member edge. The shear stress is less than half the applied direct stress.

The weld classifier should identify the weld as Class F type 2.9 if all the inputs are correct. You will refer to this weld class in the Material Info... form. Exit PFMAT. Open the Material Info... form and fill out the spreadsheet for a single material as follows: 1. Material: classF 2. Finish: No Finish 3. Treatment: No Treatment 4. Region: reference The Group called reference does not yet exist. You will need to create it. The classF entry is a component S-N curve. If you remember back to the discussion about component S-N curves you will recall that they are representative of the component’s geometry (the weld in this case) and the measured nominal stress is from a reference location away from the weld failure itself (such as where a strain gauge could be properly located). For the sake of this example, let us assume that we know only approximately where this reference location is but only within a certain tolerance. Select all the nodes on the surface one element thickness away from the weld on the flat plate and two elements thick. The analysis will assume that each of these nodes is the reference location respectively and we will make the determination of the worst case later. Call the new group reference. Note: In this particular case the reference location for this Class F component S-N curve corresponds to Node 284 on the model. This node is in the area away from the stress concentration but reasonable for definition of Class F weld.

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Loading Information Open the Loading Info... form and select the Time History Manager button. The load history, to be called LUGLOAD, consists of a single cycle with a min = 0 and max = 10. (The actual load applied is ten times greater than that applied in the FE model.) The units are Force in Newtons. The service load simulates the lug being lifted which happens two times a day. The fatigue equivalent unit should be Years with one repeat of the time history simulating 1/(2lifts*365days) = 0.00137 years. Enter this information into PTIME using Enter X-Y points where the y values are 0,10,0. To reiterate the details: 1. Filename: LUGLOAD 2. Description 1: Lug Loading 3. Description 2: whatever you want 4. Load type: Force 5. Units: Newtons 6. Number of fatigue equivalent units: 0.00137 7. Fatigue equivalent units: Years On the Loading Info... form, associate the time variation of the load that you just created to the FE load case by filling out the spreadsheet. 1. Load Case ID: 1.1-3.1-1Select the only result case available, Default, Static Subcase and Stress Tensor, NON-LAYERED. 2. Time History: LUGLOAD 3. Load Magnitude: 1.0

Job Control Open the Job Control... form and set the Action to Full Analysis and press the Apply button to run the job. Monitor the job form time to time. Since you have requested to do a Factor-of-Safety analysis, when you see the message Safety factor analysis completed successfully.

the job is complete.

Results - Factor of Safety Analysis For the Factor of Safety Analysis, the options are None, stress based, and life based. Stress based analyses are relevant to S-N analysis and apply to a class of structures where fatigue lives are essentially infinite. In the simplest sense, in order to assess the fatigue life of this class of structures, it is useful to obtain a measure of the margin between the working (applied stress) and the fatigue endurance stress (the limit below Main Index

CHAPTER 8 Design Philosophies

which no fatigue damage is incurred). The ratio of the endurance stress/working stress is known as the factor of safety. This is ideal for hand calculations using idealized loadings but for realistic variable amplitude loading, a more rigorous approach is used (see the MSC.Fatigue User’s Guide for the equations used) that estimates the overdesign or underdesign factor on the user supplied reference stress. Factors close to unity indicate the design has been achieved, factors less than unity indicate underdesign and factors greater than unity indicate overdesign. The Life based calculation is an iterative calculation used to find the magnitude of the scaling factor on the stress-time spectrum that will cause failure for a specified design life. The scaling factor is applied to all cycles and is, thus, an overall factor of safety, far better suited for random loading. To begin the analysis, open the Results... form and with the Action set to Read Results, press the Apply button to read the analysis results into the database. Next set the Action to List Results and invoke PFPOST, the tabular listing module. If you request to list the Most damaged nodes you will see that there are no damaged nodes. Therefore, for the weld, we have satisfied the design goal of 100 years of service and, in fact, have designed a fail safe structure, i.e., it will never fail. If you desire, assume that the weld is the worst class which is Class W. Re-run the analysis using this new S-N data set. You should see that there are still no damaging nodes and therefore the weld does not fail. Infinite life does not tell us much however. For this reason we ran the Factor-of-Safety analysis. Open the Results... form from the main form to make a contour plot of Safety Factor using the Factor of Safety result case. Note that the smallest safety factor (at Node 284) is around 8 1/2 times. When you expect a component to survive a very large number of significant loading cycles, e.g., around 108 cycles or more, it is not reasonable to make finite life predictions; you are too near to the fatigue limit where the amount of scatter is very large. For cases like this it makes more sense to apply a Factor-of-Safety method which aims to put the design below the fatigue limit by a selected safety margin. Two methods can be used.

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In general, the Stress Based method compares the largest stress cycle that occurs in the loading sequence to a Reference Stress (normally the fatigue limit) taking into account the mean stress. The Life Based method requires the target Design Life, the usage of a Material Cutoff value, and a Maximum Factor (default is 100) to be set. The Maximum Factor is simply used to stop the analysis for any particular location when this maximum is reached and go on to the next location. This can speed up the analysis if lowered significantly. The Material Cut-off is like the fatigue limit. It is the point beyond which damage will not be considered. If you are carrying out a life-based safety factor calculation, it is clear that if you change the cutoff you will reduce the influence of small cycles and hence get a larger safety factor. If you are doing a crack initiation based safety factor, changing the cutoff may also change the slope of the strain-life curve if you are using a surface finish or treatment correction. This is because the surface factors are applied at the cutoff. For a life-based calculations, the method is iterative. The calculation stops when the life is within a certain percentage of the target life - 5% by default. If you increase the allowable error, the number of iterations is reduced. This can only be changed by running FEFAT interactively. Note: In an earlier exercise we stated that making a contour plot of life (or safety factor in this case) from a component S-N analysis is meaningless and only the result at the reference location has any meaning. This is true, except in the case where you are not quite sure where that location is and wish to weigh the relative importance between different locations. Really only Node 284 has the correct safety factor but you can tell the relative magnitude difference if some other node were the actual reference location. With this simple loading, it is obvious that all other nodes will be less than Node 284 but with more complicated loading situations, this quickly becomes unclear.

Hint:

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PFPOST can also tabulate safety factor results. When you invoke PFPOST, type the .fos extension onto the jobname. This way it will pick up the jobname.fos result file created by a Factor-of-Safety analysis instead of the default jobname.fef file from regular life analysis.

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8.3

ε-N Analysis of Lug Now set up a Crack Initiation analysis of the lug itself. We are not concerned about where this crack will initiate since we know this due to the nature of the simple loading, i.e., the high stress area. We wish to determine the life until a crack initiates in the hole. Set the General Setup Parameters as follows: 1. Analysis: Initiation 2. Results Loc.: Node 3. Nodal Ave.: Global 4. F.E. Results: Stress 5. Res. Units: MPa 6. Jobname: lug_ci 7. Title: Crack Initiation Analysis of Lug

Solution Parameters Accept all the defaults on this form.

Material Information The material used in this analysis is BS4360-50D. This material is already in the materials database. Assume there is a good machined finish with no surface treatment. Open the Material Info... form and fill out the spreadsheet as follows: 1. Material: BS4360-50D 2. Surface Finish: Good Machined 3. Surface Treatment: No Treatment 4. Region: default_group This group contains the nodes for the entire model.

Loading Information The load history is identical to that of the S-N analysis of the lug weld problem.

Job Control Open the Job Control... form and set the Action to Full Analysis and press the Apply button to run the job. Monitor the job form time to time until it is complete.

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Results Open the Results... form from the main MSC.Fatigue setup form and read the results in. Do a contour plot of the log of the life (in Years) or list the results using PFPOST to identify the node with the shortest life. Note that the initiation life is approximately 3000 cycles at Node 7 or about 4 years assuming 2 lifts per day. On the Results... form set the Action to Optimize, select Node 7 to run the design optimization mode of FEFAT. Set the design life to 100. After reanalyzing Node 7 and entering into the main menu, do a Sensitivity analysis on surface Finishes (all). Do not forget to press the Recalculate switch. Note that a polished surface only increases the life to less than 6 years. This is obviously not sufficiently long, even with a polished surface. The Safe Life design philosophy would have us scrap this component after 4 to 6 years depending on surface finish/treatment we could apply or impose. This would be OK if the component were small and inexpensive and easy to replace such as automobile parts. However, this is not an option in the case, and the existence of a crack will not hinder the operation, nor is it a safety critical item. However, this calculation is only to crack initiation. There may still be many years of life left in this assembly depending on how fast this crack propagates.

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CHAPTER 8 Design Philosophies

8.4

LEFM Analysis of Lug Set up a Crack Growth analysis now to determine if a crack will grow in the lug and to determine at what interval an inspection may be necessary. Set the General Setup Parameters as follows: 1. Analysis: Growth 2. Results Loc.: Node 3. Nodal Ave.: Global 4. F.E. Results: Stress 5. Res. Units: MPa 6. Jobname: lug_cg 7. Title: Crack Growth Analysis of Lug

Solution Parameters Open the Solution Params... form. Before we can fill this form out completely we need additional information about the crack geometry, namely the compliance function. Run PKSOL (press the Compliance Generator button) and create a compliance function for a specimen with a double crack at a hole in tension where R = 15mm and W = 70mm. Call it LUG. These are the PKSOL steps: 1. Millimeters 2. Generate a Y function table 3. Output K-Solution Filename: LUG 4. Cracks a holes 5. Double crack at a hole in tension 6. Define 7. R: 15 8. W: 70 9. Changes: Carriage Return - no changes 10. Calculate Select Plot Y function against crack ratio to see the graph of the crack ratio.

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On the Solution Params... form set the following widgets and accept defaults for those not mentioned: 1. Select a Compliance Function: LUG 2. Initial Crack Length: 3 3. Final Crack Length: 55

Hint:

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You can plot the compliance functions from the Solution Params... form (if you have done so from PKSOL first) by selecting a compliance function from the listbox and pressing the Plot button. To remove the plot, press the Unpost or Delete buttons.

CHAPTER 8 Design Philosophies

Material Information Open the Material Info... form. Before filling the form out however, create a group with only Node 223 in it. Call the group far_field. Node 223 is chosen to indicate the area of nominal or far-field stress. It is not too close to the high stress gradient where the influence of the hole is dominant. LEFM theory is based on a nominal far-field stress. The material again is BS4360-50D and the environment is air. The properties for this material are already in the materials database manager. On the Material Info... form set the cells of the spreadsheet as follows: 1. Material: BS4360-50D 2. Environment: air 3. Region: far_field This group contains the node of the far field stress point only.

Loading Information The load time history is identical to the previous two lug analyses.

Job Control Open the Job Control... form and set the Action to Full Analysis and press the Apply button to run the job. Monitor the job form time to time until it is complete.

Results Open the Results... form and with the Action set to List Results, press Apply to invoke PCPOST. View the Results summary page and the final a-N curve. Note that the crack takes over 150 years to grow to a critical size before total failure. Then close PCPOST. The Damage Tolerant design philosophy would have us determine an inspection interval. Re-run the analysis by changing the Action to Optimize. Accept all the defaults on each setup screens presented to you by PCRACK. When the analysis begins you will notice that the crack grows, but fairly slowly. Therefore, you could feel good about a fairly long interval between inspections (say once a year).

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When the analysis has completed, make your way back to the PCRACK Post Analysis main menu. It may be of interest to change the initial crack size to something less than 3 mm. We assumed a 3 mm initial crack length based on an engineering crack size from the initiation analysis. Use the Edit analysis parameters | local Geometry option to change the crack size to 2 mm. Then Recalculate. Do it again for 1 mm. Finally put in 0 mm which flags the code to calculate the minimum initial crack length valid for LEFM in this case. Note that the crack hardly grows for a long time. Growth of the crack is quite sensitive to the initial crack length. This could have been surmised from a plot of the compliance function which shows slow growth at first but very accelerated growth at a high crack ratio. Press the Abort button when you get tired of watching the crack grow.

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CHAPTER 8 Design Philosophies

8.5

Concluding Remarks This exercise ran all three methods of fatigue analysis to investigate different aspects of life evaluation on the same component. The Total Life (S-N) analysis was performed on the lug weld to demonstrate a fail safe design. As a side note, a weld class S-N curve was used. Note that two types of S-N analysis can be performed with a weld. Either a normal S-N analysis or an S-N analysis based on the current British Standard. The British Standard can be turned on using the Materials Info... form by scrolling the spreadsheet to the right and setting the Weld cell to YES. Try reruning the analysis to see the differences in results. With Weld set to YES, various effects are taken into account according to the British Standard such as residual stress based on the weld class selected. The Crack Initiation analysis was performed on the lug itself to demonstrate the safe life approach. And finally the Crack Growth analysis was performed on the lug to demonstrate a damage tolerant approach where inspection intervals were determined based on crack growth rate. To be completely thorough you might also run a crack growth analysis at the lug weld. It is highly unlikely, though, that this would yield any crack growth since the S-N analysis gave infinite life and a safety factor of greater than 8 was achieved. For this particular component, the safe life approach really did not avail us much. The crack initiates very quickly in comparison to the time it takes to propagate the crack. From this perspective, we can ignore the Crack Initiation analysis entirely.

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MSC.Fatigue QuickStart Guide

CHAPTER

9

Multiple Loads

■ Problem Description ■ S-N Analysis of Engine Mounting Lug ■ Crack Growth Analysis of Engine Lug

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9.1

Problem Description To this point we have used simple, semi-fabricated examples to illustrate concepts. Now we revert to a more realistic, real-world example. The model is still simple but the loading is complex. This example describes a typical multiple load case fatigue analysis as applied to a safety-critical component. Because it is safety critical, both a Total Life method (to ensure that it will survive its design life) and a defect tolerant approach (to ensure that a crack will not grow to failure too rapidly) are employed. The component is a titanium alloy aircraft rear engine mounting lug. It is mounted across the rear of the aircraft wing and is used to locate a pin which constrains the rear of the engine in the x-y plane. The engine is restrained in the z-direction (direction of travel) by the front engine mounting. Thrust results in a downward distribution of pressure and is simulated by a cos2t pressure distribution over a 90 degree angle, amounting to a resultant force of around 1000 Newtons. The model is composed of 2D quadratic elements. The lug is mounted very stiffly along its top edge, simulated by constraining this edge for all degrees-offreedom. Loading around the lug hole is simulated by applying 8 different load cases. Each load is a separate FE analysis and spaced every 45 degrees. A loading spectrum is available consisting of horizontal and vertical loads that represent 1000 flights. The design life is approximately 30,000 flights.

Objectives • • • •

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To assess the fatigue life of safety critical items To demonstrate setup of multiple loading conditions To determine the critical location To assess a damage tolerant design

CHAPTER 9 Multiple Loads

Table 9-1 Chapter 9 Necessary Files File P3_HOME/mscfatigue_files/examples/mounting_lug.op2 P3_HOME/mscfatigue_files/examples/lug.mat P3_HOME/mscfatigue_files/examples/lug.ksn

Load Case 1

Load Case 2

Load Case 3

Load Case 4

von Mises Stress

Load Case 5

Load Case 6

Load Case 7

Load Case 8

As you can see, each load case gives a different stress distribution. It is not clear from these stress plots where the critical location will be when combined. To begin, start Pre&Post or MSC.Patran and import the model and results using the MSC.Nastran results file mounting_lug.op2 into a new database called mounting_lug.

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9.2

S-N Analysis of Engine Mounting Lug First we will run a Total Life analysis of the mounting lug. Open the main MSC.Fatigue setup form and set the General Setup Parameters as follows: 1. Analysis: S-N 2. Results Loc.: Node 3. Nodal Ave.: Global 4. F.E. Results: Stress 5. Res. Units: MPa 6. Jobname: mountlug_sn 7. Title: S-N Analysis of Rear Engine Mounting Lug

Solution Parameters Open the Solutions Params... form and set the Design Criterion (certainty of survival) at 96%, i.e., we are going to predict the lifetime we expect 96% of these mounting lug components to exceed. Accept the default Goodman mean stress correction method (which tends to be a bit conservative) in conjunction with the Abs. Max. Principal stress to use in the fatigue analysis.

Material Information A material test was performed for this titanium alloy and a material S-N curve created. It needs to be loaded into the materials database. Copy the file lug.mat to your working directory. Go to the Tools pulldown menu and select MSC.Fatigue (for the MSC.Patran version) or Fatigue Utilities (for the Standalone version). Under this pulldown menu select Material Management and then ASCII Materials File Reader. Use the form that comes up to read in the lug.mat file. An alternative method would be to issue the pfmat @lug.mat

command from a system prompt (not from Pre&Post or MSC.Patran). Note: If you open this file, lug.mat, and examine its contents you will find all the parameters necessary to define and load the material, lugmaterial, into the database. Any MSC.Fatigue module that runs in batch mode can accept a file of parameters to define its operation. Use the file name as the argument with the @ symbol in front of the file name. Likewise, the parameters themselves can be used as the arguments also, e.g., fefat /opt=a/inp=jobname/ov=y.

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CHAPTER 9 Multiple Loads

Open the Material Info... form. Press the Materials Database Manager button to see that a new S-N curve called lugmaterial has been created and loaded into the database, which now resides locally in your working directory. Graphically display the new S-N curve if you wish. Note the scatter band representing +/- 2 standard deviations (Plot_Type | Scatter Curve). When satisfied, quit from PFMAT.

Define a Group The fatigue problem is clearly going to be around the hole, so we can speed up the analysis by calculating fatigue damage for this area only. To create this group we are going to enlist the help of the List function. First open the Group | Create form. Create a new group called hole. In the Entity Selection databox include only the elements on the inside of the hole. The easiest way to do this is to use the graphical polygon picking. Zoom in on the hole using the View Corners icon on the top level form (you click and drag a rectangle around the area of interest). Set the Select Mechanism to pick only Quad elements. Select all the inside elements by placing the cursor just inside the inner-most layer of elements and press and hold down the control key to surround the elements as you drag the mouse. Each click of the mouse will define a new corner of your polygon. Double click to close the polygon when you get near the starting point.

View Corners Icon

Start End Drag

Select Mechanism

Drag Click

Select Quads Select FEM Entities

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Polygon

Making the Polygon Pick Hold down the Control key and click the mouse to create polygon corners. Illustration shows a partial pick.

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Press the Apply button on the Group form to create the group and close the Group form. The elements are in the group now but we really need the nodes of the elements. From the Tools pulldown menu select List | Create. Two forms will appear. On the Create List form set the Model to FEM, the Object to Quad, and the Method to Association. Set the Association to Group and pick the group we just created, hole. Press the Apply button. You will notice that the other form called List A fills with all the elements we just created in group hole. Now on the Create List form set the Model to FEM, the Object to Node, and the Method to Association. Set the Association to Element. In the Element databox type ‘lista‘ including the back quotes. Near the bottom of the form change the Target List to “B” and press the Apply button. You will notice that another form called List B appears filled with all the nodes associated with the elements of List A (the elements that are in group hole). Add the nodes to group hole by pressing the Add to Group... button on the List B form. Select hole as the group and press Apply on the List Save form that appears. Close all the List forms when you are done. All nodes from elements on the inner ring of the lug elements are now in the group.

Fill Out the Spreadsheet Fill out the spreadsheet on the Material Info... form for a single material as follows: 1. Material: lugmaterial 2. Finish: No Finish 3. Treatment: No Treatment 4. Region: hole

Loading Information In practice the load could be in any direction. However for practical reasons we can not have an infinite number of load cases, so we have applied a finite number around the perimeter of the hole and will simulate loading in any particular direction by a linear combination of the nearest two load cases. In the original FE analysis, eight load Main Index

CHAPTER 9 Multiple Loads

cases were set up around the perimeter. To simplify things for this example we use only four of these in the fatigue analysis. Of course, the more load cases you use in a case like this, the more accurate the simulation of the load distribution at any instant. In this analysis the four load cases used are in positive and negative x and y directions, respectively. Originally the x and y loadings contained both positive and negative content. The problem with this is that when you have loading via a pin as in this case. The stress distribution for a unit negative x loading is not -1 times the stress distribution for a positive x loading. Positive and negative loads have to be treated as separate load cases, with separate load histories. For this reason, the x and y loadings are separated into positive and negative parts. Hint:

This was simply achieved using a MSC.Fatigue utility routine module called MFRM (formula processor). If you have a measured or derived time variation that you wish to separate into positive only and negative only components, use MFRM.

Multi-file Display Look at the time variations of the four load cases. Open the Loading Info... form and press the Time History Manager button. This time, instead of copying from the Central database, copy from Remote and specify the directory where the example files reside such as, e.g., /mscfatigue_files/examples/

or x:\\mscfatigue_files\examples\

Do not forget the last slash (/ or \). Copy the four histories called XPOS, YPOS, XNEG, and YNEG. Note: You can select all four histories by holding down the Shift key as you select them. You will need to Change an entry | edit Details and change the load Units to kNewtons, the Number of fatigue equivalent units to 1000, and the Fatigue equivalent units to Flights for each history. Each repeat of the combined load history is equal to 1000 flights.

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To view all four histories at once, use the Multi-channel... | Display Histories option. This will run the multi-file display module, MMFD. When MMFD appears, use the List facility to select the four files above (use the Shift key to make multiple selection from the file browser). Note that the files will not appear in the databox but the number of files selected will appear below it. Accept all the other defaults on the form and press OK. The files will be displayed.

Note: If you make a mistake selecting the files for multi channel display, you can always add to or delete from the currently selected list. Simply press the List button again and a menu will appear allowing you to make modification to the list of files. If you are already in graphical display, select File | New File(s) to return to the file selection screen. Note that all the histories have positive values. The stress distributions from the FE analyses will be used to define the actual sign (positive or negative). Click on Full Plot to see the same plots as shown above. Close the graphics by selecting File | Exit and then quit from PTIME.

Fill Out the Spreadsheet On the Loading Info... form, the spreadsheet is used to establish the association between the load histories (the time variation of the load) and the FE load cases. MSC.Fatigue scales and combines the stress distributions according to the time histories, to obtain the stress history for each node. Set the Number of Static Load Case to 4 and press the Return or Enter key to effect the change, then fill out the spreadsheet as shown below. The load magnitude from each load case is around 1000 N (they vary slightly with each load case). The time variations have been defined in kNewtons whereas the FE loads use Newtons. To keep the loading consistent the Load Magnitudes are defined in kNewtons. The load cases selected correspond to Load_Case.1, Load_Case.3, Load_Case.5, and Load_Case.7.

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Load Case ID

Time History

Load Magnitude

Row 1:

2.1-3.1-2- (Load_Case.1)

XPOS

0.924

Row 2:

4.3-3.1-2- (Load_Case.3)

YPOS

1.023

CHAPTER 9 Multiple Loads

Load Case ID

Time History

Load Magnitude

Row 3:

6.5-3.1-2- (Load_Case.5)

XNEG

1.121

Row 4:

8.7-3.1-2- (Load_Case.7)

YNEG

1.218

Note: The spreadsheet is filled out in exactly the same manner as with a single load. With multiple load cases however, it is only necessary to Get/Filter Results... once. Each subsequent time you fill in a cell with a load case ID, all results remain in the selection listbox. Also note that the actual load case IDs may vary from what is shown in the table.

Job Control Open the Job Control... form and set the Action to Full Analysis and press the Apply button to run the job. Monitor the job form time to time until the job is complete. The job, because of the complexity of the loading, takes a few minutes to run.

Principal of Linear Superposition In a previous example we explored the possibility of offsetting the load by imposing a residual stress. It was treated as a multiple load case situation where one of the load cases was a simple offset of the first. Now we have four load cases all of which vary independently of one another. MSC.Fatigue uses the principle of linear superposition to combine all load cases together to determine the stress variation at each node due to the combination of all loads. This is done using the following formula: σi j ( t ) =

σ i j, k

∑ P k ( t )  -------------P f ea, k k

where the elastic FE stresses, σij, from each load case, k, are normalized by the load magnitude from the FE analysis, Pfea and then multiplied by the time variation of the loading, Pk(t). The result of summing over all load cases gives the total stress time variation at each location of the model. Or in terms of strains for Crack Initiation jobs: Time

FE Load

FE Load

Local Strain

Histories

Case Results

Case Loads

Histories

Repeated for all Load Cases

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To summarize the procedure: 1. All load cases are normalized with respect to each FE load case magnitude, i.e., converted to unit loads, if necessary. This is done by dividing the FE stresses by the load magnitudes and then multiplying by the time history. 2. All normalized stress time histories for each load case are superimposed using the principal of linear superposition. 3. The stress time history tensor is resolved down to a single scalar value versus time (as defined on the Solution Params... form - the Stress Combination value). 4. Rainflow cycle counting is performed. 5. Any reductions and corrections (surface finish, mean stress, etc.) are applied. 6. Finally, damage is summed according to the linear damage summation rules. Hint:

In order to properly do linear superposition, it is important that all the time variations used in the same analysis have the same sample rate (same number of total points). You can easily adjust the sample rates to achieve this in PTIME using the Change an entry | Sample rate adjust option.

Results The quick evaluation is to read the results in and do a contour plot of life. Do this by opening the Results... form from the MSC.Fatigue main form and press the Apply button with the Action set to Read Results. Now make a fringe plot of life. Open the Results application from the main form in Pre&Post or MSC.Patran. Select the result case called Total Life, mountinglug_snfef and select Log of Life (Flights) and press the Apply button. A fringe plot of only the first ring of elements will appear.

Critical Node 1121

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Note: If you have not noticed already, when results are read into the database a new color spectrum, fatigue_spectrum, is made which has the opposite color scheme as that used for plotting stresses. This is done to display lowest life in red just as highest stress is plotted in red. You can change the spectrum under the Display | Spectrums if necessary to revert back to the other spectrum, standard_spectrum. You may have to re-create the plot to make the spectrum active. The lowest value of around 6 in the spectrum/range means that the shortest predicted lifetime is around 2E6 Flights. This comfortably exceeds the design life of around 30,000 Flights. Close the Results application by pressing on its switch again in the top menu bar and then re-open the MSC.Fatigue main form if it is not already open. Now go to the MSC.Fatigue Results... form and set the Action to List Results and press the Apply button. List the Most damaged nodes as done in previous exercises to find the node with the lowest life (Node 1120). This will be useful later. Exit from PFPOST. Change the Action to Optimize from on the Results... form. Do not select a node on this form. Press Apply. Use the first page of FEFAT’s Design Optimization option to select the Worst Case node. Enter the design life of 30000 flights and press OK. Press the End button after being presented with the analysis results for the worst case node. First look at the cycles results Display | plot Cycles histogram and damage histograms (results Display | plot Damage histogram). You can see that the loadings in the critical region are predominantly tensile.

Cycles

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Now try a sensitivity analysis on scaling factor (Sensitivity analysis | Scaling factors), applying factors from 1 to 3 by steps of 0.2 (you can use the syntax ‘(1, 3, 0.2)’ to define these values). Use the Recalculate option to re-do the analysis. When the calculation is complete, you can X-Y plot the results (results Display | Sensitivity plot). This calculation indicates that a factor of 2.2 would Sensitivity have to be applied to the loads to cause failure according to the design criterion. However, a factor of 2.2 puts the largest cycle (and remember that there will only be a small number of these, maybe 30 or 60 in the design life) apparently above the UTS of the material. This is a shortcoming of the S-N method, due to the fact that it does not model yielding and load redistribution at all. If we wanted to design this component so that it only just survived the design life, we would need to use the local strain approach, which is more appropriate for low-cycle fatigue problems. Quit from FEFAT when done.

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9.3

Crack Growth Analysis of Engine Lug The Total Life calculation we have just carried out is for a defect-free component. Because this is such a safety-critical component we should not assume that it is defect free. The part can be inspected for cracks, and the method used to inspect it is capable of detecting cracks of 1.5 mm or more in depth. If inspection reveals no cracks, we should assume the worst case which is that there is a crack of 1.5 mm at the most highly stressed location, i.e., Node 1120. We then want to know what will happen to this crack in service. Will it grow? And if so, how long will it take to cause failure? It is assumed that the database is still open and the MSC.Fatigue main form is open. Set up the Crack Growth analysis now by setting the General Setup Parameters as follows: 1. Analysis: Growth 2. Results Loc.: Node 3. Nodal Ave.: Global 4. F.E. Results: Stress 5. Res. Units: MPa 6. Jobname: mountinglug_cg 7. Title: Crack Growth Analysis of Mounting Lug

Solution Parameters Copy the file lug.ksn to your working directory. Open the Solution Params... form. A compliance function for the specimen has been created to define the crack geometry. It was empirically derived via specimen tests and curve fit to a polynomial function. It was in this form, using PKSOL, that it was input (by defining the coefficients of User parametric definition).

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The compliance shows roughly what the crack growth rate will be. In this particular example the crack growth rate will increase as the crack gets larger relative to the width of the lug. The compliance function is used to derive the driving force (∆K) of the crack as formulated in the Paris Law, using the equation K = Y ( a ⁄ T )σ πa . T is a model dimension, in this case the width of the lug at the critical location, 24mm.

Hint:

Compliance function files(.ksn) have the same format as .dac (time history) files and can be plotted by PTIME and other MSC.Fatigue modules that do graphical X-Y displays such as MQLD.

In this case the starting crack size is the minimum detectable crack size of 1.5 mm and the final crack length is the width of the lug from the critical location. In practice of course, the fracture toughness K1C may be reached before the crack grows right through, and in any case, the Y function may not be valid at this point. For instance, the Y function used in this calculation is not valid beyond a/T=0.85. Fill out the Solution Params... form as follows: 1. Select a Compliance Function: lug 2. Stress Combination: Max. Abs. Principal 3. Crack Length Units: Millimeters 4. Initial Crack Length: 1.5 5. Final Crack Length: 24.0 Leave the defaults for all else not specified here and close the form.

Material Information Open the Material Info... form. The material information form looks similar to that for the S-N analysis, but has a few notable differences. Number of Materials is grayed out, because we can consider only one material at a time. The options to correct for surface finish and roughness are no longer appropriate, but the material may have a number of LEFM data sets for different environments. lugmaterial has only air data, but if you select BS4360-50D there are 5 sets of different environments.

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Create a Group The Region means something different here also. The software will average the stress across the calculation region for use in the crack growth calculation. In this analysis, we will use the stress from the critical Node 1120 only. Open the Group | Create form and create a group called critical_node. This stress used in the equation for K (shown earlier) is known as the far field stress or the stress that would be there if there were no crack (or notch influence). Once this group is created close the form and go back to the Material Info... form.

Fill Out Spreadsheet On the Material Info... form set the cells of the spreadsheet as follows: 1. Material: lugmaterial 2. Environment: air 3. Region: critical_node This group contains the node of the far field stress point only.

Plot da/dN Curves Now start the Material Database Manager and make sure that the data set lugmaterial is loaded as dataset 1. From the Graphical display options, choose Apparent delta k plot, entering stress ratios of 0.5 and 0.7 to see the effect of mean stress on threshold and growth rates. Also look at the Threshold:ratio delta k plot, which shows how the threshold delta K value is related to stress ratio. Now leave PFMAT and close the Materials Info... form.

Loading Information The loading information form is exactly the same as for the S-N job.

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Job Control Open the Job Control... form and set the Action to Full Analysis and press the Apply button to run the job. Monitor the job form time to time until it is complete.

Results PCRACK can also be run interactively by selecting Optimize from the Results... form menu. This is more revealing as you get an on-line display of crack growth. Do this now and accept all the defaults, and overwrite existing files. Watch the crack grow to failure. Note that failure is not predicted to occur within the 30,000 Flight design life (~49,000 Flights). The final a-N curve can usefully be used to determine acceptable inspection intervals. Note also that the crack only grew over 5 mm before the fracture toughness of the material was exceeded. There are a number of other ways of postprocessing the results. These can be accessed by running PCPOST (the List Results option from the Results... form menu.) You may like to explore these if you wish.

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CHAPTER

10

A Multiaxial Assessment

■ Problem Description ■ Geometry ■ Determine the Critical Location ■ Evaluate Results ■ Concluding Remarks

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10.1

Problem Description This example is a local-strain (Crack Initiation) analysis of a steering knuckle subjected to a complex, multiple load-case loading environment. The component is a steering knuckle from a car. It is cast from a spheroidal graphite cast iron. The obvious features are the strut mount at the top, the lower ball joint at the bottom and the steering arm on the right. The wheel spindle goes through the large cylindrical hole in the central part.

strut mount

ball joint

When the vehicle is driven through a steering arm cobblestone slalom, loads are applied to the component via the strut mount, the lower ball joint, the steering tie rod and the wheel axis. In the FE analysis the loads are applied via loading devices in an attempt to make the transfer of loads to the component as realistic as possible. This has been done using devices made from elements rather than MPCs.

Three of the Twelve Loads

The model has been constrained at the wheel center (again through element loading devices) and 12 load cases have been applied: 3 forces (1000 N in x-y-z) at the lower ball joint, the steering arm and the strut mount, and 3 moments (1000 Nmm) at the strut mount. Three of the 12 loads are plotted here. A linear combination of these 12 load cases can describe any loading condition that occurs during the test track event.

Objectives • To assess where the critical fatigue locations are in a component due to multiple loading conditions

• To explore the application of the biaxiality analysis feature and interpretation of the results

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• To assess the multiaxial stress state and make decisions on the validity of the fatigue analysis, i.e., are the original uniaxial assumptions valid, does proportional loading have to be taken into account, or does a critical plane analysis need to be done.

Table 10-1 Chapter 10 Necessary Files File P3_HOME/mscfatigue_files/examples/knuckle.out P3_HOME/mscfatigue_files/examples/knuckle*.nod P3_HOME/mscfatigue_files/examples/knuckle.nod_tmpl P3_HOME/mscfatigue_files/examples/knuckle.ses P3_HOME/mscfatigue_files/examples/knuckle_ma.fin P3_HOME/mscfatigue_files/examples/knuckle*.dac P3_HOME/mscfatigue_files/examples/knuckle_ma.fef

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10.2

Geometry Because this is a fairly large model with a time consuming analysis, and for the sake of simplifying this example, it has been semi-automated for you. This is done to help speed up the study of this exercise. However, all the steps necessary to reproduce the results manually are indicated if you so desire. To begin, start up Pre&Post or MSC.Patran and open a new database giving it the name knuckle. (Any Analysis Preference will do for this analysis, but leave it at MSC.Nastran when asked.) Initially you will need these files copied over from the central installation area to a clean, empty working directory: knuckle.out, knuckle*.nod, knuckle.nod_tmpl. (There are 12 .nod files where * = 1 through 12.)

Import FE Model and Results This has been automated by running a session file (a file full of commands to be executed). Copy the file knuckle.ses to your directory and then from the File | Session | Play command select the file and press Apply. Answer Yes to any questions. Playing this session file accomplishes the following which you can do manually if you wish: 1. Imports a neutral file containing the FE model and creates some convenient groups. (You can do this via File | Import... by setting the Object to Model and the Source to Neutral and selecting the file knuckle.out. The session file does this for you.) 2. Sets the view of the model and names it so you can recall it easily. (To name a view use Viewing | Named View Options... Press the Create View button from the form that appears. Supply a name and the current view will be stored for later recall from the Named View Options form.) 3. Reads the FE stress results into the database. (This can be done from the File | Import... pick. Set the Object to Results and the Source to PATRAN 2 .nod.... You will have to select a template file which is knuckle.nod_tmpl using the file browser that appears. The template file defines what to name each column or columns of results in the result files. Next you select the actual .nod file. There are twelve of them and you must repeat this operation 11 more times to import all files. The template file only needs to be selected the first time however.)

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Post/Create Groups The entire model should have appeared in the graphics screen including the loading devices. The neutral file that you read in the previous step contained some convenient groups called KNUCKLE_ONLY and SURFACE_NODE. Post the KNUCKLE_ONLY group from Group | Post; select KNUCKLE_ONLY and press Apply. Only the knuckle itself should appear in the viewport now. These groups were created automatically for you, however we digress a bit here to teach you how to easily create some convenient groups for subsequent fatigue analysis. These techniques are especially helpful with large solid models. This discussion is not crucial to the successful completion of this exercise. You may skip to the next step if you wish.

Group of External Elements Only To create a group of external elements, thus removing all internal elements use the list functionality - Tools | List | Create. Set the Model to FEM, the Object to Element, and the Method to Attribute. The Select Mechanism will appear from which you should select the Elements with free faces option.

Graphically surround all element of the model using the mouse by clicking and dragging from the top left corner down to the bottom right corner. All the elements with free faces will be selected. Press the Apply button to add these elements to the List A form, then on the List A form press the Add to Group... button. On the form that appears, give a new group name such as Surface_elements and press the Apply button. Press Cancel to close the form. A new group now exists with only the external elements.

Group of External Nodes Only Because fatigue damage usually only initiates on the surface of components, it is helpful to have a group of surface nodes only. The previous group we made only contains elements. By creating groups with only the surface nodes we can speed up the analysis by eliminating nodes from the analysis in which we are not interested. With the Create List form still open, set the Object to Node and the Method to Association. The Association should be set to Element Face and the Target List needs to be set to “B” (or you can Clear the List A contents). Now before proceeding, go to Group | Post and post only the group you just created, Surface_elements. Cancel the Group form when have accomplished this.

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Now on the Create List form, set the focus (click the mouse) in the Element Face databox. The Select Mechanism will appear again from which you should select Free face of element. Then surround the entire model (by clicking and dragging with the mouse) as you did before to select all free faces. Press the Apply button. The List B form will fill with the nodes associated to the free faces. Add these nodes to the group Surface_elements. Now you have a group with only the external elements and the external nodes of the model. Cancel the Create List form to close it down.

View the Stress Results Open the Results application and plot the stresses from any of the result cases. Make plots of the von Mises stress for load cases 7, 8 and 9 in turn. Note how the individual load cases cannot be relied upon to predict the fatigue hot spots.

Z-Component Stress of Load Case 1

Surface Resolved Stresses Specifically plot the Z component stresses and note that they are very close to zero over the majority of the model except at the loading points as would be expected. (A good look at these stresses would reveal model quality.) The results are surface resolved stresses, meaning the two major principal stresses lie in the plane of the surface with the third principal stress being zero (normal to the surface). This is important for models with solid elements especially given that 99% of cracks initiate on the surface. The principal stresses correspond to the X, Y, and Z component stresses. The main reason that we need surface resolved stresses is for the biaxiality analysis to properly calculate the biaxiality ratio which will be discussed later in this example. Without surface resolved stresses it would be difficult, if not impossible, to assess the multiaxial stress state of the component. Many FE analysis codes will calculate surface resolved stress or may give you the option to do so. The best approach is to first assess the magnitude of the out-of-plane component to determine if the stresses are already surface resolved. If you find that

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you need to resolve your stresses, MSC.Fatigue can do this for you with a couple of easy steps. Physically the out-of-plane stresses must be zero (unless subject to some sort of hydrostatic pressure). Note: It is always good to know in what coordinate system the stresses have been output from the FE analysis, i.e., the global system, or some defined element coordinate system.

Calculate Normals Although this is not necessary for this example, to have MSC.Fatigue surface resolve your stresses for you during a fatigue analysis you must first create a vector file (for coordinate transformations). Before submitting your fatigue job, open the Job Control... form. The Calculate Normals option is an essential precursor to running the biaxiality analysis with a solid model if you know your results are not surface resolved (z-normal is not zero). This routine determines surface normals at each surface node, and writes them to the file jobname.vec. MSC.Fatigue detects the presence of this file and uses it to define a local coordinate system at each surface node that has its z-axis normal to the surface. The stress results in the fatigue analysis input file are then written in this coordinate system, permitting the software to carry out a biaxiality analysis in the x-y plane only. Do not run this unless you have some time to spare because of the size of this model. (Besides the stresses are already surface resolved.) A graphical depiction of a normal vector calculation is shown to the side.

A Normals Calculation

During the fatigue analysis translation surface resolved stress tensor files are created with the name jobname_lc#.nod where the # is the load case number. There will be one file for each load case in the fatigue analysis setup. You can read these .nod files back into the database exactly as described earlier (using the jobname.nod_tmpl file) to evaluate the success of the surface stress resolution (by plotting the Z component stress from these files). Note: If you do run the Calculate Normals option while going through this problem, be sure to use a different jobname than the one used in the analysis described in this chapter. The analysis will detect the .vec file and use it if the job names are the same. This will not effect the fatigue results but will result in an erroneous biaxiality analysis because each nodal stress tensor is in its own local coordinate (since it is already surface resolved) which is unknown by Pre&Post or MSC.Patran which makes the local coordinate transformation invalid. Main Index

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10.3

Determine the Critical Location You are ready to set up the fatigue analysis to determine the critical location. Copy the file knuckle_ma.fin to your directory. Instead of filling in the forms as we have done in all previous exercises, read in the fatigue job setup file. Of course you can still do it manually if you wish also. Bring up the MSC.Fatigue main setup form. Open the Job Control... form. Set the Action to Read Saved Job, select the job knuckle-ma, and press Apply. The parameters from the file are read in and all the widgets on the various forms are filled in. Reading the saved job recovers all the information from the job and sets up the forms. One of the nice features of MSC.Fatigue is the ease with which job files are handled. If you have a number of similar jobs to run you can simply change the job name and make any other edits before saving the job, and then repeating this process as often as necessary. If desired, all the resulting jobs may then be run in batch mode. This is the most efficient way of working if you have a lot of analyses to carry out. What reading an old job file does not do however, is recreate the material group(s) used in the previous analysis just in case they have been removed or changed. Nor does it ensure that the referenced result cases actually exist. This is up to the user. Hint:

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Another way to easily and conveniently read in an old job setup file is to type the jobname in the Jobname databox on the main form and press the carriage return. If a file called jobname.fin is detected in the local directory, it will be read. This can be more convenient than opening the Job Control... form.

CHAPTER 10 A Multiaxial Assessment

The General Setup Parameters should appear as follows: 1. Analysis: Initiation 2. Results Loc.: Node 3. Nodal Ave.: Global 4. F.E. Results: Stress 5. Res. Units: MPa 6. Jobname: knuckle_ma 7. Title: Slalom on cobblestones, but with loads scaled by a factor of 13 Now open the various forms to see how the job has been setup.

Solution Parameters Open the Solution Params... form. 1. Analysis Method: S-W-T S-W-T (Smith-Topper-Watson) is a variant on the standard strain-life methodology which takes into account the mean stress of each cycle. 2. Plasticity Correction: Neuber Neuber is the default elastic-plastic correction method. 3. Run Biaxiality Analysis: ON This is the only real variant from previous examples. 4. Biaxiality Correction: None This is the default correction method. Correction methods will be discussed later. 5. Stress/Strain Combination: Max. Abs. Principal The Max. Abs. Principal is the default choice of Stress/Strain Combination. This is the principal strain that has the largest magnitude (in a uniaxial test, this would be the axial strain). 6. Certainty of Survival (%): 50.0 The Certainty of Survival (%) defaults to 50%. This means that the component will have a 50% chance of surviving the calculated life. The probability is based on the scatter defined in the material parameters. 7. Run Factor of Safety Analysis: OFF Many components are designed for infinite life, e.g., crankshafts; these are better analyzed using a Run Factor of Safety Analysis. This is not covered by this demonstration.

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Close the Solution Params... form when done.

Material Information Open the Material Info... form. The original material of this component is not used in this example analysis. Instead a representative material, for illustration purposes only, has been selected. The material information form is used to assign fatigue properties to different parts (groups) of the model. You can have up to 20 different groups which may have any combination of materials, surface finishes and treatments. Clicking on the material box gives access to a picklist of suitable materials from the database. Corrections can be made for surface finish and treatment. These are valid only for steels, though you can set up your own corrections if desired. In the case of this analysis, no correction is made, because the specimens were tested as-cast, i.e., with the same surface condition as the component. The region for this analysis is the group containing the surface nodes only. This speeds up the analysis. The spreadsheet on the Material Info... form is filled out as follows: 1. Material: MANTEN 2. Finish: No Finish 3. Treatment: No Treatment 4. Region: KNUCKLE_ONLY Note: You can change the Region to the group Surface_elements that you created earlier if you wish as long as the nodes exist in it also. A very common mistake that results in an error during translation is that the selected group does not contain nodes when a nodal fatigue analysis has been requested or the group does not contain elements when an element centroidal fatigue analysis has been selected. Close the Material Info... form down when done. Always use the OK button when changes have been made. If you use the Cancel button, any changes will not be saved.

Loading Information Copy the load variation signals to you local directory. They are called knuckle*.dac where * is a wild card for the twelve load cases. Open the Loading Info... form.

Load the Time History Files The spreadsheet appears filled out on the form but the actual time history files are not loaded into the time history database yet. Press the Time History Manager button to invoke PTIME.

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When PTIME appears select the Load files option. A form will appear from which you can load all files at once. In the Source Filename databox type *.dac and press the Tab key so that the Target Filename databox automatically gets a wild card * placed there. Ignore any warning messages if there are any. Put something in the Description 1 databox such as Cobblestone Loads. Press the OK button accepting all the other defaults. The files will be loaded into the database. PTIME will show you a list of the new entries that it loaded. Note: If you have been working sequentially through this document, then you may need to select Add an entry... before the option Load files is visible to you. In the Source Filename databox type knuckle*.doc, then proceed as described above. One point ought to be made here. Nine of the 12 loads are forces in Newtons. The other three are Moments in Nmm. We loaded all files as Forces (N). In practice, this makes no difference at all to the analysis. The load type and units are simply labels. It is up to the user to make sure that the loading in the time history file and the loading in the FE model use consistent and compatible units regardless of how they are labelled.

Customized Loads and Units Change the details of the three moments (KNUCKLE10, 11, and 12) using the Change an entry | edit Details option. Change the Load type to Moment then change the Units to Nmm. A problem you may encounter is that there may not be units defined as Nmm. Your choices could only be Nm or Ft lbs. If you have access and privileges to modify the installation area of MSC.Fatigue you can customize the load types and units. There are two files in /mscfatigue_files/ptime (UNIX) or on Windows: x:\\mscfatigue_files\ptime (Windows)

called ltypes.ind and utypes.ind. You can edit these files to add your own load types and/or units if they do not exist. For instance, edit utypes.ind and add the following line at the bottom of the file: 92

11

0.001

Nmm

The first number indicates the unit type ID; the second is the load type ID defined in ltypes.ind that the units are associated to; the third defines the conversion from SI units (N, m); the forth is an offset; and the fifth is the common name. See the MSC.Fatigue User’s Guide for more details.

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If you are able to modify this file and wish to edit the details to change the moment units, you will have to stop and restart PTIME for it to recognize the changes. If you don’t have access to modify these files then simply select Nm as the unit types since it will not make any difference to the resulting fatigue calculations.

Note: The only time that the actual load type and units are important is when you use the PTIME option Change an entry | Unit conversion to convert the selected time history to other units, although a compatibility check is made between the header of a .dac file and that specified in the jobname.fes file.

View the Time Histories The loads have been derived from a single test-track event, namely a slalom on cobblestones. There are 12 load histories which correspond to the 12 FE static load cases. For example, knuckle09.dac is the Z vertical load on the strut mount corresponding to static FE load case 9. Loads are forces in Newtons (N) and Moments in Nmm. Let us take a look at these time variations of the twelve load cases that are used in this example.With PTIME still running select the Multi-channel... | Display Histories option which will run the multi-file display module MMFD. Using the List facility select as many files as you would like to view. You can select all 12 but only eight will be visible at once. Use the Shift key to make multiple selection from the file browser. Note that the files will not appear in the databox but the number of files selected will appear below it. Accept all the other defaults on the form and press OK. The files will be displayed. If you displayed more than eight, use the View | Scrn_Options | Next Scrn option to view the rest of the time histories. Exit from MMFD and PTIME when you are done.

The Load Association Spreadsheet The spreadsheet in the Loading Info... form is used to establish the association between the load histories and the FE load cases. The other piece of information required is the FE load case load magnitude. This is used to ensure correct scaling of the stresses. In this analysis there are a couple of peculiarities. One is that the load case Main Index

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loads are set to 333 (N) instead of 1000. This effectively scales all the loads up by a factor of 3. This has been done to make the pictures prettier - the first pass analysis showed very little damage. The other peculiarity is the sign. This is due to a difference between the coordinate set used in the FE model and that in which the load histories were defined. The spreadsheet should be filled out accordingly. Load Case ID

Time History

Row 1:

15.1-1.1-1- (Load Case 1)

KNUCKLE01

-333.

Row 2:

16.2-1.1-1- (Load Case 2)

KNUCKLE02

-333.

Row 3:

17.3-1.1-1- (Load Case 3)

KNUCKLE03

333.

Row 4:

18.4-1.1-1- (Load Case 4)

KNUCKLE04

-333.

Row 5:

19.5-1.1-1- (Load Case 5)

KNUCKLE05

-333.

Row 6:

20.6-1.1-1- (Load Case 6)

KNUCKLE06

333.

Row 7:

21.7-1.1-1- (Load Case 7)

KNUCKLE07

-333.

Row 8:

22.8-1.1-1- (Load Case 8)

KNUCKLE08

-333.

Row 9:

23.9-1.1-1- (Load Case 9)

KNUCKLE09

333.

Row 10:

24.10-1.1-1- (Load Case 10) KNUCKLE10

-333.

Row 11:

25.11-1.1-1- (Load Case 11)

-333.

Row 12:

26.12-1.1-1- (Load Case 12) KNUCKLE12

Hint:

KNUCKLE11

Load Magnitude

333.

There is a toggle called Fill Down on the Loading Info... form. If you have many load cases, it becomes a tedious task to fill out each cell in the spreadsheet. If you turn this toggle ON when you select anything such as a Load Case ID or Time History, all cells below the active cell will also be filled in by selecting the next Load Case ID or Time History available. This is a very convenient tool.

Note: Depending on the coordinate system in which your stresses are defined, you may want or need to set the Transform to Basic option ON in the Loading Info... form. This will have the effect of transforming all results into the global system such that all results are in the same coordinate system before nodal averaging. This ensures proper nodal averaging and that the subsequent surface resolution will be as good as possible.

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Note also that the Results Transformations is set to No Transformation. This is because the results are nodal and in surface resolved coordinates and we wish them to remain so. Close the Loading Info... form when done.

Job Control If you do not want to wait for the analysis to run, copy the file knuckle_ma.fef to your directory and go on to the Evaluate Results (p. 197). Otherwise, open the Job Control... form and set the Action to Full Analysis and press the Apply button to run the job. Monitor the job form time to time until the job is complete. Because of the complexity of the loading, the job takes a while to run. Note: If you do not want to wait that long, you might want to do the fast analysis run instead.

Fast Analysis Your analysis can be made to run faster by selecting the Simplified Analysis toggle in the Job Control... form for a multiple load case analysis and turning it ON. The analysis will perform peak-valley-slicing to reduce the time histories and run the analysis using these reduced time histories. This quickly identifies the nodes with the most damage and then the original time histories are used in a complete analysis on only the identified locations. Note: This does make it more difficult to view the critical locations in the form of a contour plot because only the damaged locations are retained in a Simplified Analysis. The contour plot will not be continuous over the entire model.

CPU Times There are certain thing that will affect the CPU time it takes to run a fatigue analysis. These are: 1. Number of Analysis Locations (Nodes or Elements). Selecting only a certain group of locations can certainly speed up the operation. Knowing which areas to include in the group(s) you create is the challenge if you do not know where the critical locations are before hand. 2. The Number of Load Cases. There is not much you can do about this. The number of load cases required is generally the number of load cases required. However you may be able to eliminate some load cases if they have no influence on the life. 3. The Number of Time History Points. The number of points in each time history is a significant factor. The longer the time histories, the more computationally intensive is the rainflow cycle counting procedure. Peakvalley-slicing can be used to reduce time histories and still retain the damaging events.

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4. The Processor Speed. The final influence on the CPU time is the processor speed of course.

Peak-Valley-Slicing The original load histories which were around 44,000 points, have been reduced using a multi-channel peak-valley-slicing program called MPVXMUL. They have been reduced down to around 1600 points. Peak-valley-slicing is a fairly simple mechanism which tracks and extracts the peaks and the valleys of all signals to be used in an analysis. Whenever a peak or a valley is encountered in one of the signals, the corresponding points from the other signals are also retained. This procedure can be accomplished directly from PTIME using the Multi-channel... | Peak Valley Extract option, which will run MPVXMUL. You may wish to try this while the analysis is running. Open PTIME from the Loading Info... form and invoke PTIME from the Time History Manager button. Then select the Multi-channel | Peak Valley Extract option. When MPVXMUL appears select DAC as the file type. The next screen asks for the input files (channels). Accept all the defaults. The names of the files must have the same (generic) name in front of the channel numbers (KNUCKLExx.DAC). The output file names will have a .pvx extension. The next screen is the Analysis Set-up where you specify by which method to do the slicing. Accept the defaults and see the MSC.Fatigue User’s Guide for detailed descriptions of these methods. Finally a spreadsheet is presented to you with the names and statistics of the signals to be sliced. There are two editable column, F and G. You must fill in one of these columns in order to affect a change in the original signals. In the first cell of column G (Gate %) enter 10 and press the carriage return. A 10 will appear under the File pulldown menu. Press the Copy button. This will copy 10 down the column for all the signals.

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A percentage gate specifies a percentage of the total stress or strain range of the time history. For example 3 if the largest range is 1000MPa and the gate is set to Gate 10%, then any cycles encountered with ranges below 4 this gate (100MPa) will be ignored. The program does not actually count cycles but during the course of the peak-valley extraction process, the number of turning 2 points detected is restricted by imposing this hysteresis “gate”. This gate corresponds to the smallest difference between adjacent turning points that can be accepted. For turning points to be counted, they must be separated by a distance greater than the specified gate. By these means, small disturbances or “noise” in the time series may be “gated out” from the set of extracted turning points. 1

5

To perform the slicing, select File | OK. Load the files back into the Time History Database Manager by doing the Add an entry | Load files operation again but this time specifying *.pvx as the Source Filename. You may wish to use the Multi-channel... | Display Histories to compare the before and after files in MMFD. To the right shows the time history for the first load case where 49% gate has been used. After you have finished with this exercise you may wish to re-run the analysis using the *.pvx files to see the difference in the speed of the analysis and the accuracy of the answers.

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knuckle01.pvx

knuckle01.dac

CHAPTER 10 A Multiaxial Assessment

10.4

Evaluate Results The quick evaluation is to read the results into the database and do a contour plot of life. You will need to copy the file knuckle_ma.fef to your directory to do this. Open the Results... form from the MSC.Fatigue main form and press the Apply button with the Action set to Read Results. Then make a fringe plot of Log of Life, Repeats (of slaloms) from the Crack Initiation, knuckle_ma result case using the Results application as has been done in previous exercises. If you have rotated the model for any reason, select the named view nice_view from Viewing | Named View Options... to restore the original view when the session file was played. The shortest lives (greatest damage) appear to be around the loading devices, notably at the end of the steering arm, but this is spurious and should be ignored. The real hotspot is at Node 7977 which is on the left most of the two ribs running down from the strut mount.

Critical Location

On the MSC.Fatigue Results... form change the Action to List Results and run PFPOST to list the most damaged nodes. The first few are all around the loading devices. Note that Node 7977, the node of interest, gives a life of around 330 Repeats. If you list fatigue lives for all nodes, you can see that most nodes are “beyond cutoff,” meaning that no damage accumulates. Hint:

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All your listings have been written to the file pfatigue.prt, which is an ASCII file that acts as a report file for all MSC.Fatigue activity.

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If you are so inclined you may run FEFAT’s design optimization mode to view rainflow histogram plots or do sensitivity analyses or a number of other activities. This is done from the Results... form with the Action set to Optimize. Since these operations have been covered in detail in previous exercises, this will be left out of this discussion. Hint:

The influence of individual load cases can be investigated using FEFAT from the Job Control... form with the Action set to Interactive.... From the Preprocessing and analysis pick in FEFAT you can specify a single node to process and then edit individual load cases to change scaling factors or even the time histories themselves. Be aware that the jobname.fpp and jobname.fef files will be overwritten each time however.

Biaxiality - a Multiaxial Assessment What we really want to know now, (and this is what “biaxiality analysis” will tell us) is whether the analysis we have carried out is appropriate to the states of stress occurring in the component. A biaxiality analysis accomplishes the following: 1. First we are concerned with stresses and strains at free surfaces, where a state of plane stress exists, i.e., the stress state is two dimensional in the plane of the free surface. We therefore simplify our attempts to understand the free surface by transforming the stress results to local coordinate systems at each location where the x-y plane is the plane of the surface, i.e., surface resolved stresses. 2. The principal stresses are re-ordered from the conventional order where σz is the surface normal stress (should be 0) and σ1 and σ2 are ordered in magnitude. σ1 is the largest in-plane principal (in absolute value) and σ2 is therefore the other in-plane stress. 3. The biaxiality ratio is calculated for every location at every time point: ae = σ2/σ1. The angle, φp, that σ1 makes with the local x-axis is also retained for each location at every time point. 4. The surface stress state is therefore described completely by σ1, ae and the angle, φp. 5. ae and the angle, φp, get a bit unstable when the stresses are small, so when we calculate statistics of these parameters a gate is applied to filter out these small stresses. It is obvious that the biaxiality ratio, ae, can take on any number between -1 and +1. There are two reasons that we are interested in biaxiality. 1. One is that we need to know what the biaxiality is to calculate the stressstrain response correctly. 2. The other is that it affects the type and severity of fatigue damage.

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When the biaxiality analysis is negative (as indicated by the Mohr's circles of stress), the maximum shear plane where cracks tend to initiate is oriented as shown in the diagram. In the early stages of initiation the type A cracks grow mainly along the surface in Mode 2 (shear), before transitioning to Mode 1, normal to the maximum principal stress. When the biaxiality is positive however the cracks tend to be driven more through the thickness. These are therefore more damaging for the same levels of shear strain. Uniaxial loading is a special case.

Biaxiality Negative Type A Cracks

Biaxiality Positive Maximum shear plane

Uniaxial Loading

Type B Cracks

Biaxiality Ratio - (ae) = σ2/σ1

What is Multiaxial Loading? This table below describes what is meant by proportional and non-proportional multiaxial loadings. “Loading” in this case means the local stress state variations, not the global loading environment. Fortunately we very often find that although the global loading environment has a complex set of out-of-phase loads, the local stressstate variations in the critical locations are much simpler. This is often dictated by geometry - for instance the stress state at the edge of a thin metal sheet will always be uniaxial.

φp

ae

Rarity/Difficulty

Uniaxial

constant

Most common and easily dealt with. Only one principal stress exists, σ1. Standard methods OK.

Proportional Loading

constant

–1 ≤ ae ≤ 1

Less common but easily dealt with by knowing ae to correct from a uniaxial case.

Non-proportional Loading may vary may vary

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Most rare and tricky to deal with.

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Biaxiality Indicators A biaxiality analysis calculates three main indicators available for graphical plotting along with the damage and life. 1. Mean Biaxiality Ratio: Plot this result type from the Results application. This is the average value of the biaxiality ratio over the entire combined time signals for every location. The average is carried out throughout the loading history, except values are ignored if the stress does not exceed a gate value set by default to 20% of UTS. Zero indicates uniaxial (or below gate), -1 pure shear (torsion), +1 equi-biaxial and 0.3 plane strain. We can see here that most of the model remains below the gate, the critical region is very close to uniaxial and the steering arm experiences considerable shear (torsion). If you have ratios of +0.3 or more, it may be better to use the signed Tresca strain combination method, as this will be more conservative. 2.Biaxiality Ratio Standard Deviation: This parameter provides a measure of the variability of the biaxiality ratio, i.e., is the loading proportional or not. Small values (close to zero) denote proportional loadings. Non-proportional loadings are more difficult to handle, and the results may be misleading. If you plot this (use the standard spectrum selection on the form - Display | Spectrums... ) you will see that once again, the critical area presents no problem and all the action seems to be on the steering arm. Proportional loading indicates that the magnitudes of σ1 and σ2 vary proportionally to one another. Large standard deviations in the biaxiality ratio indicate non-proportionality between these two stresses.

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3. Angle Spread: This gives an indication of the mobility of the absolute maximum principal stress (range φp=0-180). 45 degrees or so is not a big problem. Movements of around 90 degrees or more is either another indication of nonproportionality of loading or may occur when there is pure shear, when the stress will “flip” through 90 degrees. When this occurs, MSC.Fatigue may give misleading results, although when the problem is due to shear stresses, the predictions will tend to be conservative. Again, in the critical location the angle spread is less than 45 degrees. In this component the extra information provided makes us feel quite comfortable about the assumptions of uniaxiality used in the initial fatigue analysis. To further enhance this confidence there are more ways to look at the above results.

Biaxiality Cross-Plots Close down the Results application (by selecting the Results switch again on the main form) and go back to the main MSC.Fatigue setup form. Open the Job Control... form; set the Action to Interactive... and press Apply to run the FE fatigue analyzer FEFAT. FEFAT will start by presenting you its main menu. Select Assess multiaxiality. The next screen presented will ask you what location to assess besides the jobname and output file which should be defaulted to knuckle_ma. Enter 7977 as the node number for the location to assess multiaxiality. Press the OK button to proceed accepting all the other defaults. The analysis will present you with a summary form. Press End to close this form down and be placed in the main Analysis Postprocessing menu for assessing multiaxiality.

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There are four main choices on this menu. Select each one to view the results: 1. Plot all outputs: This plot simply displays the time variation of all the parameters such as the biaxiality ratio, ae, and the angle, φp, for the critical location, Node 7977. The time variation of these parameters can be interesting, however the more useful plots are when each of these is cross-plotted against the principal stress for all time points.

2. Biaxiality vs. Principal: This a cross-plot of the biaxiality ratio vs. the maximum absolute principal stress for all time points at the critical node, Node 7977. The interesting thing to note is that the biaxiality ratio, ae, tends to line up vertically close to zero for this node indicating a uniaxial condition for the higher stress values. The lower stress values should be gated out.

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3. Angle vs. Principal: This is a cross-plot of the angle, φp, vs. the maximum absolute principal stress for all time points at the critical node, Node 7977. Again note that for the higher stress levels, they tend to line up vertically at a particular angle suggesting that the mobility is minimal and uniaxial conditions exist. The smaller stress cycles do show quite a bit of mobility but they should be gated out as they are of no consequence to the damage of the component. Note: The gate value used was zero (the default). To properly check for mobility you should set a reasonable gate value to exclude small stress/strain cycles that may mislead you in the interpretation of the angle spread which will be reported larger than it really is for the damaging cycles only. 4. Angle Distribution: This is another way of looking at the stress tensor mobility. This plots displays the number of times each angle, φp, appeared during the loading sequence. A spike indicates the predominate angle. The other angles that appeared occasionally are generally due to the lower stress cycles as indicated by the previous plot.

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Finally repeat these steps for a different node where there is a lot of mobility in the stress tensor, say Node 1045 which is located on the steering arm. A multiaxial condition results in plots as seen at this node: random and scattered for ae and φp not constant (flops back and forth between two predominant angles indicating a shear condition). Note however that the stress range is much less than that at Node 7977 and therefore is not of concern to us. Exit from all programs when you are finished.

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CHAPTER 10 A Multiaxial Assessment

10.5

Concluding Remarks This exercise has dealt with a FE model with complex geometry subject to multiple loading inputs. The critical location was determined and a stress state evaluation was done to determine the amount of multiaxiality at the critical location. From this the validity of the fatigue analysis was determined based on the assumption of a uniaxial stress state. Had the loading been proved to be other than uniaxial the following approaches must be taken:

Accounting for Proportional Loading Proportional loading, which means ae is non-zero but constant and the stress tensor mobility is minimal, can be accounted for on the Solution Params... form by setting the Biaxiality Correction method. Two methods exist, both of which modify the uniaxial material properties using ae. 1. Material Parameter: The Material Parameter method basically makes a new set of parameters (E, K' and n') for each state of stress. For example, Young’s Modulus becomes E* = E/(1-νae). It assumes the ratio of the principal strains remains fixed and that the von Mises stress and strain yield criteria obey the cyclic stress strain curve postyield. It is only valid to use with a maximum strain based combination parameter (Max. Abs. Principal). 2. Hoffman-Seeger: The Hoffmann-Seeger method makes the same basic assumptions, but makes the Neuber correction in equivalent stress-strain space. It has the advantage that it predicts all the principal stresses and strains and can therefore be used in conjunction with any equivalent stress or strain combination parameter. See the MSC.Fatigue User’s Guide for more details on these correction methods.

Accounting for Non-proportional Loading There is, as yet, no general agreement about how to fully deal with non-proportional loadings - it is still a major research topic. A full multiaxial fatigue analyzer is included as part of MSC.Fatigue however, and can be run externally once an initial global location fatigue analysis has been run (at least through the translation stage, i.e., the creation of a jobname.fes file). The module is called FEMLF and can be invoked from the system prompt using the symbol, femlf or from the Tools pulldown in Pre&Post. This module has a few different methods and you are referred to the MSC.Fatigue User’s Guide for detailed description of its usage. In general, fatigue life estimation from a non-proportional loading situation can only be properly determined by doing a critical plane analysis. This entails doing multiple analyses at representative angles of φp. A new rainflow cycle counting procedure is also adopted which takes Main Index

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into consideration that a cycle may begin on one plane and close on another. The notch correction procedure of correcting for plasticity also becomes complicated and uses a kinematic hardening model (the equivalent of using Neuber and Masing’s hypothesis for a uniaxial stress state). It is an advanced subject and beyond the scope of this text. But procedures do exist in MSC.Fatigue. Note: As of the release of this version of MSC.Fatigue, the multiaxial analyzer, FEMLF, although available, has not been fully validated. This is mainly due to the fact that the theoretical basis is not well established or fully accepted in the circles of fatigue experts. For this reason a number of methods are included in FEMLF. The moral of the story is to not assume a non-proportional loading situation just because the external loading and geometry are complex: 1. First assume a uniaxial stress state and perform the fatigue analysis. 2. Run a biaxiality analysis to produce the stress state parameters needed to evaluate multiaxiality. 3. Evaluate the biaxiality parameters at the critical locations to determine if any corrections need to be made for proportional or non-proportional loading. The evaluation criteria for proportional or non-proportional loading is as such: 1. If ae is close to zero and φp is constant, uniaxial assumptions stand. 2. If ae is non-zero but constant and φp is constant, a state of proportional loading exists. Compensation can be made by using the Material Parameter or Hoffman-Seeger methods to modify the uniaxial material properties. Hint:

For ae=0 Signed Tresca, Signed von Mises and Max. Abs. Principal should give close to the same results. If ae is negative, Max. Abs. Principal is the best choice. If ae is positive, Signed Tresca is the best choice. These comments apply to the crack initiation approach. If using stress life it is best to stick with Max. Abs. Principal.

3. If neither ae or φp are constant but vary significantly above the stress gate, a state of non-proportional loading exists. Compensation must be made by using the full multiaxial fatigue analyzer, FEMLF to do critical plane analysis. Note: Critical plane analyses can be computationally expensive since they requires multiple calculations at every location.

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Run FEMLF from a system or DOS prompt if you wish and select Crack initiation life analysis. Accept the jobname, knuckle_ma as default and enter 1045 as the node of interest. Remember this is the node on the steering arm that appeared to have some degree of multiaxiality. Accept all the defaults and press OK to run the analysis. A summary form appears. Accept it by pressing OK to go to the Display Menu. From here you can plot cycle/damage histograms or damage polar plots to see the results of the critical plane analysis. Try rerunning the analysis at this node for all the different methods to see the variability. The table below summarizes the results

Multiaxial Method

Life

Uniaxial Solution

~97,300

Normal Strain

~106,000

SWT-Bannantine

~316,500

Shear Stress

~18,500

Fatemi-Socie

~27,000

Wang-Brown

~30,500

Wang-Brown + mean ~26,000

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A Typical Polar Plot of Damage From one of the Critical Plane Analyses at Node 1045

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Re-run the analysis using Node 7977. This was the critical node from our uniaxial analysis. The biaxiality analysis showed us that the most popular angle, φp, at most time points for values above the stress gate was around -40 degrees. This means that if we were to perform a critical plane analysis we would see the majority of the damage at -40 degrees in a polar plot. This is indeed the case as shown to the right.

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MSC.Fatigue QuickStart GuideLK

CHAPTER

11

Welding

■ Introduction ■ Problem Description for Spot Weld Analysis with Spot Welds Modeled as Stiff BARS ■ Geometry and FE Results ■ Define a Group of CBARS ■ Spot Weld S-N Analysis ■ Problem Description for Spot Weld Analysis with Spot Welds Modeled as CWELDS ■ Problem Description for Spot Weld Analysis with Spot Welds Modeled with CHEX/MPC ■ Concluding Remarks ■ Problem Description for a Seam Weld Analysis ■ Geometry and FE Results ■ Setting up the Seam Weld Analysis ■ Concluding Remarks

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11.1

Introduction The Spot Weld Module supports three commonly used methods for modeling Spot Welds: 1. Spot welds modeled as stiff MSC.Nastran CBARS. 2. Spot welds modeled as MSC.Nastran CWELD elements. The ALIGN, GRIDID and ELEMID options on the CWELD connectivity are supported. 3. Spot welds modeled with MSC.Nastran CHEX/MPC elements. Each of these methods are illustrated with examples.

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CHAPTER 11 Welding

11.2

Problem Description for Spot Weld Analysis with Spot Welds Modeled as Stiff BARS A portion of a vehicle body is analyzed to determine the dependability of the spot welds that hold the metal sheets together in various locations. The model is fixed at one end and at the center hub area and subject to horizontal and vertical forces and a torque at the other end as shown in the plot to the right. Each load varies with time independently of the other two. The Spot welds are modeled as stiff CBARS between the sheets. Only MSC.Nastran results are presently supported by this analyzer. The forces and moments of these CBAR elements are recovered in the FE analysis and used in a subsequent spot weld fatigue analysis. The actual fatigue analysis of the spot welds is based on the Total Life or S-N method.

Objective • To illustrate spot weld fatigue analysis setup and usage • To determine the location of the weakest spot welds due to the imposed loading conditions

Table 11-1 Chapter 11 Necessary Files File P3_HOME/mscfatigue_files/examples/spot.op2 P3_HOME/mscfatigue_files/examples/horizontal.asc P3_HOME/mscfatigue_files/examples/vertical.asc P3_HOME/mscfatigue_files/examples/torque.asc

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11.3

Geometry and FE Results The geometry and FE results are contained in the file spot.op2. Copy this file over to a clean directory and open a new database called spotweld. Import both the model and the results in the typical way for MSC.Nastran as illustrated in most of the previous exercises. Because the CBAR are very small in comparison to the overall model dimensions, they have been plotted to the left as small spheres to visualize where on the model they are located. The spheres have been color coded according to element force magnitude for the first load case (horizontal force). Because the spot welds are modeled as stiff CBAR elements, only a coarse mesh is required. The CBAR are used as force transducers to obtain forces and moments transmitted through the spot weld. MSC.Nastran CBAR forces and moments are used to calculate structural stresses in the actual fatigue analysis. The spot welds are placed between the sheets joining the mid-planes of the two sheets of shell elements, and perpendicular to both. The length of the spot weld and the sheet separation should therefore be half the sum of the sheet thicknesses. There is no need for any refinement of the mesh around the spot-welds. The only requirement for the shell elements used to model the sheets is that they transmit the correct loads to the bar elements. In fact, the best results are achieved when the dimensions of the shell elements are quite large - more than twice the diameter of the weld nuggets. z

x

Point 3

CBAR element coordinate system y q Point 2 Sheet 2 Sheet 1

Weld nugget

Point 1

A typical spot-weld is illustrated above. The shaded part is the spot weld nugget. Again, the length of the CBAR element must be 0.5(s1+s2) where s1 and s2 are the thicknesses of sheets 1 and 2 respectively. Point 3 is on the axis of the weld nugget and at the interface of the 2 sheets,

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CHAPTER 11 Welding

i.e., 0.5s1 from Point 1. All forces and moments are taken to be in the MSC.Fatigue CBAR element coordinate system illustrated below. This is taken to be a Cartesian system with the Z axis going from Point 1 to Point 2. Plane 2 y z

y

Plane 1 x z

x

MSC.Nastran element coordinate system

MSC.Fatigue spot-weld coordinate system

The translator extracts forces and moments Fx,y,z and Mx,y,z in the MSC.Fatigue coordinate system, and in the conventional right-handed sense, from the results in the database, for each of the three specified points. These forces and moments (except Mz) are used to calculate nominal stresses (structural stresses) on the inner surface of sheet 1 and sheet 2, and in the weld nugget at the interface of the two sheets, at intervals around the circumference of the spot weld (θ=0 degrees to 360 degrees by increments of 10 degrees). The forces and moments at points 1 and 2 are those applied by the spot welds on the sheets, and the forces and moments at point 3 will be those applied by the upper section (between point 3 and point 2) on the lower section (between point 1 and point 3). Fy My

Sheet 1

Fy My

Nugget

Fz

Fy Fz Fz

Fx Sheet 2

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Mx

My

Fx

Mx

Fx

Mx

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11.4

Define a Group of CBARS Before we commence on the exercise, create a group of CBARS that represent the Spot welds. In this example we are going to keep things simple by defining a single group of bar elements. This means that all spot welds in this model have the exact same nugget radii and flange sheet thicknesses. Note:

d ifferent groups may be created for spot welds that connect across different flange D pair thicknesses. However, care must be exercised to ensure that duplicate elements (CBARS) do not exist in different groups. Overlaps are not permitted. If this does occur, the characteristics of the CBAR from the last group will be adopted. Automatic group creation, meaning grouping CBARS that connect across the same flange thickness pairs, is feasible when the Material form is filled out (see next section) if the bulk data file for a model is read into the Patran database.

Open the Group form from Group | Create on the main form of Pre&Post or MSC.Patran. Call the new group beams. Set the focus in the Entity Selection databox. The Select Mechanism should appear. Set the selection to pick only CBAR elements from the graphics screen. With the mouse and the cursor, surround the entire model by clicking and dragging from one corner to the opposite. All the CBAR elements will be selected. Press the Apply button to create the group. Close the Group form when you are done.

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CHAPTER 11 Welding

11.5

Spot Weld S-N Analysis To set up the Spot Weld fatigue analysis follow the instructions in this section. Start by opening the main MSC.Fatigue setup form and set the General Setup Parameters as follows: 1. Analysis: Spot Weld 2. Results Loc.: Both This is automatically set to Both. Since we are working with CBAR elements, both the element number and the node IDs associated with the elements are necessary. Fatigue calculations will occur at the two end nodes and the element centroid (or weld nugget). 3. Nodal Ave.: Global This setting does not have any bearing on a Spot Weld analysis. The default is set. 4. F.E. Results: Force This setting is also automatically set for you to Force. Forces and moments will be extracted from the database as opposed to stresses as with all other fatigue analyses. The stresses will be determined from the forces and moments. 5. Res. Units: N, mm Units are now in forces and moments and not stresses. The default is N, mm (Newtons and millimeters). 6. Jobname: spotweld 7. Title: Spot Weld Fatigue Analysis Example

Solution Parameters Open the Solution Params... form. There is only one setting on this form for a Spot Weld analysis, namely the design criterion, or Certainty of Survival. By default it is set at 50%. This parameter has been discussed in detail in previous exercises. It is the association with the scatter of the S-N curve. To be 90% confident of reaching the design life, set this value to 90. For our example problem simply accept the default of 50%.

Material Information Setting up the material information for a Spot Weld analysis is similar to other fatigue analyses with a few differences. The major difference is that you must define groups with CBAR elements only. The spot weld nuggets themselves can differ in radius, and the sheets to which

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they attach can differ in thickness. For each combination of spot weld nugget radius and sheet thicknesses you must define a group. Also each location on the spot weld (sheet1, sheet2, nugget) can be associated with a different S-N curve.

Spot Weld S-N Curves S-N curves for Spot Weld analysis are determined by testing specimens similar to that shown. The system requires an S-N curve for each metal sheet and for the weld nugget at load ratio R=0, plus a mean stress sensitivity factor and a standard error parameter (used when setting the Certainty of Survival on the Solution Params... form). The formulation of the S-N curve is as follows:

∆ S = SRI1 ( N f )

b1

for Nf < Nc1, the transition life. For Nf > Nc1 a second slope b2 is used. It is possible to correct each cycle with amplitude S and mean stress Sm to calculate an equivalent stress amplitude S0 at R=0: S + MS m S 0 = ----------------------M+1 The MSC.Fatigue materials database contains around ten S-N curves for specific spot weld types. In this example we will use the generic spot weld S-N curves for the nugget and the sheets. You can view these SN curves by pressing the Material Database Manager button. When PFMAT starts Load | data set 1 with spot_nugget_generic and Load | data set 2 with spot_sheet_generic. Then do a Graphical display to view the S-N curves. Exit from PFMAT when you are done.

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Fill Out the Material Spreadsheet Now on the Material Info... form, fill out the spreadsheet as follows:

1. Number of Groups: 1 Up to 165 different combinations of spot weld nugget diameters, sheet thicknesses, and corresponding S-N curves can be created. If you change this setting, be sure to press the carriage return to affect the change. The spreadsheet rows will update to the number you specify here. 2. Group: beams Click on Group and pick BAR as the element type. Pick the group beams that you created in the previous step and click on Fill Cell. We will not use the create Sub Group option as we are assuming that all welds have the same properties, and default values will be filled in the cells. You may want to experiment with this using your own model by reading in a model that has element properties and experiment with the Sub Group option. This option splits a selected group into a series of groups based on the thickness pairs found at the end of the bar elements and load up the cells automatically. The next cell will become active. 3. Diam: 4.8 This is the spot weld nugget diameter for the specified group of spot welds. If the properties for this model were in the database this would have been calculated automatically from a look up table (see MSC.Fatigue User’s Guide). Note that the units must be consistent with that specified in the General Setup Parameters. The diameter is specified in millimeters (and the sheet thicknesses). Enter the value and press the carriage return. The next cell will become active.

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4. S-N(nug): spot_nugget_generic This is the S-N curve for the spot weld nugget. 5. S-N(sh1): spot_sheet_generic This is the S-N curve for the top sheet of the spot weld. 6. T(sh1): 1.8 This is the thickness of the top sheet. Enter the value and press the carriage return to accept the number. The next cell will become active. 7. S-N(sh2): spot_sheet_generic This is the S-N curve for the bottom sheet of the spot weld. 8. T(sh2): 1.8 This is the thickness of the bottom sheet. Enter the value and press the carriage return. The last cell will become active. 9. SF: 1.0 This is an additional scale factor you may apply in the form of a Kf if desired. Accept the default of unity by pressing the carriage return. If more groups of spot welds had been defined, the next row would become active for data entry just as we have filled out the first row. Close the form by pressing the OK button when finished.

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Loading Information This is identical to all other fatigue analyses using the pseudo-static method. Transient FE results are also supported. Open the Loading Info... form. You will notice that it appears identical to all other analyses discussed thus far.

Define the Load Service In our analysis we have three FE loads, thus the need to associate three time variations of the loading. Press the Time History Manager button to invoke PTIME. When PTIME appears, use Add an entry... | ASCII convert + Load to load the following time histories: HORIZONTAL, VERTICAL, TORQUE. You will need the three files horizontal.asc, vertical.asc, and torque.asc which are available from the usual location. One at a time, load these ASCII files into the load history database and set the details as shown below. The first two (HORIZONTAL and VERTICAL) represent transmission (horizontal) and suspension (vertical) loading and are forces in Newtons. The third is a moment due to a bracket loading (TORQUE) in Nmm. To set the details of one of these do the following on the details page: 1. Select Add an entry... | ASCII convert + load. 2. Select horizontal.asc as the ASCII filename. Accept all other defaults and press OK. 3. Add at least something to Description 1, such as Horizontal Load. 4. Change the Load type to Force. 5. Change the Units to Newtons. 6. Accept all other defaults and press OK.

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Repeat this operation for the other two ASCII files. Remember the torque time history is a moment and the moment units are Nmm. If these units are not available, see A Multiaxial Assessment (Ch. 10) for a discussion on customizing loads and units. You may have already customized the units to include Nmm if you followed this previous exercise fully. Use the Multi-channel... | Display Histories to invoke MMFD to view all three histories at once if you wish. Quit from PTIME when you are done.

Fill Out the Loading Spreadsheet Fill out the form and spreadsheet on the Loading Info... form as follows: 1. Number of Static Load Cases: 3 There are three static load cases representing the horizontal, vertical, and torque load cases. 2. Fill Down ON: ON Turn this toggle on to make it easier and more efficient to fill out the spreadsheet.

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3. Load Case ID: 2.1-2.1-2Activate the first cell by clicking in it. Press the Get/Filter Results... button and on this form press the Select All Results Cases toggle and then the Apply button. All the available results cases will appear in the left most listbox under the spreadsheet. Select the first load case from this listbox and in the right most listbox, select either Bar Forces, Rotational or Bar Forces, Translational. Both must exist but only one needs to be selected. Finally press the Fill Cell button. Note that all three cells fill out with the three load cases because you had the Fill Down toggle turned ON. The next cell in the first row becomes active. 4. Time History: HORIZONTAL The time history database is queried and all available time histories are displayed in a spreadsheet below the main spreadsheet. Select the row containing HORIZONTAL. Again all rows are filled in because the Fill Down toggle is ON. Make sure HORIZONTAL, VERTICAL, and TORQUE are associated with Load Cases 1, 2, and 3 respectively. 5. Load Magnitude: 1000 These are the load magnitudes as applied in the FE analysis. Enter 1000 as the load magnitude in Newtons. Press the carriage return. Again all cells fill with this value. This is appropriate for load cases 1 and 2 but load case 3 needs to be set at 100,000 Nmm. Activate the bottom cell in the Load Magnitude column and change the value to 100000 and press the carriage return to accept the value. The Loading Info... form is now complete. Press the OK button to accept the form.

Job Control Open the Job Control... form and set the Action to Full Analysis and press the Apply button to run the job. Monitor the job from time to time until the job is complete. The job, because of the complexity of the loading, and the number of spot welds takes a few minutes to run.

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Stress and fatigue damage are calculated at 10 degree intervals around the spot weld for the two sheets. This also increases the calculation times. Note:

By default, calculations at the nugget are not done. You must run the Spot Weld analyzer, SPOTW, interactively to do nugget calculation or to reduce the number of angle calculations around the spot weld.

As already mentioned, this method requires spot welds to be modeled as stiff CBAR elements in MSC.Nastran. The forces transmitted through these CBAR elements are used to calculate the structural (nominal) stresses in the weld nugget and the adjoining sheet metal at intervals around the perimeter of the nugget. These stresses can then be used to make fatigue life predictions on the spot weld using a S-N (Total Life) method. Life is calculated using Linear Damage Summation (Miner’s rule).

Results Evaluation Open the Results... form and Read Results into the database. Two results files are created by a Spot Weld analysis: The jobname.fef (spotweld.fef) file is the normal result file that is similar to other result files and is the result file read into the database. It contains ten columns corresponding to worst damage, life, and log of life for each spot weld including the angle of failure, the node ID (sheet 1, 2 or the nugget ID=0), and the maximum force encountered. The other results file is called jobname.spt (spotweld.spt). It is an ASCII file that is queried by the actual spot weld analyzer, SPOTW, with result reported at all angles and locations.

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Identify Critical Spot Weld Open the Results application from the main form. You will see a new result case called SPOT WELD ANALYSIS, spotweld. If you select this you will see all the result values that you can plot. However because the spot welds are very small in comparison to the rest of the model dimensions and because the results are stored as elemental results, the Results application is not practical to use. (You will not be able to see a contour plot.) It is better to graphically view the results at these CBAR by making element marker plots. This is done in the Insight application. Before invoking the Insight application make sure that the group beams is posted (Group | Post) along with the default_group. Then press the Insight switch on the main form of Pre&Post or MSC.Patran. A new graphical window will open and the Insight application will appear. Set the Action to Create and the Tool to Marker as shown to the side. Then follow these instructions: 1. Press the Results Selection... button. 2. From the form that appears, select the SPOT WELD ANALYSIS as the Current Load Case. Press the Update Results button and select the Marker Result, Log of Life (Repeats). Accept all the defaults on the Result Options form if it appears. Close the form with the OK button. 3. Press the Marker Attributes... button. 4. Change the Type to Sphere and the Scale to Screen. Set the Scale Factor to 0.03 and then close the form with the OK button. All the spheres will appear the same size. 5. Change the Target to Elements. 6. Turn the Use All Posted toggle OFF so you can select a group. 7. Select the group beams only.

Marker Plot Here

8. Press the Apply button to produce a marker plot similar to that shown to the side. Using color mapped markers, you can easily and quickly identify the critical spot welds in the model. To experiment with the marker plot you can change the Action to Modify and the Object to Marker. Select the marker plot you just made (the default name is Marker_1) and Main Index

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change any setting you wish such as the marker type, the scale factor, and the color mapping. Also try plotting some of the other results. When you are finished, press the Insight toggle switch again to close Insight and return to the standard graphics screen.

Results Listings Return to the main MSC.Fatigue setup form and on the Results... form set the Action to List Results. This runs the Spot Weld analyzer SPOTW in its List global results mode. Exit SPOTW and go to the Job Control... form and run SPOTW from the Interactive... action. You can now see all the options of SPOTW. The analysis was performed in batch mode through Pre&Post or MSC.Patran, however you can also run it interactively using the first selection, Estimate fatigue life. Listing the results from the Results... form put you directly into the List global results option. Another result listing option is the lisT.spt file. Press this button to list results. The Results Filename SPOTWELD.SPT will be read. Keep all defaults and select OK. A Results summary of the worst damaged element will be displayed.

Polar Plots From SPOTW’s main menu select Results polar plot. Accept all defaults and press OK. This will spawn a graphical program called MPOD that displays polar plots of damage. These plots show life for the nugget and the two sheets around the circumference of the spot weld showing you at which angle the worst damage occurs. It is very much like a critical plane analysis display. The plot to the left shows life from the worst case element.

If you run Results polar plot again and this time select Maximum stress rate for the plot parameters, you will get a different plot.

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The plot to the right shows the maximum stress ranges for the sheets and the nugget. Note that this plots shows three curves, one for the nugget and each sheet. Sheet 2 shows the most stress range and thus the reason for damage appearing from only sheet 2 in the first polar plot above. The stress range in the other sheet and the nugget do not cause much, if any, damage. Note:

You may not see a third curve corresponding to the nugget calculations as shown in the plots above when you do this on your own. This is because by default the nugget is not calculated. If you want the nugget calculations, you have to turn them on when running SPOTW interactively.

Return to the SPOTW main menu when you are done.

Sensitivity Analysis SPOTW also has a Design optimization mode very similar to that of FEFAT. In fact, most of the same options exist. Only those that are different are discussed here. It can be accessed from the Results... form directly when the Action is set to Optimize or you can enter it from the Job Control... form when you run SPOTW in Interactive... mode. You can play “What if games” on the sheet thicknesses, nugget diameter, loading...etc. To enter Design optimization you must supply an element number or specify the WORST element. The calculation will proceed with a summary page and then you are placed in the Design Optimization main menu where you can do a multitude of things similar to the FEFAT fatigue analyzer optimization mode.

For example, try a Sensitivity analysis | sheet 2 thickness and enter “(1.0,3.0,0.5)” which indicates sheet thicknesses from 1 mm to 3 mm by increments of 0.5 mm will be analyzed. Then go to result Display | Sensitivity plot to do a graphical display of the results. You can experiment with other options as you see fit and when you are done, exit SPOTW and quit from Pre&Post or MSC.Patran. Note:

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Although the sensitivity analysis of differing sheet thicknesses is insightful, changing the thicknesses of the sheets necessarily changes the moments which are not taken into account. Therefore this is simply an approximation. A full validation should be done by changing the thicknesses in the FE model and recalculating the forces and moments.

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11.6

Problem Description for Spot Weld Analysis with Spot Welds Modeled as CWELDS A simple CWELD element model is used to demonstrate the procedure. The model consists of 2 Channels and a middle sheet Spot welded at the location indicated by the arrow. The three sections are connected by 2 CWELDs using the GRIDID option, with each CWELD connecting a Channel section and the middle sheet. One channel section is loaded with 25N loads in the X, Y & Z directions while the legs of the other channel are clamped at the edges. The 3 sheet connection used in this and the next exercise will be useful in the discussion on three sheet calculations.

Objective • To illustrate Spot Weld fatigue analysis with CWELDS • Reading CWELD results from the MSC.Fatigue or Fatigue Utilities menu • To illustrate the auto spot weld group creation on the Materials form. Table 11-2 Necessary Files for Section 11.6 File P3_HOME/mscfatigue_files/examples/spot_cweld.bdf P3_HOME/mscfatigue_files/examples/spot_cweld.op2

Reading in the Model and CWELD Results Create a new database called cweld and read in the Model input file (the BDF file) and results (the OP2 file)

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Create CWELD groups Just like in the previous exercise, we have to create a group that represents the CWELD Spot welds. From the Group menu, create a group by selecting the connector elements (appears as a sphere on the menu) and dragging the mouse over the entire screen. Name the group cweld_elem. The cweld_elem group can be displayed from Group | Post for verification.

Set Up the Spot Weld Analysis Set up the main form just like in the previous example but this time give it a new Jobname. Accept the defaults on the Solution parameters form.

Material form The third objective of the exercise will be illustrated here. Open the Material form and do the following: 1. For the Element Type select CWELD. 2. Select the cweld_elem group created above. 3. Turn Create Sub-groups on. 4.

Click on the Fill Cell button.

The Material form cells will load up automatically with 2 groups, with each group of spot welds connecting a different pair of flange thicknesses, determined from the properties in Model input file.

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From the form on the right it can be seen that 2 CWELDS groups are created connecting a thickness pair of (1.2,0.6) and (0.6,1.2) mm respectively. These are based on the orientation of the CWELD defined in the input deck. The diameters are also filled in automatically and are calculated values based on the flange thicknesses.

Note:

The diameters are calculated from the flange sheet thicknesses from an internal table in the software and are based on mm units. Users may define their look up table in a Spotweld.sys ascii file and place the file in the run directory

For this exercise, change the diameters of both groups to 5.4839 mm. The reason for this will be evident in the next exercise. Press OK to accept the inputs.

Loading Form As per the previous exercise, select the Loading Info… button on the main MSC.Fatigue form and fill it out as follows: 1. Select the available Static Loadcase (this time there will only be one) 2. Pick the Weld Forces (translational or rotational) for the Load Case 3. Pick HORIZONTAL for the time history 4. Normalize the loading history by specifying the maximum value of 999 in the load magnitude cell. 5. Press OK to accept the inputs.

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Run the Spot Weld Analysis From the Job Control form, set the Action to Full Analysis and press Apply to run the job. Once the job has completed read the results into the database. From the MSC.Fatigue Results form, set the Action to List Results and press Apply. Enter the name of the job you just ran and click on OK. Spot Weld will then display the results form. The minimum life of 6149 repeats is reported in the middle sheet (0.6 mm thickness), as expected.

For more complex models, Users may want to read the results into the Patran database and display results using marker plots as per the previous exercise.

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11.7

Problem Description for Spot Weld Analysis with Spot Welds Modeled with CHEX/MPC This modeling technique utilizes HEX elements to model the spot weld where the weld is connected to the flanges by MPCs. MSC.Nastran Grid Point Force (GPFORCE) results must be recovered on the HEX8 nodes for MSC.Fatigue to extract Spotweld forces and moments for analysis. The same model from the previous exercise is used with the CWELD connections replaced by CHEX/MPC connections. The model plot is shown below:

Objective • • • •

To illustrate Spot Weld fatigue analysis with CHEX/MPC Converting CHEX/MPC results to equivalent BAR results To illustrate the auto spot weld group creation on the Materials form. Displaying Analysis results on the CHEX Spotwelds.

Table 11-3 Necessary Files for Section 11.7 File P3_HOME/mscfatigue_files/examples/spot_chex.bdf P3_HOME/mscfatigue_files/examples/spot_chex.op2

Reading in the Model and CHEX/MPC Results Create a new database called chex and read in the Model input file (the BDF file) and results (the OP2 file). The model is shown below.

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Convert CHEX/MPC to Equivalent BARs The next step is to convert the CHEX/MPC results to equivalent BAR results. For Patran installations, invoke Tools| MSC.Fatigue| HEX to BAR Results or Tools | Fatigue Utilities | HEX to BAR Results for the standalone version. The form shown below will be displayed. The select pulldown has two options. If “All HEX8 SW” is selected, then all HEX8 elements that are attached to MPCs will be grouped and processed. The second option, “Hex8”, allows the User to specify the HEX elements that are to be processed as Spotwelds. A warning message is issued if the HEX elements selected do not have any MPCs associated with them. For this exercise, select All Hex8 SW and select the single Results case.

Note:

Only the element GPFORCE results belonging to the selected results case will be used.

The Options button activates another form, as shown below, for accessing additional utilities. A new primary results name can be specified to distinguish the extracted Spotweld forces and is useful for assigning names for results from multiple loadcases. Since we only have a single loadcase, we will accept the assigned name. The Create Moments for Check toggle when selected generates nodal moments that can be used for visualization and checking purposes. The Spot weld elements are placed in List A, that can be viewed and processed from Tools | List | Create. For this exercise, we will not create moments for check but the User may exercise this option independently. The tolerance is used to check equilibrium of the forces in the Spot Weld. Close the form and hit Apply button on the HEX to BAR Results form.

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Element forces and moments of the selected spot weld elements are calculated. The results for the selected result case will be named 'Spot Weld Forces, Rotational' 'Spot Weld Forces, Translational'

Set Up the Spot Weld Analysis Set up the main form just like in the previous example but this time give it a new Jobname. Accept the defaults on the Solution parameters form.

Material form Open the Material form, select CHEX as the modeling method and press Fill Cell. The Material form is filled automatically with the Spot Weld Groups.

Note:

The same number of Spot Weld groups are created as per the previous exercise. The flange thickness associations are also the same. The only difference is that the diameters of the Spot weld nuggets are calculated from the volumes of the HEXA elements instead of through derivation from the flange sheet thicknesses. We will use this diameter for results comparison with the previous exercise.

Press OK to accept the inputs. At this point it is worth examining the Groups that have been created. From Group | Post notice that there are 2 groups – SPOTWELDS and Groups labeled SW_PSXX_PSYY. The SPOTWELDS contains all the extracted Spot Weld CHEX elements. The SW_PSXX_PSYY are the equivalent CBARS that and are located between the first and fifth node of each CHEX element, where PSXX and PSYY are the PSHELL identifiers for the flanges the Spot Weld connects to.

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Loading Form As per the previous exercise, the steps for filling out the loading form are exactly the same. 1. Select the Static Loadcase 2. Pick the Spot Weld Forces (translational or rotational) for the Load Case 3. Pick HORIZONTAL for the time history 4. Normalize the loading history by specifying the maximum value of 999 in the load magnitude cell. 5. Press OK to accept the inputs.

Run the Spot Weld Analysis From the Job Control form, set the Action to Full Analysis and press Apply to run the job. Read the results into the database and list the results as per the previous exercise.

Note:

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Apart from the damage angle, the results are identical to the previous exercise. The damage angle, is correct in both cases as it is relative to the definition of the orientation of the weld.

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Additional Postprocessing for CHEX/MPC Spot Welds An additional postprocessing menu, apart from the regular post processing tools used in the previous exercises, is available that allows display of results on the faces on the Spot Weld Hex8 elements. Users are encouraged to exercise this menu on their own by reading the results into the database and posting the Spotweld group to the display. For Patran invoke the postprocessing menu from Tools | MSC.Fatigue | Post Processing Hex8 Spotwelds or Tools | Fatigue Utilities | Post Processing Hex8 Spotwelds for the standalone version. From the form shown at the right, select the Result Case and the Result Type in order to display the results on the posted group.

Three Sheet Calculations As is the case above, you may find the necessity of analyzing spot welds that connect three sheets. In general, joints with three or more sheets spot welded together are more difficult to make efficiently and undesirable from a durability point. They should be avoided as much as possible in design. Sometimes they may be unavoidable, or alternative designs may be uneconomic. As demonstrated in this Chapter, there is no reason why one should not analyze cases where three sheets are spot welded together by treating them as two separate welds, as demonstrated above, but the analysis methods currently used are not validated for these cases. This problem is the subject of current research and until a validated solution has been found, a temporary fix, called a 3 sheet correction has been provided. In the example problems used above, the lowest life is on the middle sheet (in this case it is logical since the loads are the same on all sheets and the middle sheet has the lower thickness) but generally for 3 sheet connections with all parameters being equal (sheet thicknesses and loads) failures are predicted in the middle sheet, which rarely occur in practice. For this reason, a simple fix has been implemented. We will use the results from our last exercise to demonstrate this fix as the results from a 3 sheet connection are always required before this can be used. Go to the Job Control form and set the Action to Interactive and press the Apply button. Select the 3 sheet correction option on the SPOTW form. The jobname.spt result file just created will detect three sheet spot welds in the .spt file and will create a new jobname.fef results file

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in which failure at the middle sheet (the common node between the two spot welds) is ignored. The worst result for the remaining calculation points is written to both spot weld elements in the resulting .fef file. The figures below show the results of the 3 sheet connection correction.

This option makes postprocessing the results easier by eliminating spurious predicted failures at the middle sheet. Note however, that if the middle sheet really would fail (as we have noted above that the life predicted on the middle sheet is indeed correct as it has the lowest thickness), this will not be predicted either! However this does not appear to happen much in practice.

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11.8

Concluding Remarks The Spot Weld analysis module is robust, easy to use, and is an integrated part of MSC.Fatigue. It finds all the spot welds that cause problems. The method is generally applicable and handles multiaxial loadings. Using the generic material properties, predictions can be somewhat conservative however. Some of the other spot weld S-N curves may be more appropriate or you may need to derive or create your own.

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11.9

Problem Description for a Seam Weld Analysis This simple example is used to demonstrate the use of Seam Weld Analysis in MSC.Fatigue.

The tubes are welded as shown and a point load of 394N is to the top of the upper tube.

Objective • To illustrate seam weld analysis setup and usage • To demonstrate the unique capability of automatically extracting the seam weld analysis group

• To determine the most damaged nodes on a seam weld The .op2 files necessary for the example problem are included in the installation as indicated in the following table.

Table 11-4 Necessary Files for Section 11.9 File P3_HOME/mscfatigue_files/examples/seamw.op2 (Windows) P3_HOME/mscfatigue_files/examples/seamw_unix.op2 (UNIX)

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11.10 Geometry and FE Results Copy the file seamw.op2 to a clean directory and open a new database called seamw1. Import both the model and results in the typical way for MSC.Nastran as illustrated in most of the previous exercises. Note:

This .op2 file contains the results from a MSC.Nastran run using the STRESS(cubic) case control command that extrapolates element centroidal results to the nodes using a cubic extrapolation function and PARAM,SNORM,22.5 which generates shell normals to improve the accuracy of results in curved shells. Users should refer to the MSC.Nastran User’s Guides for more information on these commands.

Creating a Weld Group In order to carry out the fatigue analysis of the weld we need to create two groups, one containing the weld elements and the other containing the plate elements. To demonstrate the unique group extraction capability of this module, we will only create the weld group and use the default group for the plate group. First, let’s create a group containing the weld elements. From the main menu select Group | Create. Enter the name weld for the group and then enter Elm 1422:1477 for the entity selection. Click on Apply to define the group, the group should be highlighted in the plot.

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Viewing the Stress Results Select Results from the toolbar. Choose the first loadcase, select the quantity (max principal) and click Apply to see the results. The Maximum Principal stresses for the first loadcase are shown in the following figure. The second loadcase is identical to the first case except that the load is applied in the opposite direction.

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11.11 Setting up the Seam Weld Analysis To set up a fatigue run, open the main MSC.Fatigue setup form and set the General Setup Parameters as follows: 1. Analysis = SEAM_Weld 2. Results Loc.: Both This value is automatically set for you. 3. Nodal Ave.: Group This value is automatically set for you. 4. F.E. Results: Stress This value is automatically set for you. 5. Res. Units = MPa 6. Jobname = demo_qsg 7. Title = Seam Weld Example The result locations are at nodes, averaging is performed using the elements, except the weld elements connected to an analysis node and only FE nodal stress results are permitted.

Solution Parameters Click on the Solution Parameters... button. The Mean Stress correction, that is based on the Haigh Diagram, can be set ON or OFF. For this example we will set it to OFF. The certainty of survival can be adjusted but we will accept the default 50%. Select OK to continue.

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Material Information This form is used to create the seam weld analysis groups and select the fatigue properties of the weld material. Fatigue analysis is carried out at the toe of the weld using the nodal stresses. To make life easier for the user, the software will automatically create a group for these nodes. The user needs to enter the weld group and the plate group (or the default group) and the software creates a group of the shared nodes between these. Open the Materials Info... form, click on the group entry. The Create Weld Group form should now appear as shown in the following form.

Select the appropriate weld and plate groups and create a new group titled weld_toe. Note:

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Internally, MSC.Fatigue appends “MW_” to the beginning of this name. If you wish to create a node list for plotting results, this must be selected before creating the MW group. We will not do this for this example. Click on Apply - a warning message about the contribution from triangular elements that end up in the created group (MX_toe.weld) is displayed. This message can be ignored. A new group containing the nodes and elements along the weld_toe is now posted in the display window as shown below. Note that only the toe elements are extracted which is the unique feature of this module.

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Note:

The new group should be automatically posted to the viewport and look like the diagram below. If not, you can post it by going to Group | Post on the main MSC.Patran form an selecting the group MW_weld.toe.

The new group is stored in the database with a MW_groupname. Note that groupname cannot have any spaces, either leading, trailing or anywhere in between. There is no limit on the number of MW_groupname groups that may be created and analysis is performed and results are reported on the MW_groupname groups. If a MW_groupname group exists in the database the group cell in the main form get populated automatically. Note:

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If the Create node list was turned on, the user can now create a node list for plotting various results by clicking in the Node list text box and selecting nodes from the display of the MW_group (e.g., 1025, 139, 1040, 135, 1041, 142, 1038, 132, 1039, 148, 1042, 144, 999, 151, 995). The result plotted is in the order in which the nodes are selected, i.e. the x-axis is the node list. Selecting Create node list creates a node list file and the list may be reversed by clicking on the reverse node list button. Use the Cancel button to return to the main form.

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Complete the Materials Information form by assigning the following values to the various cells of the spreadsheet: SN Flexible = SEAM_STEEL_FLEX SN Stiff = SEAM_STEEL_STIFF Flex Ratio = 0.5 - bending/total stress ratio. This controls the cut-off for which S-N curve to use M1/M2 Ratio value = 2.5 – M1/M2 is a fatigue material property used in mean stress correction based on the Haigh Diagram. Multiplier = 1.0 Offset = 0.0

Loading Information Open the Loading Info... form. In this example we have carried out a quasi-static FE analysis on a single load case with a point load of 394N. We now need to determine how long the weld will last when subjected to a time varying load with a maximum of 394N. Pick the Load Case ID cell. This will update the form. Now select the Get/Filter results button and pick the Select All Results Cases toggle. Press Apply and the 2 FE load cases appear in the form. Select the first load case (in fact either load case can be picked and the final results should be the same) and the stress tensor and click on Fill Cell, the load case is now entered into the table. Complete the rest of the spreadsheet as follows:

• • • •

Time History = SINE01 Load Magnitude = 1.0 Scale Factor = 0.5 Offset = 0

Click OK to submit the loading information.

Fatigue Analysis and Results Click on the Job Control button and set Action to Full Analysis. Press Apply to submit the fatigue analysis. The job can be monitored by setting Action to Monitor Job. When the job has completed click Cancel to return to the main Fatigue form.

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Plotting Fatigue Results The Insight application will be used to plot the results of the seam weld model. Go to the main form of Pre&Post or MSC.Patran and press the Insight button. The Insight application will appear. Set the Action to Create and the Tool to Marker. Fill out the rest of the form as follows: 1. Press the Results Selection... button. 2. From the form that appears, select the Seam Weld Analysis, demo_qsgfef as the Current Load Case. Press the Update Results button on select Log of Life (Cycles) as the Marker Result. Close the form with the OK button. 3. Press the Marker Attributes... button. 4. Change the Type to Sphere and set the Scale Factor to 0.05. Close the form with the OK button. 5. Accept all other values as is and press the Apply button to produce a marker plot similar to the one shown below.

Exit Insight by selecting the button on the main form.

Listing the Fatigue Results Press the Results button on the main MSC.Fatigue form and set Action to List results. Press Apply and select the jobname.demo_qsg. Click on OK. This brings up the PFPOST form that allows the User to filter the results listing. In this example, nodes with Damage>0 have been requested. Set the Filter to damage and Notebook Output to On and Click OK. Select the most damaged nodes (or the desired option) in the PFPOST menu and Click OK. Main Index

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The worst damage occurs at node 87 with 5.856E6 repeats to failure. Press OK to exit from the results listing program and then exit PFPOST. The listed results can also be plotted from the Interactive menu of the solver (SEAMW) by selecting the Job Control button from the main MSC.Fatigue form and setting Action to Interactive. Press Apply. The SEAMW menu is displayed. Selection of the Plot Damage Distribution produces the GUI shown in the figure below. Nodes can be listed using standard input. Enter the Nodes 995 and 1025 as shown on the form. It is important to note that the nodes must exist in the database for the plot to be created.

Accept all other default values and press OK.

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The software then reads the list and extracts the required values from the FEF file and creates a DAC file for plotting in MQLD.

Note that the x-axis increment is unity and the increments correspond to the node list in the Plot Damage Distribution form. If you created a node list file earlier, then go ahead and select the Plot Damage Distribution option again. This time set the Method of Node Selection switch to ASCII File and select your node list. The filename will be demo_qsg_MW_xxxxx.ent where xxxxx is the group name you gave it when you created it. Accept all other defaults and press OK. The following MQLD form will be displayed.

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11.12 Concluding Remarks The seam weld module is a powerful tool for the fatigue analysis of continuously welded thin sheet structure. The analysis group extraction functionality is extremely useful as it removes the burden on the analyst on having to create weld groups on the toe side of the weld. Predictions using the generic material properties will in general be conservative but users may modify the properties or create their own.

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MSC.Fatigue QuickStart Guide

CHAPTER

12

Wheels Module - Analysis of Rotating Structures

■ Problem Description ■ Geometry and FE Results ■ Setting Up the Wheels Analysis ■ Fatigue Analysis and Results ■ Concluding Remarks

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12.1

Problem Description This simple example is used to demonstrate the Wheels module in MSC.Fatigue. To aid in quick familiarity with this module .op2 files for Windows and Unix platforms are included for the model shown below.

The model is an 80 in (L) by 20in (R) hollow cylindrical modeled with cquad4 elements (t=0.01in.) meshed using a 10 in. by 10 degree increment mesh. The tube is clamped at each end and point loads of 0.5 lb. have been applied at 10-degree increments in separate subcases on the peripheral nodes. The reason for the application of the load of this magnitude is due to the fact that the wheels module, treats the output stresses in KSI units and any significant stresses in the plates will lead to erroneous interpretation of reported lives.

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CHAPTER 12 Wheels Module - Analysis of Rotating Structures

12.2

Geometry and FE Results Start MSC.Patran or the stand-alone version of MSC.Fatigue and select File | New. Enter a new database name as qsg_demo.db as shown in the New Database form. On the Model Preferences form, select a default Nastran | Structural run. This time you will also need to set the Model Dimension to 80 before you press OK.

Now load the data file by selecting the Analysis option from the menu bar or use the Analysis button in prepost standalone. When the form appears, set Action to Access Results, the Object to Read Output2, and Method to Both (model and results); then, press the Select Results File button, select the file cylinder_model.op2, and press Apply. The model will then appear on the screen.

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Viewing the Stress Results Select the Results option from the menu bar. Action = Create Object = Quick Plot Select Result case = Default, Static Subcase Select Finge Result = Stress Tensor Quantity = Max. Principal 2D Click Apply and the following results will be displayed.

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CHAPTER 12 Wheels Module - Analysis of Rotating Structures

Note: The figure above shows the results for the first Result case. If you were to select the first nine subcases using the same Fringe Result and Quantity value, you would see an 80-degree “Rotation” of the load in 10-degree increments.

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12.3

Setting Up the Wheels Analysis To set up a fatigue run select Tools|MSC.Fatigue, from the main menu. Select Wheels and specify a Jobname:

• Analysis = Wheels • Jobname = qsg_demo Element or nodal results may be processed and results averaged globally or on the selected group. For this example, we will set the Results Loc. to Node, the Nodal Ave. to Global, and the Res. Units to PSI.

Solution Parameters Click on the Solution Parameters button. The form shown is shown below. Set the Mean Stress Correction to Goodman. Biaxiality analysis or the stress combination methods are not available, since the Wheels module performs a critical plane S-N analysis using surface resolved stresses at every surface node. Although this is not necessary for this model, other models, in particular solid element models, will require stresses to be surface resolved. The option to generate surface normals is available from the job control menu. The surface angle selects the increment at which the analysis will be performed. In this case surface stresses will be resolved in 10-degree increments as well as the analysis. Make sure that the Surface Angle in Degrees databox is set to 10. The certainty of survival is not selectable in this module – the S-N curve is used without any modifications. Press OK and close the form.

Material Information The materials form is shown below. Select: Material - 7075_HV_T6 Finish – No Finish Treatment – No Treatment Region – default_group

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CHAPTER 12 Wheels Module - Analysis of Rotating Structures

Select the default values for the remaining fields and press OK to close the form.

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Loading Info Click the Loading Info Button and the following form will appear.

This form allows us to specify the loading environment. The default number of load conditions is 5 and for this module it is important to understand the concept of a load condition. The load condition represents a particular type of loading. The first load condition could be the set of subcases for one revolution that define a straight roll, the second could be a set of subcases for one revolution that define a turning condition, and similarly for the third, fourth and fifth load conditions. A typical usage profile for a wheel is shown in the table below.

Loading Condition

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Vertical Load [lb.]

Lateral Load [lb.]

Required Mileage

Straight Roll

11,300

42,500

15% Inboard Turn

11,300

1,700

3,250

15% Outboard Turn

11,300

-1,700

3,250

30% Inboard Turn

11,300

3,400

250

30% Outboard Turn

11,300

-3,400

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CHAPTER 12 Wheels Module - Analysis of Rotating Structures

In our example, 36 subcases (loads at 10 degree increments – 360/10=36 subcases) constitute the first and only load condition. This means that only one loading condition will need to be entered. Change the Number of Loading Conditions to 1. The .op2 file contains the results for the 36 subcases that will be used as the load condition. The table below illustrates how the stress at every surface angle and rotational angle (this is merely the same load applied in the next subcase at the node corresponding to a 10-degree rotation) is extracted and damage computed for the load condition. The stress time history at an analysis node is a sequence of stresses extracted from each subcase for every surface angle for each rotational increment (subcase). In the table below the stress time history for the analysis node for a surface angle θ is the column associated with the surface angle. It is also worth noting that since the stress time histories are created from the subcases there are no .DAC files required for analysis.

Rotational Angle Φ = subcase #

Surface angle θ 10°

20°

350°

360°

0° (subcase 1)

σ1,10

σ1,20

σ1,350

σ1,360

10° (subcase 2)

σ2,10

σ2,2

σ2,350

σ2,360

20° (subcase 3)

σ3,10

σ3,2

σ3,350

σ3,360

350°(subcase 36)

σ36,1

σ36,2

σ36,350

σ36,360

Damage DθL

D1,L

D2,L

D36,L

D36,L

The loading form allows user selectable units for reporting life. Here, approximately 500 repeats of the loading is equivalent to 1 mile. Enter the respective quantities in the cells as shown in the Loading Information form.

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Click on the loading conditions cell. The form is updated. Now select the Get/Filter Results... button and a new form is displayed. See below.

Select the item Default, 36 subcases and click on the Filter button to populate the bottom list box. Here all the subcases are lumped into one but it is possible to remove subcases, that appear in the lower window, by picking the subcase and clicking on the remove button. The Clear button clears the entire selection while the remove button allows the analyst to pick and remove the selected subcases. Since, we are using all subcases we will accept them by clicking on the Add button. The overwrite button allows the analyst to change the selection in case a mistake is made. Click on the Close button to accept and load the Select Loading Condition Results listbox loading information form with the line “Default 1. (1.36)” or something similar. Now select this value and the Stress Tensor value and press the Fill Cell button. The Loading Condition ID cell will now be filled in. Since we previously set the life reporting units to miles, we will set the design life to 1000 miles. The design life is the target distance and allows reporting a factor of safety on the calculated life. Set the loading factor to 1.0 and press OK.

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12.4

Fatigue Analysis and Results Click on the Job Control button and select Full Analysis form the form. Click on Apply to submit the fatigue analysis. The job can be monitored by setting Action to Monitor Job. When the job has completed click Cancel to return to the main Fatigue form. Note: The Calculate Normals option is available. This would be used to generate surface normal files for solid element models. The qsg_demo.fef file contains the following items: 1. Node 2. LC1 Angle – angle of lowest life at the node for load condition 1 3. LC1 Damage – worst damage at the node for load condition 1 4. LC1 Life – life in equivalent units at the node for load condition 1 5. LC1 Log of Damage – log of damage at the node for load condition 1 6. LC1 Log of Life – log of life at the node for load condition 1 7. Log of Worst Damage – For a single load condition, this is identical to (5). For multiple conditions this is the log of the sum of damage from all load conditions. 8. Log of Worst Life – Log of (11). 9. Worst Angle -- For a single load condition, this is identical to (2). For multiple conditions this is the surface angle which has the highest accumulated damage from all load conditions. 10. Worst Damage – For a single load condition, this is identical to (3). For multiple conditions this is the sum of damage from all load conditions. 11. Worst Life – For a single load condition, this is factor of safety. For multiple conditions this is sum of the factors of safety of each load condition.

Plotting the Fatigue Results From the Results menu change the Action to Read Results. Click on Apply, then on Cancel to return to the main Fatigue form. The results are now loaded into the MSC.Fatigue database. The qsg_demo.fef file contains the results that may be plotted. Select Results from the main Toolbar and fill in the form as follows: Action = Create Object = Fringe Select Result Cases = qsg_demofef Select Results = LCI Log of Life Click Apply

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The log of life fatigue fringe plot is shown in the figure below.

The fringe plots show the expected results – since the boundary conditions do not change and the loading is identical for every subcase, the maximum and minimum principal stresses for every subcase are identical. Consequently, we see progressive concentric circles of increasing life from the lowest in the middle of the cylinder (area of maximum stress under the applied load) to highest in between the constrained ends and the middle. Note: Plots of the other items (from the Select Results listbox) are not shown here. The user may want to exercise these other options and review their plots.

Wheels Interactive Menu On the Job Control form, select the option Interactive to display the Wheels Interactive menu. The FEROT main form appears. We are now going to analyze the FES file and create a ROT file. Select Analyse and the following menu is displayed.

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CHAPTER 12 Wheels Module - Analysis of Rotating Structures

Select a .fes file that has been previously created. The output filename defaults to Jobname.fef (which for our example is qsg_demo.fef) but an alternative name may be supplied. Click on Yes to create the ROT file. This file provides the user with useful information on the stress contribution and damage for each load condition at the analysis nodes for each rotational increment of the wheel (remember this is dependant on the number of subcases that were specified to define a revolution) and each surface angle increment. For large models, this listing can be quite extensive and the option is provided to turn this OFF. A sample output of the .rot file is shown below

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Each cell represents a stress from the surface angle and the subcase (rotational angle). Each row is a stress time history for the node at the particular surface angle and may be plotted by using the extract time histories from the interactive menu. The lower figure shows the damage values for each surface angle increment for load condition 1 and the maximum damage. The row labeled D0 shows the damage sum from all load conditions for each surface angle increment. When the option Results postprocess is selected on the main FEROT form and the OK button is pressed, the following form is displayed.

This menu allows further postprocessing of the results that requires the .ROT file for input to generate a new output results file (username.fef file). This file can either be listed or read back into the MSC.Fatigue database by specifying the same Jobname in the main form as the output filename. The extraction options are:

• Specific Angle – Extract results at a specific angle at all analysis nodes. Since the surface angle selected for our example case was 10 degrees, results may only be extracted for angles in ten-degree increments up to 360 degrees. Main Index

CHAPTER 12 Wheels Module - Analysis of Rotating Structures

• Worst Case – Extract the worst (lowest life, maximum damage) results for all conditions including the worst case (accumulated damage from all conditions) results. When the option Extract Time Histories is selected on the main FEROT form and the OK button is pressed, the following is displayed.

Click on the Input FES filename browse button and pick the FES file QSG_DEMO.FES. Press OK. Click on the Calculation points browse button to display the calculation nodes. Enter the node number 501. Click on the load conditions browse button to display the available load conditions. Enter the load condition number 1. Enter the surface angle at which the stress time history is desired. For this example, we will use 0.

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Enter the output filename (e.g., QSG_DEMO). An extension will be added to the filename that identifies the node number, load condition and the surface angle. A plot of the stress time history will be automatically generated as shown below.

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12.5

Concluding Remarks The wheels capability allows fatigue analyses on wheels for a variety of loading conditions and can also be applied to any rotating body. This tool is particularly powerful as it performs a critical plane analysis at the analysis surface nodes for multiaxial loading. Another feature which is unique to this module is the loading concept (load condition/subcases) that provides the analyst the capability to define duty cycles (sometimes referred to as an event spectrum) and making damage assessments due to particular events.

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MSC.Fatigue QuickStart Guide

CHAPTER

13

A Software Strain Gauge

■ Problem Description ■ Geometry and FE Results ■ Time History Extraction ■ Correlation Techniques ■ Concluding Remarks

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13.1

Problem Description A physical prototype of the engine mounting lug that was studied earlier is placed in a test laboratory and subject to an external service loading environment. A hardware strain gauge rosette is placed on the prototype in a strategic location near a suspected failure location and the strain time history is captured. The prototype is also created as a finite element model. A software strain gauge is created in the same location as that of the physical prototype. FE results are extracted and converted to the same coordinate system as the rosette. A subsequent fatigue analysis is done on both the physically measured strain time history and that simulated by the finite element model for comparison and correlation purposes.

Strain Gauge Rosette Placement

Objective • To create a software strain gauge on the FE model in the same location and orientation as the physical strain gauge

• To extract FE results in the same coordinate system as the gauge • To synthesize the measured strain history from the FE model • To run fatigue analyses on both measured and simulated strain histories for comparison purposes

• To assess the stress state in the prototype at the measurement location Table 13-1 Chapter 13 Necessary Files File P3_HOME/mscfatigue_files/examples/mounting_lug.op2 P3_HOME/mscfatigue_files/examples/soft_sg.fin P3_HOME/mscfatigue_files/examples/soft_sg_m1.dac P3_HOME/mscfatigue_files/examples/soft_sg_m2.dac P3_HOME/mscfatigue_files/examples/soft_sg_m3.dac

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CHAPTER 13 A Software Strain Gauge

13.2

Geometry and FE Results To begin the exercise, copy the Output2 file, mounting_lug.op2, to a clean working directory. Like the previous exercise using spot welds, this specialty tool requires (and only works with) MSC.Nastran stress/strain results. Open a new database called softsg and import both the model and FE results from the Output2 file in the usual way as explained in previous exercises.

The Gauge Tool The Software Strain Gauge (Soft S/G) application in MSC.Fatigue is a special tool, mainly for use with measured strain results, to do test-analysis correlations, comparisons and validations. Soft S/G allows a direct correlation to be made between measured strain histories obtained using resistance strain gauges and predicted strain histories from the surface of finite element models. This is achieved by applying simulated strain gauges to the surface of the FE model in the same positions as real strain gauges on the corresponding component. The simulated gauges consist of one or more thin shell elements which are fitted to the surface of the FE model. The gauges can then be used to extract the results of previously carried out FE stress/strain analyses at the locations and in the orientations defined. Then, using MSC.Fatigue and one of its modules, called SSG, it is possible to synthesize the stress or strain histories from the gauges in a way which is directly comparable with direct strain measurements. Hint:

Users may also find the Soft S/G useful simply for obtaining static stress and strain results from particular locations within elements, or in particular directions.

Open the main MSC.Fatigue setup form and set the Analysis to Soft S/G. You will notice that the form changes appearance from the normal setup form and only displays three main buttons. Each of these buttons will be invoked in turn starting with Gauge Tool.... The first button will invoke the Gauge Tool application for placement, creation and modification of the software gauges. Open this form now.

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Create a Soft S/G Creating a Soft S/G is a two step process: 1. Select a point on the surface of your model to place the gauge and define the gauge orientation. 2. Define a surface area on which to place the gauge. These are all done conveniently from the Gauge Tool. Once the gauge is created you can always modify its location and orientation or delete it if necessary. On the Gauge Tool form, set the Action to Create and fill out the form as follows: 1. Object: MM-120WR This is the gauge name. The names that appear in this pulldown are fully customizable with a gauge definition file. See below for more details. 2. Gauge Number: 1 Give the gauge a number. It should be set to 1 and will increment automatically. The gauge number will be padded with zeros so as to always be three digits. 3. Elastic - Plastic: Plastic Turn the Plastic toggle ON. When the time comes this will be a flag used to indicate that elastic-plastic correction is requested and the resulting output time histories will have had the correction applied to them. If Elastic is selected the resulting time histories will remain purely elastic. 4. Select a Point: Node 1175 Activate this databox by clicking in it with the mouse. The Select Mechanism will appear. You can use any of the standard mechanisms to graphically select or define a valid point. The point however, must exist on the surface of the model for proper creation. For the purposes of this exercise, type Node 1175 in the databox. 5. Select Gauge X Axis: Coord 0.1 Set the focus into this databox and select the orientation of the x-axis for the gauge. Again the Select Mechanism appears and you can use any of the standard graphical selection methods. For this exercise type Coord

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CHAPTER 13 A Software Strain Gauge

0.1. This specifies to use the global x-axis as the gauge x-axis. You can achieve the same by selecting the 1-direction in the Select Mechanism and then selecting the global axis from the viewport. 6. Press Apply When you press Apply, a yellow marker will appear on the location that you selected to create the gauge. The form will update for the second step which is to define the area for the gauge. You may wish to zoom in on the area of interest for a better view. Use the View corners icon on the top form. 7. Element Type: 2D: Shell elements You can either select shell elements or the free faces of solid elements. Set this to 2D: Shell elements. 8. Select Shell Elements: Elem 166 167 178 179 Set the focus in this databox. The Select Mechanism changes to allow selection of either shell elements or faces of solid elements depending on the Element Type setting. Type Elem 166 167 178 179 in the databox or graphically select the four elements around the point of interest.

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. Note:

The only requirement for selecting an area of elements around the point is that the area be large enough to contain the gauge. If the area is not large enough an error will occur. Note that the relative angles, gauge length, and gauge width are displayed on the form for informational purposes. Also, if there is too much curvature, the gauge creation may fail. You should choose relatively flat areas for gauge placement.

9. Press Apply Pressing the Apply button a second time will create the gauge.

The Gauge Definition File All gauges that appear as selections under the Object pulldown are defined in a file called gauges.def that exists in the main MSC.Fatigue installation area for UNIX in /mscfatigue_files/gauges.def

or for Windows in x:\mscfatigue_files\gauges.def

where x: is the drive on which MSC.Fatigue was installed. This file is fully customizable to allow additions or changes to gauge types. You simply need to define the gauge type (single, tee, rosette), whether it is stacked or planar, the configuration (rectangular, delta, other), the units, and the coordinates, besides giving it a name. See the MSC.Fatigue User’s Guide for details or use the file contents as a guide to customization.

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CHAPTER 13 A Software Strain Gauge

The file can exist in the local work directory, your home directory or in the installation area and will be recognized in this order also. A variety of gauge types is shown above.

The Gauge Group When a Soft S/G is created it appears graphically on the screen as one, two, or three quadrilateral elements. Additionally a special group is created for each strain gauge. The name of these groups take on the form: dms_n_m_oo_ppp

where dms is the German abbreviation for strain gauge n is “t” or “b” indicating whether the results set is extracted from the top or bottom of underlying shell elements making up the Soft S/G m is the gauge type number (its unique type identifier) oo is either “el” or “ep” indicating elastic or elastic-plastic ppp is the number of the gauge applied to the model, e.g., 001, 002, etc.

Go to Group | Post if you wish, and you will see any Soft S/Gs that have been created. The element and node numbers are contained in the groups. The rest of the necessary information resides in the name of the group and in the subsequent results extraction.

Modify the Soft S/G Our gauge that we have created thus far is not quite what we want. Change the Action to Modify in the Gauge Tool. The gauge needs to be translated and rotated since the node where we placed it and the orientation do not match the exact spot that it exists on the prototype. On the prototype, the gauge was placed two millimeters to the left from the current location and the gauge needs to be rotated 30 degrees counterclockwise. 1. Select Gauge to Modify: 001 Select 001 as the gauge to modify. We are not changing the type of the gauge but simply the location and orientation. 2. Delta X: -2.0 This is the displacement to move the gauge in the x-axis direction of the existing gauge. 3. Delta Y: 0.0

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This is the displacement to move the gauge in the y-axis direction of the existing gauge. 4. Delta Theta: 30 This is the rotation in degrees that the gauge is to be rotated relative to the current orientation. 5. Element type: 2D: Shell elements Again select 2D: Shell elements as the means to define the surface. 6. Select Shell Elements: Elem 166 167 178 179 Select the same elements as before to define the surface where the modified gauge will be placed. To properly modify the location and orientation, you must select a surface area that will contain the new location and orientation of the modified gauge or an error will occur, e.g., if you translate the gauge off of the defined area. 7. Reverse normal: OFF If necessary you can reverse the normals of the gauges. The gauge outward normals are calculated as the average of the outward normals of the selected elements or faces. 8. Press Apply Now that the gauge has been created and modified to the proper location and orientation, close the Gauge Tool form by pressing the Cancel button.

FE Results Extraction The next step is to extract the results from the FE result sets and create new result types in the location and orientation of the Soft S/G. Press the Results Extraction... button on the main MSC.Fatigue setup form with the Analysis still set to Soft S/G. Another form will appear listing all available result cases that contain MSC.Nastran stress results and all available Soft S/Gs that have been created. The process is simple:

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1. Select the necessary Available Loadcases. Press the Select All button to select all of them. Make sure you extract results from all the necessary result cases if you have multiple load inputs. If the results are from a transient analysis, make sure you select all time steps. For this exercise, we will Select All. 2. Select the required Strain Gauges. In our case there should only be one, 001. 3. Press the Apply button. A message window will pop-up reminding the user that “The current group should contain all nodes and elements with valid model results.” It will ask you if you want to proceed. Just select the Yes button on the form. Two new result types are created after the results extraction is complete for each stress analysis load case selected. Under each selected set of load case results, the two new subcases are: Gauge Stress, Average

and Gauge Stress, Centroidal

The results in the Gauge Stress, Average subcase are, for each element, the average of the results from the four corners of the element and the element centroid. For Gauge Stress, Centroidal, they are the results from the origin of the gauge coordinate system. For a rosette or tee gauge, the average results will, in general, be a little different for each element, depending on the stress field in which the gauge is placed. For Centroidal results, the same results should be written to all the gauge elements. Select Cancel to close the form.

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13.3

Time History Extraction In order to proceed we must create an MSC.Fatigue input file and then use this file as input to the Soft S/G (SSG) application module that will produce our synthesized stress/strain time histories. The goal at this point, is not to do a fatigue analysis but to create the combined time history at the gauge location and in the gauge orientation.

Fatigue Analysis Setup To create the MSC.Fatigue input file, change the Analysis to Initiation and set the General Setup Parameters as described below or the user can expedite things by copying the MSC.Fatigue setup file soft_sg.fin and reading it in using the Job Control, Read Saved Job option: 1. Analysis: Initiation 2. Results Loc.: Element You must set this to Element. Results exist at the element centroids. 3. Nodal Ave.: Global 4. F.E. Results: Stress Only stresses are extracted into the gauge coordinates. 5. Res. Units: MPa. 6. Jobname: soft_sg 7. Title: Soft S/G Analysis Example -- Elastic/Plastic Note: If you read the soft_sg.fin file in, then you can skip ahead to page 281 and run the translation.

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CHAPTER 13 A Software Strain Gauge

Solution Parameters Open the Solution Params... form. Set the parameters as follows: 1. Analysis Method: S-W-T This parameter is ignored in the analysis. 2. Plasticity Correction: Neuber You can use either Neuber, MertensDittmann, or Seeger-Beste methods according to which elastic-plastic correction method you wish to use in the Soft S/G. If you select Mertens-Dittmann or Seeger-Beste you will be required to enter shape factors on the material information form. Analysis for any gauges which are elastic will ignore this selection. If you want to estimate elastic-plastic strains, you should ensure that the gauges have ep included in the group name. We will use Neuber for our example. 3. Run Biaxiality Analysis: ON This should be set to ON. There will be no need to execute the Calculate Normals option because the stress analysis results written to the gauges are already in a surface resolved coordinate system. 4. Biaxiality Correction: Hoffmann-Seeger Select Hoffmann-Seeger. The option Material Parameter is not allowed for Soft S/G. Analysis for any gauges which are elastic will ignore this selection (the same as setting it to None). The rest of the information on the Solution Params... form is ignored, so it is OK to accept the defaults.

Material Information Open the Material Info... form. This form is used to assign material and other information to the individual gauges. The strain gauge software requires that each gauge be a group, consisting of one shell element for each leg. Valid group names take the form dms_m_n_oo_pp as described previously.

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The strain gauge software will assume for a tee gauge or a rosette that gauges 1-3 are in numerical order of elements. Gauges are numbered in an counterclockwise direction. The Material Info... form and spreadsheet should then be filled in as follows: 1. Number of Materials: 1 This should be set to the number of strain gauges that are to be processed. For our example, it will be set to 1. Note: There is a limit of 20. If more that 20 are to be processed you will have to break the analysis up into multiple analyses. Therefore, on the spreadsheet, there will be one row for each gauge or rosette. 2. Material: RQC100 This is the material on which the gauge is positioned. 3. Finish: No Finish This is not used, so any setting is OK. 4. Treatment: No Treatment This is not used, so any setting is OK. 5. Region: dms_t_4_ep_001 Select the name of the group defining the gauge. 6. Kf: 1 This is a surface finish correction factor and is not used by the strain gauge software. Leave blank or set it to 1. 7. Shape factor: 0.0 This is the shape factor (Formzahl) or plastic strain concentration factor required for the Mertens-Dittmann and Seeger-Beste methods. Valid values are greater than 1. Typical values are around 1.5 to 3.0. Zero (0) can also be used and is interpreted as infinity. In this case both methods reduce to the Neuber method. Leave this blank or set it to 0.0. Main Index

CHAPTER 13 A Software Strain Gauge

8. Multiplier: 1.0 This should not be used. Leave it blank or set it to 1.0. 9. Offset: 0.0 This should not be used. Leave it blank or set it to 0.0. Close the form by pressing the OK button when finished.

Loading Information The Loading Info... form should be used in the same way as for any other MSC.Fatigue job. The results can be from a transient analysis (time step analysis) or from a set of static load cases which will be associated to time variations in the normal way (which are defined using PTIME). When selecting actual results, the user should choose one of the following result types for the extracted strain gauge results: Gauge Stress, Average

for the stresses averaged from the four corners and the centroid of each strain gauge element, or Gauge Stress, Centroidal

for the stresses at the centroid of each gauge element. If neither of these exist, it means you have not extracted results to the software strain gauges from the FE model results as done in the previous section. Use the exact same time histories as in a previous example of the engine mounting lug. See Multiple Loads (Ch. 9) for a review. The basics are repeated here for convenience sake: Open the Loading Info... form and press the Time History Manager button. These four histories have been provided in the examples directory. Copy from Remote the four histories called XPOS, YPOS, XNEG, and YNEG. Specify the remote directory /mscfatigue_files/examples/ (UNIX) or x:\\mscfatigue_files\examples\ (Windows). Do not forget the ending slash (\ or /). Change the units to kNewtons to be consistent with the “Multiple Loads,” Chapter 10 example. However, these are simply labels and will not affect the analysis.

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To view all four histories at once, use the Multi-channel... | Display Histories option. This will run the multi-file display module, MMFD. When MMFD appears, use the List facility to select the four files above (use the Shift key to make multiple selection from the file browser). Note that the files will not appear in the databox but the number of files selected will appear below it. Accept all the other defaults on the form and press OK. The files will be displayed. Close the graphics by selecting File | Exit and then quit from PTIME. On the Loading Info... form, set the Number of Static Load Case to 4 and press Return or Enter, then fill out the spreadsheet as shown below. The load cases selected correspond to Load_Case.1, Load_Case.3, Load_Case.5, and Load_Case.7. Remember that you must use the Get/Filter Results... button to display the FE load cases before you can select them.

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Load Case ID

Time History

Load Magnitude

Row 1:

2.1-4.1-2- (Load_Case.1)

XPOS

0.25

Row 2:

4.3-4.1-2- (Load_Case.3)

YPOS

0.25

Row 3:

6.5-4.1-2- (Load_Case.5)

XNEG

0.25

Row 4:

8.7-4.1-2- (Load_Case.7)

YNEG

0.25

CHAPTER 13 A Software Strain Gauge

Note: For this example you must select the Gauge Stress, Average stress tensor in order to fill the Load Case ID cells out correctly in the spreadsheet. Also note that the actual load case IDs may differ from those shown here.

The Loading Info... form is now complete. Press the OK button to accept the form.

Job Control Open the Job Control... form and set the Action to Translate Only. This will save or create the MSC.Fatigue job (soft_sg.fin) file and run the PAT3FAT translator to produce the intermediate (soft_sg.fes) input file required by SSG when the Apply button is pressed. Answer Yes to any questions. When the translation is complete you are now ready to run SSG.

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Run Soft S/G (SSG) Set the Action to Soft S/G again on the main MSC.Fatigue setup form. Press the SSG Analysis... button to launch the software strain gauge analysis.

When SSG appears, you will be asked for the MSC.Fatigue input file you created in the previous section. Select soft_sg.fes and accept all other defaults on the SSG input screen and press OK a few times to start the software strain gauge extraction process. The process will take place quickly. Note: The extracted strain time history can now be used to correlate with the direct test signal.

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CHAPTER 13 A Software Strain Gauge

13.4

Correlation Techniques Once the strain histories have been extracted in the gauge coordinates, there are a number of things that may be done to compare the software strain gauge results directly with the measured strains. The most obvious is direct signal comparisons using the multi-file display module (MMFD). Before embarking on the mini-exercises in this section, copy to your working directory three signals that represent the actual measured time histories. These files are soft_sg_m1.dac, soft_sg_m2.dac, and soft_sg_m3.dac, one for each leg of the rosette. The Soft S/G analysis, using SSG, also created a simulated strain signal for each leg. If you look in your working directory you should also see three files called soft_sg00101.dac, soft_sg00102.dac, and soft_sg00103.dac.

Overlays and Cross Plots Either invoke MMFD from a system prompt by typing mmfd, or from PTIME using the Multi-channel... | Display Histories option. When MMFD appears, use the List mechanism to select all six time histories of interest mentioned above. Three are from the Soft S/G analysis and the other three represent measured strains that you just copied to your directory. Select Overlay as the Display Type and set Alter Setup to Yes before pressing the OK button. When you request to alter the setup, an additional screen appears. On this screen we want to request that only two plots per page be displayed. Set Plots Per Page to 2 and press the OK button. A final setup page will be displayed that specifies which signals are displayed on which page. We want to overlay gauge leg one of the measured signal with gauge leg one of the synthesized signal, and two with two and three with three respectively. So set the leg one signals to display on page one, leg two files to display on page 2 and leg three files to display on page 3. This done by clicking in the cell under the Page (C) column and a pulldown menu appears allowing you to set the page number. Select File | OK when done. Press OK to accept the defaults on the Overlay Setup form. Press OK to accept the defaults of the Y-axis Alignment form. The first overlay plot for leg one will be displayed.

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To display the next plot use the View | Scrn_Options | Next Scrn pulldown selection. You will need to press OK several times (accepting the defaults) before the next plot screen is displayed. In cases where there appears to be an almost one-to-one correspondence, it might be better to show a cross-plot instead. You can change any of the overlay plots to cross-plots using Plot_Type | Crossplot. You will need to press OK several times (accepting the defaults) before the cross-plot is displayed. You can go back to overlay by using Plot_Type | Overlay. The overlay and cross-plots for the three gauge legs are shown below comparing the measured to the synthesized signals.

Leg 1

Leg 2

Leg 3

Select File | eXit to close the overlay plots or cross plots.

Signal Statistics Another means of comparison is to look at the signal statistics that are displayed by MQLD. This plot shows the maximum, minimum, mean, standard deviation, and RMS values for the displayed plot.

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CHAPTER 13 A Software Strain Gauge

Note: Only one of the dac files can be selected at a time using this method. The MQLD for can be brought up in three ways: 1. Run MQLD from the system prompt by typing mqld. 2. Select MQLD (Quick Look Display) from the Tools | MSC.Fatigue | Graphical Display Utilities menu (installation with Patran) or Tools | Fatigue Utilities|Graphical Display Utilities menu (standalone). 3. Open the Loading Info form, select the Time History Manager button, and then select the Plot an entry option.

Rosette Analysis Now perform a rosette analysis. You can do this by invoking MSSA from the system prompt (mssa). When MSSA appears, select the first option: 1 - Strain Gauge Rosette Analysis | Analyze. A rosette analysis requires the strain signals from the different legs of the gauge as input. Run two rosette analyses, one with the three synthesized signals and the other with the three signals representing the measured strains. On the first form that appears, accept all the defaults by pressing the OK button. Our strain gauge rosette is Rectangular and Stacked and the Output Type requested is strain. The next screen requests the three signals representing each leg of the rosette. Select the first three measured signals, soft_sg_m*.dac, using the List button and also set the Biaxiality Ratio calculation to Yes. Accept all other default values and press OK. The rosette analysis will commence. After the analysis, you are placed into a postprocessing menu where you can plot the outputs of the rosette analysis. The outputs are maximum, minimum, absolute

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maximum, signed shear strains, and the angle, f, and biaxiality ratio, ae, as a function of time. All of these outputs can be plotted. Use Plot all outputs to see all these signals. Not all of the plot will appear on the same screen; use View | Scrn_Options | Next Scrn to view the next page of plots. Select File | Exit when you are done viewing all the plots. Successively plot Biaxiality vs. Principal, Angle vs. Principal, and angle Distribution. Note the general uniaxial nature of this particular problem. Repeat this operation for the other three synthesized signals, soft_sg0010*.dac and compare the outputs from the first run with the measured signals as shown below. Note: The output that you get out of a rosette analysis is dependent on the type of gauge and how many legs it has.

Main Index

Angle Distribution

Select eXit to leave close the MSSA form.

Main Index Cross Plot, ae vs. Max. Abs. Prin.

Synthesized Rosette Analysis Output

Cross Plot, f vs. Abs Max. Prin.

Measured Rosette Analysis Output

All Outputs vs. Time

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Single Location Uniaxial Life Analyzer MSC.Fatigue also provides a single location analyzer for use directly with measured strains called MCLF (for crack initiation analysis - critical life fatigue). It is based on uniaxial assumptions. MSLF is the equivalent stress based single location analyzer (stress-life fatigue). You can feed any of the strain signals into MCLF or any of the outputs from the rosette analysis to calculate a fatigue life. Invoke MCLF by typing mclf at the system prompt. When MCLF first appears you must give it a job name or select an existing job. Since none exist yet, type in soft_sg_uniaxial as the jobname. You will be asked whether you would like to create this new job. Answer Yes. A number of input screens will be presented to you. The input for each of these screens is mentioned below.

Service Loading Environment Form Filename: soft_sg.abs Use the List button to select the maximum absolute principal strain output signal from the rosette analysis using the synthesized strain gauge results. We could use any one of the output signals from the Soft S/G analysis or the rosette analysis. To see the file with the file browser you will have to change the filter to view files with extensions .abs. Note: MCLF assumes elastic-plastic correction has taken place already by default, otherwise you must set the Strain Type to Fully Elastic to invoke a notch correction procedure. Accept all other defaults and press OK.

Model Parameters Form Mean Stress Correction: All Set the mean stress correction to All. The strain signal we are using has a very tensile mean (positive). Since it is easy enough to analyze all mean stress corrections, let us do so. We should notice the S-W-T method giving the most conservative answers. Accept all other defaults and press OK.

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Material Data Input Form Material Name: RQC100 Use the List button to choose the material RQC100 or simply type in the name. Accept all other defaults and press OK.

Geometry Definition Form Accept all defaults for this page and press OK. The analysis will commence and a summary page of results will be displayed. Note that S-W-T (Smith-Watson-Topper) gives a bit over 400 Repeats of the signal as the life.

Post Processing Options Now run the analysis again with the maximum absolute principal signal from the rosette analysis using the measured signals. From the Post Processing Options form, select Loading environment. Change the Filename to soft_sg_m.abs and press the OK button.Then doubleclick or press the Recalculate switch. A summary page of results will be displayed. The table below compares the life values from the measured signals to the synthesized signals:

Mean Stress Method

Measured

Simulated

None

~1120

~750

Smith-Watson-Topper

~565

~435

Morrow

~715

~535

Single Location Multiaxial Life Analyzer MSC.Fatigue also provides a single location multiaxial analyzer for use directly with measured strains called MMLF (for crack initiation analysis - multiaxial life fatigue). You can feed the three strain signals from the Soft S/G analysis directly into MMLF to calculate a fatigue life based on multiaxial techniques. Invoke MMLF by typing mmlf at the system Main Index

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prompt. When MMLF first appears you must give it a job name or select an existing job. Since none exist yet, type in soft_sg_multi as the jobname. You will be asked whether you would like to create this new job. Answer Yes. A number of input screens will be presented to you. The input for each of these screens is mentioned below.

Rosette Input Options Form Accept all the defaults on this page and press OK. The rosette of interest to us is Rectangular and Stacked.

Service Loading Environment Form Gauge 1: soft_sg00101.dac Gauge2: soft_sg00102.dac Gauge3: soft_sg00103.dac Use the List buttons to select the three signals generated by the Soft S/G analysis. Each one of these represents one of the three legs of the software strain gauge. These files are soft_sg0010*.dac. Note: MMLF assumes elastic-plastic correction has taken place already. It has no notch correction procedures built into it as does FEMLF or MCLF. Accept all other defaults and press OK.

Calculation Parameters Form Damage Accumulation Method: 7. All (1-6) Set the calculation method to All (1-6) so we can see the difference between the various analysis types and see which gives the most conservative answers. Accept all other defaults and press OK.

Material Data Input Form Material Name: RQC100 Use the List button to choose the material RQC100 or simply type in the name. Accept all other defaults and press OK. The analysis will commence and a summary page of results will be displayed after a some what lengthy calculation. Note the lives given.

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CHAPTER 13 A Software Strain Gauge

Postprocessing Options Form Now run the analysis again with the measured signals. From the Post Processing Options select Loading environment. Change the Gauge file names to soft_sg_m*.dac. Gauge 1: soft_sg_m1.dac Gauge2: soft_sg_m2.dac Gauge3: soft_sg_m3.dac Use the List buttons to select the three measured signals or type them in by hand. Accept all other defaults and press the OK button. Press the Recalculate button. The table below lists the life values from the measured vs. the synthesized signals:

Mean Stress Method

Main Index

Measured

Simulated

Normal Strain

~1120

~755

SWT/Bannantine

~270

~230

Shear Strain

~960

~655

Fatemi-Socie

~1070

~695

Wang-Brown

~650

~435

Wang-Brown + Mean

~265

~210

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13.5

Concluding Remarks Once the strain histories have been generated from the FE model, they may readily be compared with the corresponding measured strains from the real component. The possible methods for comparing them shown in this exercise are:

• • • • •

Multi-File Display (MMFD) to overlay or cross-plot the data Comparison of signal statistics (max, min, RMS, etc.) Strain gauge rosette analysis option (MSSA) Single location uniaxial fatigue analysis (MCLF) Single location multiaxial fatigue analysis (MMLF)

Correlation is a very important aspect of reliable durability calculations. If a correlation exercise indicates that there is poor qualitative and quantitative correlation between predicted and measured strain histories, any fatigue calculations are also likely to give poor results. Likely causes of poor correlation are:

• Errors in setting up the MSC.Fatigue job, particularly in matching the correct channels to the correct load cases with the correct scaling factors

• • • • • • •

Main Index

Errors in calculating the loading histories Poor definition of the loads and boundary conditions, or missing loads Inadequate meshing Inaccurate strain gauge placement Inappropriate analysis (e.g. quasi-static when the problem is dynamic) Poor materials Non-proportional loadings together with high levels of plasticity

MSC.Fatigue QuickStart Guide

CHAPTER

14

Dynamic Fatigue

■ Introduction ■ Analysis Using Transient Results ■ Modal Superposition Method ■ Vibration Fatigue ■ Comparison Studies

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14.1

Introduction All fatigue is dynamically induced. That is, there must be some level of dynamic loading in order for fatigue damage to occur. It is probably a true statement to say that nothing in real life is actually static, or not moving at all. Even slight changes in temperature will cause stress fluctuations in an otherwise apparently static structure. Some dynamic loading is hardly detectable, changes very slowly, and is quite repeatable while other types are quite noticeable and very random in nature such as engine noise from an automobile. The pseudo-static approach for calculating a stress time response, where unit stresses are associated with load time histories, is valid if the frequency of the input loading is below the lowest natural frequency of the structure. However, for cases where the dynamic response of the structure comes into play, the usage of transient response or random response is appropriate to compute fatigue life.

Objective • • • •

Perform analysis using transient results Perform analysis using the modal superposition method Random Vibration Fatigue analysis Run comparative studies

Table 14-1 Necessary Files for Transient Results File P3_HOME/mscfatigue_files/examples/key_tran.op2 P3_HOME/mscfatigue_files/examples/key_stat.op2 P3_HOME/mscfatigue_files/examples/key_tran.asc P3_HOME/mscfatigue_files/examples/transient.fin P3_HOME/mscfatigue_files/examples/static.fin

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CHAPTER 14 Dynamic Fatigue

Table 14-2 Necessary Files for Vibration Analysis File P3_HOME/mscfatigue_files/examples/bs_fresp_v.op2 P3_HOME/mscfatigue_files/examples/bs_fresp_h.op2 P3_HOME/mscfatigue_files/examples/bs_fresp_t.op2 P3_HOME/mscfatigue_files/examples/7d_44-50.dac P3_HOME/mscfatigue_files/examples/8d_44-50.dac P3_HOME/mscfatigue_files/examples/9d_44-50.dac

Table 14-3 Additional Files Needed for Comparative Studies File P3_HOME/mscfatigue_files/examples/bs_modal.op2 P3_HOME/mscfatigue_files/examples/bs_static.op2 P3_HOME/mscfatigue_files/examples/bd_modal.op2 P3_HOME/mscfatigue_files/examples/bd_fresp_v.op2 P3_HOME/mscfatigue_files/examples/bd_fresp_h.op2 P3_HOME/mscfatigue_files/examples/bd_fresp_t.op2 P3_HOME/mscfatigue_files/examples/abarun.fil P3_HOME/mscfatigue_files/examples/bd_trans_v.op2 P3_HOME/mscfatigue_files/examples/bd_trans_h.op2 P3_HOME/mscfatigue_files/examples/bd_trans_t.op2 P3_HOME/mscfatigue_files/examples/bd_trans_vth.op2

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14.2

Analysis Using Transient Results Up to this point we have strictly used linear elastic FE results from static load cases where we have associated the time variation of the loading to externally defined time histories. This is the most common usage of MSC.Fatigue and perfectly valid for most components and structures which are fairly stiff in nature. Thus the name quasi-static. The assumption is made that dynamic effects are third or fourth order contributions to fatigue life and therefore ignored. There are times, however, where the dynamics of the structure can significantly effect the fatigue life of the product especially when the mass of the structure is large and the operating loads approach or even pass through the natural frequencies of the structure such as the dynamics of an entire vehicle body as shown by the bus to the right. P(t) In these cases it is generally better to use a dynamic FE analysis to capture all the important dynamic effects. All time variations of the loading are defined directly in the FE model and a direct or modal dynamic transient analysis is performed. There is no need for any externally defined and associated time histories as with the pseudo-static method. The drawback however, is that you cannot separate the loads. They must all be defined in the same FE analysis. Investigation of the influence each load may have on fatigue life requires a new FE analysis to be run each time.

To illustrate the use of transient results in MSC.Fatigue, follow this mini-exercise:

Transient Keyhole Job The geometry is the same keyhole model. Open a new database called keyhole and import the MSC.Nastran Output2 file call, key_tran.op2. In addition to this transient analysis, we are also going to compare the answers to an equivalent pseudo static analysis, so also read in the Output2 file, key_stat.op2. Remember to read the model and results for the first file and only the results for the second file in the order specified here. In this version of the keyhole model, the static load case results were determined using a 30 Newton loading at the same point of application as the original keyhole problem, the results from which, when scaled by the load time history should give roughly equivalent stress time histories for all nodes as does the modal transient analysis. This of course does not take into account any dynamic effects that the mass distribution may have on the dynamic behavior and resulting stress results, however with this simple model and a very evenly distributed mass, there should not be a large difference. Open the main MSC.Fatigue setup form and read in the saved job called transient using the file transient.fin. You will also need static.fin, so copy this file while you are at it. Systematically open the Solution Params..., the Material Info..., and the Loading Info... forms and follow the explanations of each to understand the setup. Note that we are running a Crack Initiation analysis.

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CHAPTER 14 Dynamic Fatigue

Solution Parameters Open the Solution Params... form. All the defaults have been selected for this analysis.

Material Information Open the Material Info... form. The material chosen is the MANTEN steel alloy with a Polished finish and the treatment set to No Treatment. The material already resides in the materials database.

Loading Information Open the Loading Info... form. This is where the analysis setup differs when using transient FE results. For the transient analysis the loading time history is defined by the FE analysis in the input deck and therefore it is not necessary to create externally. You will notice that the Loading Info... form appears quite a bit different than for a pseudo-static setup. The following observations are made: 1. The Results Type is set to Transient. This is the controlling setting for the appearance of this form. 2. Note that no access to the Time History Manager is available when set to Transient since this is unnecessary. 3. Results can be extracted from, this is the results from widget, three different sources: Database, MSC.Patran FEA, or External, with the Database being the most common source. Note:

External PATRAN Results files can be accessed in the same manner as for pseudostatic cases with multiple loading except the # symbol in the file name now refers to the time step number. They must start at 1 and exist up to the number of time steps indicated, e.g., filename1.node, filename2.nod, ...

4. A scale factor is allowed to uniformly scale the FE results for all time steps selected.

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5. For results extracted from the database, you must filter all the time steps you wish to include in the analysis using the Get/Filter Results... button. Only the time step you want must appear in the Result Time Steps listbox. The Number of Time Steps selected is indicated in the databox below the Scale Factor and is dimmed and unchangeable since the number of items in the listbox determines the number of time steps. For example if you open the Get/Filter Results... form and select LOAD_CASE.1 in the top list box, and set the Filter Method to Global Variable with the Variable set to Time and press Filter, you will get all time steps associated with this Result Case. You can individually remove time steps you do not want by selecting them in the lower listbox and pressing the Remove button. Press the Add button to place all selected time steps into the Result Time Steps listbox on the Loading Info... form. Press the Close button to close down the Select Result Cases form. 6. Finally you must select the stress or strain tensor (and layer if applicable) to use in the analysis. This is simply done by selecting one of the time steps in the Results Time Steps listbox. Then you select the stress or strain tensor in the adjacent listbox. There is no need to fill in any spreadsheet as with the pseudo-static method. To summarize the Loading Info... form for Database results:

• Supply a Scale Factor if desired • Get/Filter Results... to include only the time steps of interest • Select one of the time steps in the Results Time Steps listbox (all will be used in the analysis)

• Select a stress or strain tensor from the Select a Stress/Strain Tensor listbox • Select a Layer if necessary • Check that the No. of Time Steps agrees with your expectation Job Control Open the Job Control... form and submit the analysis. Monitor it if desired.

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Static Keyhole Job We also wish to run a fatigue analysis from pseudo-static results for comparison purposes. Now run the equivalent analysis using static FE analysis results. First a time history must be created by reading in an ASCII version of this time history that is equivalent to the force time history used in the transient analysis. Copy over the file called key_tran.asc. The following PTIME keystrokes will accomplish this for you: 1. Invoke PTIME either from the system prompt or from the Loading Info... form by pressing the Time History Manager button (Results Type = Static). 2. Use Add an entry | ASCII convert + load 3. Select the ASCII file key_tran.asc. 4. Accept this form by pressing OK. 5. Enter at least one descriptive title on the next form. Accept the defaults for the rest of the fields and press the OK button. 6. Plot the time history if you wish and exit when finished. Now read in the static fatigue setup from the static.fin file. View the setup if desired and then open the Job Control... form and submit the job.

Evaluate Results Finally read or list the results from the two jobs to see that they are approximately the same (2100-2600 repeats). There is a difference between the two which is due to the fact that the modal transient takes into account the dynamic effects of the mass distribution whereas the pseudo-static does not, as mentioned earlier. The differences would be even greater for models with more mass and for loading services that approached the natural frequencies of the structure.

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14.3

Modal Superposition Method This technique enables the dynamic response of the structure to be simulated without the disadvantage of storing the transient response for each node or element of the model. Transient FE analysis can be very time consuming and require large disk storage. The subsequent fatigue analysis must then access all this time dependent data for each time step and each location requested which can be time consuming. By using the modal participation factors (generalized coordinates) linearly combined with the stresses from each mode the method is exactly analogous to the quasi-static method in MSC.Fatigue, as shown in the table below, but requiring much less disk space, memory and computation time.

Static Superpositioning

Modal Superpositioning

Static Analysis

Transient Analysis

Stress Input

Stress for unit load case i

Stress for mode shape i

Loading

Loading function for channel i Modal Participation factors for Mode i An advantage of this method is that due to the similarity with the quasi-static method, this technique can be used in S-N, E-N, Spot weld, Seam Weld, Crack Growth and Multi-axial analysis. Here is an example of how to do modal superposition using MSC.Nastran. Analogous methods exist for other solver codes. Some familiarity with each solver code is required to extract the correct information. 1. Run MSC.Nastran modal analysis (SOL = 103) for your model and request stress to be written to the .op2 or .xdb file (STRESS = ALL). Use the EIGRL card to select frequency range of interest and / or number of modes. 2. Run MSC.Nastran modal transient (SOL = 112) with same EIGRL card. Define the time history loading in the MSC.Nastran deck using TABLED1 cards in the normal manner. Request the output to be SDISPLACEMENTS (PUNCH) = ALL. This gives the modal participation time histories for each mode of interest. If six (6) modes are solved for, this will give an ASCII punch file with six (6) time histories. Note:

In the latest version of NASTRAN steps 1 and 2 can be combined into a single run by specifying the first subcase in the SOL 112 run as the normal modes run.

3. Run MSC.Fatigue Pre&Post or MSC.Patran and import the .op2 file from the SOL 103 run (step 1) into the database. Fill the Solution Control and Materials forms as required.

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4. Open the Loading information form, enter the number of modes recovered in the number of static conditions data box, and turn the fill down option on. Click on the Load Case ID cell and the Get/Filter Results button to display the stress results from the SOL 103 run. Select the first mode and the associated stress tensor and click on Fill Cell. This should load up the cells in the first column of the spreadsheet with the modal stress IDs corresponding to the number of modes recovered. 5. The creation of the Time histories off the modal participation factors in the punch file is achieved by clicking on the Read Punch button. This button will be displayed on this form when the user clicks on the Time history cell. A local Time history database that contains the time histories from the participation factors in the punch file is created in the local run directory. The local database can now be used to load the cells in the second column by clicking on the first cell in the Time history column and selecting Fill Cell (MSC.Fatigue accesses the local database automatically). 6. Run fatigue analysis as usual.

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14.4

Vibration Fatigue The techniques described in this section deal with random vibration induced fatigue, which is calculated from random vibration and/or frequency response FE analysis results. A Power Spectral Density Function (PSDF or PSD) is the most common way of representing the loadings or responses in the frequency domain. The transformation between time domain, i.e., the time history of the loading, and the frequency domain, i.e., a PSD, should not trouble the reader. The PSD simply shows the frequency content of the time signal and is an alternative way of specifying the time signal. It is obtained by utilizing the Fast Fourier Transform (FFT). Figure 14-1 shows this equivalence for a typical structural response signal. Transforming from the frequency domain to the time domain is also a relatively easy task which can be done using the Inverse Fourier Transform (IFT). However, when transforming in this direction the random phase angles attributable to each frequency component (which have not been kept when converting to the frequency domain) have to be generated or re-generated. This can be done such that a statistically equivalent signal can be reproduced.

DISPLAY OF SIGNAL: SAESUS.DAC

DISPLAY OF SAESUS.PSD

25061 points. 400

2E6

9 pts/sec

Displayed:

from pt 1

Strain (uE)

Full file data:

Max = 345 at 2270 sec Min = -999

RMS Power (uE^2. Hz^-1)

25060 points.

at 0 sec

Mean = -206.6 S.D. = 134.6 RMS = 246.6

-1000 0

nCode nSoft

Time (sec)

2784

0 0

nCode nSoft

Frequency (Hz.)

1

Original Title : Strain

Figure 14-1 PSDs and the Transformation Between Time and Frequency Domains It is not the intention of this manual to teach the user all there is to know about random vibration. For those unfamiliar with random vibration techniques, refer to Vibration Fatigue Theory (p. 625) in the MSC.Fatigue User’s Guide.

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CHAPTER 14 Dynamic Fatigue

Definitions These are some of the terms you might come across when going through the Vibration Fatigue example:

• • • • • • • • • •

Power Spectral Density (PSD) Transfer function Irregularity factor Narrow band Wide band White noise Probability Density Function (PDF) Expected mean crossing Expected number of peaks Spectral movement

All of these terms are defined in Appendix A, Glossary Terms (p. 436).

Frequency Domain Life Estimation - General Procedure This section provides a brief summary of techniques for computing fatigue life, or damage, from a PSD of stress or strain. These fall into two broad categories: those that estimate fatigue life directly and those that compute rainflow cycle PDFs as an intermediate stage. All of the approaches have now been brought together in MSC.Fatigue.

General Fatigue Damage Equation The general Fatigue Damage equation for the Frequency Domain is shown below. P(S) is obtained with the appropriate vibration fatigue modeler instead of with rainflow cycle counting used in the time-based approach. Refer to the MSC.Fatigue User’s Guide. m E [ P ]T Fatigue Damage = ---------------- ⋅ ∫ S P ( S ) dS K

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Comparison of Time Domain with the Frequency Domain Model

To obtain a time history of stress or strain response, either a steady state or transient analysis would be required. For the random response history indicated this would obviously be a transient analysis. In the frequency domain a transfer function would first be computed for the structural model. This is completely independent of the input loading and is a fundamental characteristic of the system, or model. The PSD response caused by any PSD of input loading is then obtained by multiplying the transfer function by the input loading PSD. Further response PSDs caused by additional PSDs of input loading can then be calculated with a trivial amount of computing time. An essential requirement of a structural analysis in the frequency domain is that it results in a PSD which is equivalent to the time history obtained using the transient approach. The rest of the design process is then concerned with using the vibration fatigue tools to compute fatigue life directly from these PSDs of stress. These tools either estimate rainflow histograms (or PDFs), or fatigue life directly. These are shown schematically in the dashed box in the figure above under the heading fatigue modeler. This is intended to show that the time and frequency domain processes are actually very similar. The only differences being the structural analysis approach used (time or frequency domain) and the fact that a fatigue modeler is required to transform from a PSD of stress to the rainflow cycle histogram. In this context the vibration fatigue modeler can be envisaged as just another form of rainflow cycle counting.

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CHAPTER 14 Dynamic Fatigue

Vibration Model Setup A simple bracket, shown to the left, is subject to random vibration excitations defined by loading power spectral density (PSD) functions, which induce serious fatigue damage around the attachment location (the circular hole). The bracket is subject to three input loads, a vertical and horizontal force and a twisting moment, at the far end of the slot. The model is constrained around the circular hole. A random vibration analysis is performed by combining FE frequency response analysis results using three unit loads combined with the loading input PSDs. Fatigue damage is calculated due to each independently and all three simultaneously.

Open a new database and call it bracket.db. Press the Import toggle switch (Analysis in MSC.Patran) on the main form. When the form appears, set the Action to Access Results, the Object to Read Output2, and the Method to Both; then, press the Select Results File button and select the file bs_fresp_v.op2. Press the Apply button to read in the file. Now set the Method to Result Entities and select the file bs_fresp_h.op2. Press Apply. Repeat for the bs_fresp_t.op2 file.

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Vibration Fatigue Analysis Setup Now set the General Setup Parameters as follows: 1. Analysis: Vibration 2. Results Loc.: Node The fatigue lives will be determined at the nodes of the model as with any other fatigue analysis. 3. Nodal Ave.: Global Accept the default which means element contribution will be averaged as with any other fatigue analysis. 4. F.E. Results: Stress Vibration fatigue uses S-N curves which require stresses; you do not have a choice. 5. Res. Units: MPa Model dimensions are millimeters and forces are in Newtons, therefore stress units are MPa. 6. Jobname: bs_fresp_v 7. Title: Fatigue due to Vertical Force PSD

Solution Parameters Open the Solution Params... form. Set the parameters as follows: 1. Analysis Method: Dirlik The default is Dirlik which is the recommended method. If you select All, all the analysis methods mentioned in the theoretical background section will be used. 2. Mean Stress Correction: None This is set to None in order to compare to the pseudo-static analyses which were also set to None. The mean stress correction is based on the same principles as that done for pseudo-static S-N fatigue analysis. 3. Stress Combination: Max. Abs. Principal

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CHAPTER 14 Dynamic Fatigue

This is the default. In general this is the combination method that makes most sense. In actually, the ability to determine the principal stresses and their directions from the transfer function of stress components is a very unique feature of the vibration fatigue capability. Most FEA codes do not have this ability. 4. Certainty of Survival: 50% This parameter is identical to that used in regular time based S-N analysis using the scatter in the S-N data to adjust the life prediction based on a probability of survival. Press OK to proceed and close the form.

Material Information Open the Material Info... form. This form is used to assign material and other information to regions of the model. In fact it is identical to the time domain S-N material set up form which you should be familiar with from previous exercises. The Material Info... form and spreadsheet should then be filled in as follows: 1. Number of Materials: 1 2. Material: MANTEN 3. Finish: Polished 4. Treatment: No Treatment 5. Region: default_group Accept the defaults for anything else on the form and close the form by pressing the OK button when finished.

Loading Information Open the Loading Info... form. Before completing this form we need loading input PSDs. These PSDs will be created from the time signals we used in the pseudo-static runs. On the Loading Info... form press the PSD Manager button. This will spawn PTIME, the loading database manager.

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When PTIME appears, select Add an entry... | creaTe psd from time. This will spawn a utility module called MASD for creating auto spectral density functions. This module has multiple functions which are beyond the scope of this text. We wish only to create a Power Spectral Density function from a time series using MASD. A number of screens will be presented to you. Accept the defaults for all items except those indicated below: 1. Input Filename: 7d_44-50.dac Press the OK button to accept this file and continue filling out the screen. 2. Output Type: Power Spectral Density Press the OK button to proceed to the next screen. 3. FFT Buffer Size: 1024 : 0.9766 Hz width This setting determines the number of points to define the PSD over the full frequency range. The full frequency range is from zero to 500 Hz which will give 1.024 pts/Hz or 512 points. Press the OK button to proceed to the next screen. 4. Output Filename: 7d_44-50 A file called 7d_44-50.psd will be created. 5. Plot Output: Yes Press the OK button. The PSD will be created and a summary page will be shown. When this is closed the PSD will be plotted using the graphic module MQLD (quick look display). 6. View | Window X: Min=0, Max=50

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To get a better look at the PSD, select the View | Window X menu pick and set the minimum frequency to 0 and the maximum frequency to 50. Use File | Exit to quit. This will return you to PTIME. Note the frequency content of this signal tapers off and is almost zero by 50 Hz. 7. Description1: Vertical Load 8. Number of fatigue equivalent units: 1 9. Fatigue equivalent units: Repeats These last two inputs are ignored for a vibration fatigue analysis. But something must be supplied. All fatigue lives are reported back in seconds, hours or years. Repeat these steps for the other two time histories creating 8d_44-50.psd and 9d_4450.psd for the horizontal and twist loads respectively. Then quit from PTIME. The original signals and their corresponding PSDs are shown below. Hint:

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PSDs can be created in a number of ways. They can be created as shown here from existing time signals. They can also be imported as ASCII text files (Add an entry... | ASCII convert + load) or they can be created manually by supplying xy points (Add an entry... | x-Y psd entry).

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Once the PSDs are created you can proceed to fill out the appropriate information on the Loading Info... form: 1. Results Type: Transfer Function The choice here is either Transfer Function or Power Spectrum. We are using transfer functions from FE analysis. It is also possible to calculate response PSDs directly in the FE analysis. In that case we would specify Power Spectrum. This will be covered later. 2. Results Transformation: Transform to Basic This is the default setting. FE tensor results are transformed to the basic coordinate system to sum and average nodal contributions from adjacent elements. This must be done in a consistent coordinate frame. Unless you have a specific need we suggest you leave the default.

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3. Load Input: Single For this first example using the vertical loading PSD, we only have a single input. Multiple inputs will be covered later. 4. Frequency Resp: 6.(5-30)2.1-2Clicking on the cell just below this title will activate a number of widgets on the bottom of the screen. This is where the Transfer Function from the FE analysis is selected. This is a multi-step operation so continue reading. 5. Get/Filter Results... Open this form to select the Transfer Function of interest. You will see all Result Cases in the upper listbox. Select the BS_FRESP_V vertical Transfer Function Result Case. Press the Filter button. This will display all subcases (frequencies) associated with this Transfer Function in the lower listbox. If you press the Add button, the Result Case IDs will be transferred to the Loading Info... form listbox. This form is quite versatile. You can remove various frequencies if you wish. You can filter based on various criteria. You can do multiple selections and fill the Loading Info... form listbox with multiple transfer function results (which will be necessary for a multiple input load analysis). It is suggested that you play with this form a bit to understand its usage. Press the Close button when you have successfully filled the listbox on the Loading Info... form with BS_FRESP_V,6.(5:30)- representing all the frequencies in the Transfer Function Result Case. 6. Select a Results Load Case: BS_FRESP_V,6.(5:30)Back on the Loading Info... form select this Result Case that you just filled in using the Get/Filter Results... mechanism. Once this is selected you will see the tensor results associated with this transfer function in the adjacent listbox. 7. Select a Stress Tensor: 2.1-Stress Tensor, Select the only available tensor from this listbox. The layer information will update. 8. Select a layer: 2-At Z1, This is displayed by default. Accept the default which is the top layer of stress of the shell elements. Main Index

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9. Fill Cell Press the Fill Cell button. This will fill the Frequency Resp. cell with the appropriate IDs in the spreadsheet above. The Input PSD cell then becomes active. 10. Select a PSD File Name: 7D_44-50.PSD Select the PSD representing the vertical force which we created earlier. The Loading Info... form is now complete. Press the OK button to accept the form. Before going on, however, a word or two on loading input PSDs is appropriate. Vibration fatigue analysis makes certain assumptions of loading input. Those assumptions are that the signal is random, stationary and gaussian in nature. Random means that the signal contains no deterministically dominant event such as a spike occurring occasionally or a superimposed dominating sine wave. Truly random signals can only be characterized by their statistics such as root mean square (rms) and mean levels. Stationary means that those statistics are not changing significantly with time. Any section of the signal should show very close statistical agreement. Gaussian means that the peak and amplitude probability density function are gaussian in nature or follow a bell shaped curve as shown here. If you draw tram lines through a signal and count the number of times the signal passes through it and plot that as a density function it is gaussian if it follows a bell shape. An example of a nongaussian signal is a pure sine wave. However adding multiple sine wave together quickly becomes gaussian.

Gaussian

non-Gaussian

Hint:

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If you ever have the need to check the stationarity of a time signal, use the MSTATS utility module. MSTATS will give you running statistics of a signal and plot them for you. The increment of time history and overlaps can be specified. This is a very useful mechanism to determine stationarity.

CHAPTER 14 Dynamic Fatigue

Job Control Open the Job Control... form and set the Action to Full Analysis and press the Apply button to submit the job. Change the Action to Monitor Job and press the Apply button occasionally to monitor the job. This analysis will create the usual files: the job parameter file, bs_fresp_v.fin, the fatigue input file, bs_fresp_v.fes, and the fatigue results file, bs_fresp_v.fef. Also a message and status file are created (bs_fresp_v.msg, bs_fresp_v.sta). Unlike a standard time domain solution there is no intermediate rainflow count file, bs_fresp_v.fpp. When the job is complete open the Results... form and with the Action set to Read Results, press the Apply button. This will read the results into the database for later viewing.

Additional Job Setups - Multiple Load Inputs Now that you have seen how to set up the vertical load vibration analysis job you can repeat the setup procedures for the other two single input load (horizontal and twist loads). For the three single load input jobs you can follow the table below for Vibration analysis. Use default values if parameters are not specified.

Vertical Load

Horizontal Load

Twist Moment

General Setup Parameters: Jobname:

bs_fresp_v

bs_fresp_h

bs_fresp_t

Title:

Vertical Load

Horizontal Load

Twist Moment

Solution Params Form: Analysis Method: Dirlik Mean Stress Correction: None Stress Combination: Max. Abs. Principal Design Criterion: 50% Materials Info Form: Material: MANTEN Finish: Polished Treatment: No Treatment Region: default_group Loading Info Form: Result Type: Transfer Function Load Input: Single Frequency Resp: Input PSD:

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6.(5.30)-2.1-2-

7.(31-56)-2.1-2-

8.(57-82)-2.1-2-

(BS_FRESP_V)

(BS_FRESP_H)

(BS_FRESP_T)

7D_44-50.PSD

8D_44-50.PSD

9D_44-50.PSD

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Again, after each fatigue analysis is finished, read the results into the database under the Results form in the main MSC.Fatigue form with the Action set to Read Results.

Correlated and Uncorrelated Loading When each of these jobs is done we can now set up a multiple load input job with all three loads acting simultaneously. It is at this point however, that we have to decide whether the individual load inputs are correlated or uncorrelated. Simultaneously acting loads are said to be fully correlated if, in the time domain, the peaks and valleys from each signal occur simultaneously. This is normally the case for random load signals. Fully uncorrelated signals have the opposite true. Peaks and valleys do not occur at the same time and may cause a cancelling effect. Thus you would expect correlated loads to be more damaging than uncorrelated loads. Since we are dealing with correlated loads, we need some way of determining the cross-correlation PSDs that will relate one input load PSD to another. If you have the original time series, this can be done with a MSC.Fatigue module called MFRA (frequency response analysis). Start MFRA from a system prompt by typing mfra, or by selecting it from the Tools | Fatigue Utilities | Advanced Loading Utilities pulldown menu in Pre&Post or the Tools |MSC.Fatigue | Advanced Loading Utilities pulldown menu in MSC.Patran. To get cross PSDs from MFRA follow these instructions after selecting Transfer Function Analysis from the main menu (use the defaults if not specified): 1. Input File: 7d_44-50.dac Select the vertical load time history. 2. Response Filename: 8d_44-50.dac Select the horizontal load time history. Press the OK button to accept these two file names. 3. Output Type: Power Spectral Density Press the OK button to accept this screen and move to the next. 4. FFT Buffer Size: 1024 : 0.9766 Hz width Select this buffer size so that there are the same number of points in the resulting cross-PSDs as in the PSDs created thus far. Press the OK button to accept this screen and go to the next. 5. Generic Output Filename: 7-8d_44-50 Give this cross term the name 7-8d_44_50 to indicate that the vertical load (7d) had been correlated with the horizontal load (8d). 6. Zero/Zero in Gain File: Zero Press the OK button to proceed with the analysis.

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Repeat this process to correlate 7d_44-50.dac with 9d_44-50.dac and 8d_44-50.dac with 9d_44-50.dac and use the output file names of 7-9d_44-50 and 8-9d_44-50 for the two respectively. Exit from MFRA when you are finished or you may plot the results using Results Display on the main menu of MFRA. You will have in your directory three files called 7-8d_44-50.sxy, 7-9d_44-50.sxy and 8-9d_44-50.sxy. These are the cross PSD terms. Next, invoke PTIME and load these three new PSD files in using the Add an entry | Load file option so that they exist and are known in the PTIME database. Use a wild card to specify all three at the same time, i.e., *.sxy. Accept all defaults and press OK. Exit from PTIME when you are finished.

PSD Matrix File One last step must be performed before the Loading Info... form can be properly filled for a multiple input load job. We must create a matrix file that relates the cross PSD terms with the input load PSDs. There are two ways to do this. Since we want to look at the effect of both uncorrelated loads and correlated loads we will introduce you to both methods by creating two matrix files. Perhaps the easiest method is to manually create the file and then load it into PTIME. Create a file using any editor that looks like this in your working directory and call it cor789.pmx: 3 7d_44-50.psd 7-8d_44-50.sxy 7-9d_44-50.sxy 7-8d_44-50.sxy 8d_44-50.psd 8-9d_44-50.sxy 7-9d_44-50.sxy 8-9d_44-50.sxy 9d_44-50.psd

Note that the contents of this file are the names of the input load PSD files on the diagonal terms and the names of the cross PSD files on the off-diagonal terms. The first line indicates that there are three input loads and therefore the matrix is to be 3x3. This file can be loaded into PTIME by using the option Add an entry | ASCII convert + load. Once in this option do the following: 1. ASCII Filename: cor789.pmx 2. Data Type: psd Matrix 3. PSD Matrix: cor789 Press the OK button to accept this screen and move to the next. 4. Description 1: correlated loads Press the OK button to accept this screen and load the file.

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The second method is a direct method within PTIME. Use the option Add an entry | Psd matrix. Follow these instructions: 1. Filename: uncor789.pmx 2. Description 1: uncorrelated loads Press the OK button to accept this screen. 3. Enter Matrix Size: 3 Press the OK button to accept this screen. A spreadsheet will appear. In the diagonal cells of the spreadsheet type the names of the load input PSD files: 7d_44-50.psd, 8d_44-50.psd, and 9d_44-50.psd. Leave the other cells blank since this is meant to be uncorrelated. You must press the return or enter key for the file name to be accepted. Also the file must have been loaded into the PTIME database and physically exist. When you have filled out the spreadsheet select File | OK. This will load the new matrix file uncor789.pmx. If you look at the contents of the second file it should look like this: 3 7d_44-50.psd NONE NONE NONE 8d_44-50.psd NONE NONE NONE 9d_44-50.psd

Loading Information - Multiple Load Inputs Finally you can return to the Loading Info... form and set the job up for a multiple input load analysis. 1. Results Type: Transfer Function 2. Results Transformation: Transform to Basic 3. Load Input: Multiple This will cause a listbox to appear with the PSD matrix files listed. 4. Select a PSD file: COR789.PMX When you select the matrix file it will automatically update the spreadsheet on the form to indicate the number of input loads (number of rows). You must then supply a transfer function for each input load.

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5. Frequency Resp: 6.(5-30)-2.1-2Clicking on the first cell just below this title will activate a number of widgets on the bottom of the screen. This is where the Transfer Functions from the FE analysis are selected. This is a multi-step operation. 6. Get/Filter Results... Open this form to select the Transfer Functions of interest. This operation is identical to what you did for a single load input except this time you need to fill the listbox with all three Transfer Functions. Select the BS_FRESP_V vertical Transfer Function Result Case. Press the Filter button. This will display all subcases (frequencies) associated with this transfer function in the lower listbox. Press the Add button, the Result Case IDs will be transferred to the Loading Info... form listbox. Do the same for BS_FRESP_H, and BS_FRESP_T Transfer Function Result Cases pressing the Add button to add them to the listbox. Press the Close button when you have successfully filled the listbox on the Loading Info... form with BS_FRESP_V,6.(5:30)-, BS_FRESP_H,7.(31:56)-, and BS_FRESP_T,8.(57:82)-. 7. Select a Results Load Cases: BS_FRESP_V,6.(5:30)Back on the Loading Info... form select this Result Case that you just filled in using Get/Filter Results... mechanism. Once this is selected you will see the tensor results associated with this transfer function in the adjacent listbox. 8. Select a Stress Tensor: 2.1-Stress Tensor, Select the only available tensor from this listbox. The layer information will update. 9. Select a layer: 2-At Z1, This is displayed by default. Accept the default which is top layer of stress of the shell elements.

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10. Fill Cell Press the Fill Cell button. This will fill the Frequency Resp. cell with the appropriate IDs in the spreadsheet above. The next cell then becomes active so you can associate the next transfer function to its corresponding input load PSD. Repeat steps 7 through 10 for the horizontal and twist loads selecting the appropriate Transfer Function respectively. Close down the Loading Info... form. Go to the General Setup Parameters and give the job the name bs_fresp_vth_c and the title correlated loads. Submit the job as you did for the single load input jobs and when the job is complete read the result into the database as done before. Run one last job before investigating the results. Go into the Loading Info... form and change the matrix PSD file to UNCOR789.PMX. Give it the jobname bs_fresp_vth_u with the title uncorrelated loads and submit the job and read the result when finished. Do not forget to read the results in from these two jobs.

Results Perhaps the most obvious thing to do first is to make contour plots of fatigue life from the various jobs run so far. All of the fatigue analyses should have been run and the results imported into the database, so open the Results application switch on the main menu bar (remember not to confuse this with the Results... button on the main MSC.Fatigue form). When the Results application appears, make sure the Object is set to Quick Plot. You will see many Result Cases in the top listbox. Scroll all the way down to the bottom and select the Result Case that corresponds to the vibration fatigue analysis called Vibration Analysis, bs_fresp_vfef. Select Log of Life (Seconds) and press the Apply button to produce a plot. The plot is shown below. vibration

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Design Optimization Let us now investigate the design optimization (sensitivity) capabilities of the vibration fatigue analysis. This is analogous to those capabilities in the S-N and strain-life analyzer FEFAT. The design optimization feature can be invoked directly from the Results... form with the Action set to Optimize or from FEVIB started at the system prompt by typing fevib and then entering the Design optimization menu pick. Do this with any of the vibration fatigue jobs completed thus far. After specifying a jobname, if necessary, and selecting a node of interest and supplying a design life, the program will proceed to a summary report screen reporting the same life as the global analysis. When the summary report is closed you are placed in the main menu of the design optimization mode. The operation is identical to that of FEFAT’s design optimization mode and is therefore left to you to investigate its many options. The only unique option to FEVIB’s design optimization mode is its ability to calculate life due to all the analysis methods (Dirlik, Narrow Band, etc.). This is done under Sensitivity analysis | Analysis methods (all). To see the statistical nature of the vibration analysis, you may want to plot the rainflow cycle count histogram, which is really a probability density function of rainflow ranges. In order to plot the histogram you will need to do the following from the Design Optimization main menu: 1. Select Original Parameters: This will reset everything to the original parameters in case you have changed anything while investigating this tool. 2. Select Change Parameters: Enter the Change Parameters screen and change the next two items below. 3. Mean Stress Correction: Goodman 4. Global Offset Stress: 0 Keep this set to zero. We must run a Dirlik plus mean stress correction in order to obtain a histogram plot. Press the OK button to return to the main menu. 5. Select Recalculate 6. Select results Display |plot Cycles histogram

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This will plot the histogram. Change the view to 2D viewed from the left so you can see the stress ranges. This is done by selecting Plot-type | View Left.

Stationarity Checks As a last exercise before we go on to the second model of the bracket, let us look at another feature of the vibration fatigue analysis module FEVIB. Again invoke FEVIB from the system prompt and this time select the Output power spectrum option. Or on the Results... form set the Action to Extract PSD and press Apply. Supply a jobname if necessary. The job we want to extract a PSD from is the multi-input correlated load case, bs_fresp_vth_c at Node 72. Do the following after supplying the proper jobname: 1. Generic Output Filename: You can accept the default here. However, be aware that all file names created from this option will have the node number appended to the output filename. 2. Nodes/Elements to Select: 72 3. Combination Method: Abs Max principal 4. Interpolation Method: Linear

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5. Stationarity Check Output: Yes Be sure to turn this on. Press the OK button to continue. A result summary screen will be presented. Press the End button to continue. 6. plot Power v. Frequency At this point you are presented with three options for displaying different types of plots. Plot each one of them separately. The first is the stress response PSD as calculated by FEVIB at Node 72. This is, of course, calculated by multiplying the input PSD by the Transfer Function. For multiple inputs this becomes a matrix operation. The second plot (angular deviation v. Load case) shows the total angular spread of the principal stress axes for each load case. The solid red line is plotted through the median value at each load case. The yellow error bars indicates the total deviation for each load case; the first load case being the first yellow error bar, the second, the second load case, and the third, the third load case, from left to right.

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The third plot (angular deviation v Load Frequency) shows how the principal stress axes change with respect to frequency for each load case - actually three different plots.

vertical

horizontal

twist

The angle vs. frequency plot for the vertical load case shows a total angular deviation of the principal stress axes of only about two degrees according to the y-axis labels. The horizontal load case is very small, almost zero, and the twist shows a total of about 20 degrees. These corresponds to the single yellow error bars on the angle vs. load case plot for each load case. The yellow error bars on the angle vs. frequency plots indicate how the stress axes change due to differential damping at each frequency. In other words, it represents how the principal stress axes change subject to a sine wave load input at that frequency. All angle spreads reported on these plots are relative to an arbitrarily selected angle. The angle vs. load case plot shows the angles relative to each other. You can see that there is about a 45 degree difference between the vertical load case and the horizontal and twist load cases, these two being very similar. This is as expected also in that the horizontal and twist loads are inducing a shear

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state at Node 72 whereas the vertical load case is not. This is confirmed by plotting the principal stress at Node 72 using the Results application. Hint:

To make these plots in Pre&Post or MSC.Patran using the Results application, set the Object to Marker, the Method to Tensor, and select the appropriate Result Case. Show the tensor as 2D Principal and turn off the Min principal. It also may be desirable to only show the arrows and not the tensor box (under Display Attributes). The principals can also be animated to see the change in angle over frequency when all frequencies have been selected from a particular Transfer Function. The above plots were made from the static load cases.

These angular spread plots are characteristics of the model. To see whether or not there may be a problem with stationarity of the principal stress axes, you must look at the regions of interest on the response PSD (between zero and 25 Hz) and the corresponding frequency locations on the stationarity plots. For all three load cases, there is little movement of the stress axes in this region.

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14.5

Comparison Studies Pseudo-Static vs. PSD Approach Now that a general background to the frequency domain approach has been given, we can begin to illustrate the concepts introduced with our bracket model. Copy the following files over to a clean working directory from the examples directory of your installation and then start MSC.Fatigue Pre&Post or MSC.Patran: bs_modal.op2, bs_static.op2, bs_fresp_v.op2, bs_fresp_h.op2, bs_fresp_t.op2, 7d_44-50.dac, 8d_44-50.dac, 9d_44-50.dac

Open a new database and call it bracket_s.db.

FE Model and Analysis In this exercise we will be investigating two versions of the same model. The first model (called bs_*) has had the mass density modified such that no modes under 50 Hz are present. This has been done so that virtually no dynamic effects will influence the fatigue life because the loading input does contain frequency content below 50 Hz. In this manner we can directly compare a pseudo-static, time domain fatigue analysis approach with the frequency domain technique. To see this, input the modal analysis of the bracket model: Press the Import toggle switch (Analysis in MSC.Patran) on the main form. When the form appears, set the Action to Access Results, the Object to Read Output2, and the Method to Both (model and results); then, press the Select Results File button and select the file bs_modal.op2. Press the Apply button to read in the file. While you have this form open, read in the results from the other result files: bs_static.op2, bs_fresp_v.op2, bs_fresp_h.op2 and bs_fresp_t.op2. Set the Method to Result Entities and then select each of these files one by one and press the Apply button each time. Read the files in the order listed here. Now you can view the results. Press the Results toggle on the main form. Note: The first result case shown is a single mode with frequency greater than 50 Hz, which confirms that the bracket model has no modes under 50 Hz. Why this is important will become clear momentarily. If you plot the displacement vector of this mode, you will see that it represents the first bending mode of the bracket.

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Set the Object to Fringe. Besides the single mode shape, you will notice that there are three static result cases and three kinds of frequency response result cases. The three static results were obtained by applying a vertical unit force (_v), a horizontal unit force (_h) and a unit twisting moment (_t) at the end of the slot. The three frequency response results were obtained with the same unit forces and moment but applied across the frequency range of zero to 50 Hz. These analyses were done in MSC.Nastran. The frequency response analyses used a damping ratio of 5% of critical. This is inconsequential however, since no dynamic modes will be excited. The frequency response results are the transfer functions for the three load cases. In order to obtain transfer functions from MSC.Nastran, the load magnitudes must be unity in the analysis. Because no modes exist in the frequency range of interest (0-50 Hz), the stress results from the frequency response analyses should be very close to those of the static analyses for the lower frequencies. This is easily confirmed by plotting the stresses from these. For example do the following: 1. Select Result Case: BS_STATIC_V, Static Subcase Select this result case. Press this icon to view all subcases from every Result Case. 2. Select Fringe Result: Stress Tensor 3. Quantity: Max Principal 4. Target Entities: Change the mode to select target entities. 5. Target Entity: Elements Change the target entity to Elements. Place the cursor in the databox called Select Elements and click the mouse to gain focus in the databox. Then go to the graphics screen and box select all the elements below the slot. See the pictures below. 6. Press Apply 7. Select Result Case: BS_FRESP_V, Freq.=0. Go back to the Select Results mode of the form and select the first subcase of the vertical frequency response analysis. And press Apply again.

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You should see almost identical plots. The reason for only plotting the area around the hole is for better comparison purposes due to spurious results around the loading area. Repeat this for the horizontal and the twist load cases if you wish.

Vertical Load Static Analysis

Frequency Response

If you plot higher frequencies you will begin to see a small divergence from the static cases. This is due to the dynamic influences of the first mode shape. In fact if you make an XY plot of the transfer function at the high stress area of interest (Node 72) you can see this divergence.

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Horizontal Load Static Analysis

Frequency Response

Twist Load Static Analysis

Frequency Response

To make the XY plot of the transfer function at the high stress area of interest, set the Action to Create and the Object to Graph. Select all the Result Cases for one of the frequency responses (e.g., BS_FRESP_V, Freq=*) and make sure the Y axis is set to Result, Quantity is set to Max Principal, the X axis is set to Global Variable, and set Variable to Frequency.

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Two more steps are necessary. Under Target Entities, the Target Entity must be set to

Vertical Load Plotted as a Function of Frequency for Node 72

Path with Node 72 specified and under Plot Options, you must change the Complex No. as: optionmenu to Magnitude. Then you can press the Apply button. Note: The transfer function contains frequencies from zero to 50 Hz by increments of two, or in other words, 26 evenly spaced frequencies. The frequency resolution of the transfer function is very important in order to obtain accurate fatigue results. This will be illustrated later in this exercise.

Pseudo-static Fatigue Analysis Setup Before proceeding on to the vibration fatigue analyses, we wish to run the equivalent pseudo-static fatigue analyses for comparison purposes later on. Four pseudo-static jobs need to be run, one for each of the load cases and a fourth with all three loads applied simultaneously. Before doing this however, you will need to run PTIME and load the three loading time histories, 7d_44-50.dac, 8d_44-50.dac and 9d_44-50.dac. In PTIME use the Load files option for each file. Note: If you have been running through this document sequentially, then you will need to first select Add an entry... and then you can select the Load files option. These files represent a six second slice (44 sec. to 50 sec.) of very large measured random input loadings. A six second slice was removed out of convenience for making the jobs manageable in a tutorial guide. In order to compare against the vibration fatigue results, give each time history set the Fatigue equivalent units to Seconds, set the Number of fatigue equivalent units to 6, enter a description, set the Load type to Force, and the Units to Newtons. Press OK. You need to do this for each of the three files listed above. Now we can set up the Pseudo-static jobs. The job set up is briefly described here in order for you to recreate the results. No details are given since pseudo-static analysis has been thoroughly covered in previous chapters. Open the main MSC.Fatigue form

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from the Analysis switch in Pre&Post or select MSC.Fatigue from the Tools pulldown menu in MSC.Patran and fill out the form according to the table below for the four jobs of interest. Accept all defaults if not otherwise specified. The Analysis type should be set to S-N.

Horizontal Load

Vertical Load

Twist Moment

Combined Run

General Setup Parameters: Analysis = S-N Jobname:

bs_static_v

bs_static_h

bs_static_t

bs_static_vth

Title:

Vertical Load

Horizontal Load

Twist Moment

Combined Run

Solution Parameters Form: Mean Stress Correction: None Materials Info Form:Material: MANTEN Finish: Polished Treatment: No Treatment Region: default_group Loading Info Form: Number of Static Load Cases: Load Case ID:

Time History:

1

1

1

3

5.4-2.1-23.2-2.1-24.3-2.1-2-

5.4-2.1-2-

3.2-2.1-2-

4.3-2.1-2-

(BS_STATIC_V) (Stress Tensor, At Z1)

(BS_STATIC_H) (Stress Tensor, At Z1)

(BS_STATIC_T) (Stress Tensor, At Z1)

7D_44-50

8D_44-50

9D_44-50

7D_44-50 8D_44-50 9D_44-50

Load Magnitude: 1.0 Note: The Load Case IDs correspond to the various load cases (vertical, horizontal, and twist). The actual Load Case IDs are dependent on the order in which they were read into the database. If you read them in the order in which they have been listed in this exercise then they should be as indicated. In any case you must select the indicated result for the proper IDs to be selected regardless of what is listed in the above table.

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Once a job is set up, go to the Job Control... form and do a Full Analysis. After the analysis is completed, go to the Results... form and set the Action to Read Results. Repeat these steps for each job. The results of these analyses will be investigated later. For now go on to set up the vibration fatigue runs.

Results Open the Results application switch on the main menu bar. When the application appears, set the Object to Quick Plot. Scroll down to the bottom and select the Result Case corresponding to the first pseudo-static job we ran called Total Life, bs_static_vfef for the vertical load case. Select Log of Life (Seconds) and press the Apply button to produce the contours. Make a note of this plot. Now compare this plot with the one we looked at in the previous section. Select the Result Case Vibration Analysis, bs_fresp_vfef. Next select Log of Life (Seconds) and press Apply to produce the plot. The two plots are shown below. Notice the disparaging difference between them. .

Fatigue Life due to Vertical Load pseudostatic

vibration

This difference is due to the endurance limit imposed on the material MANTEN that we used in the analysis. With an S-N analysis, any locations with stress ranges below this endurance limit will be reported as infinite life. The vibration analysis, because of its statistical nature, has many more stress range bins, tending to spread the life contours out and appear not to be as sensitive to the endurance limit. The plots are, in

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actuality, very similar. This can be seen by removing the endurance limit for MANTEN and rerunning the pseudo-static analyses. If you feel so inclined you may do this. The corresponding plots then look much more similar as shown below.

Fatigue Life due to Vertical Load Endurance Limit Removed pseudostatic

Hint:

vibration

To remove the endurance limit run PFMAT and Load the material MANTEN into data set 1. Turn off the Material checking under Preferences and then Edit data set 1 which contains MANTEN. Do not supply a password to modify the central database. Simply press the return key and a local copy of the database will be created. Proceed to the screen with E-N Data and change the Cut-off to 2E12. This will remove any fatigue limit from the S-N curve. (Do not be confused that strain-life data is being used here. Only the S-N (elastic) portion of the strain-life curve is used.)

The rest of the plots are shown below comparing horizontal, twist and combined loading pseudo-static versus vibration fatigue analyses. These plots are left for you to create at your leisure. Note that all the pseudo-static plots were created after removing

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the endurance limit. Also note that the area at the end of the slot contains spurious results due to the singularities caused by the loading and should be ignored. We are really only interested in the critical location around the circular hole (Node 72). Fatigue Life due to Horizontal Load pseudostatic

vibration

Fatigue Life due to Twist Load pseudostatic

vibration

Fatigue Life due to Combined Loads pseudostatic

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PFPOST Listings The table below shows the results from all the jobs run thus far (calculated using the smallest bin size of 32) at Node 72. You can view a listing of damage and fatigue life by running PFPOST. You can either invoke this utility from the system prompt by typing pfpost or set the Action to List Results in the Results... form on the main MSC.Fatigue setup form. Once PFPOST has appeared do the following: 1. Jobname: bs_static_v Start with the vertical pseudo-static analysis. Press the OK button twice to proceed until the form appears as to the right. 2. User specified nodes: 72 Select the option User Specified nodes. Enter 72 as the node of interest to view and press OK. Press OK to close the table after you have viewed the results. Hint:

To view the results at Node 72 for the other analyses press the Cancel button when you return to the form at the right. This will allow you to enter another jobname and repeat the steps above.

Pseudo-Static

Vibration

Factor

Vertical Load

6.5E5 Seconds

2.1E6 Seconds

3.2

Horizontal Load

9.8E8 Seconds

3.2E9 Seconds

3.3

Twist Load

9.1E8 Seconds

9.4E7 Seconds

9.6

Combined Load

3.7E4 Seconds

4100 Seconds

9.0

Uncorrelated

N/A

1.2E5 Seconds

The results are with the endurance limit removed as explained earlier. As you can see the results are fairly good with a couple of the cases being out by a factor of ten or so on life. The shorter the life, the more discrepancy there can be because of sensitivity due to the logarithmic nature of the problem. Small differences in stress can mean large differences in life. Note also that the uncorrelated run is much less damaging as we expected.

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There are many factors that can influence this accuracy some of which may be: 1. The coarseness of the FE model and accuracy of the stresses. 2. Some possible cancelling effects due to the combined loading which would be evident in the pseudo-static case because of only partial correlation of the input loads. 3. Shortness of the signal; perhaps not long enough to fully characterize as fully stationary and gaussian. 4. Influence of the first mode shape as evident in the earlier plot of the Transfer Function at Node 72. 5. Frequency resolution of the Transfer Function to fully capture the influence of the input PSD.

Frequency Resolution To illustrate the importance of the last point above, do the following: 1. Run PTIME again and convert the original time signal 7d_44_50.dac to a PSD again. Select Add an entry... | creaTe psd from time option. Give the new output the name test.psd. This time however, use an FFT Buffer Size of 2048 : 0.4883 Hz width. This will essentially create twice as many points in the resulting PSD. 2. Plot the PSD to see that it looks much more jagged than when the buffer size was set to 1024. Zoom in from zero to 25 Hz for a good view. 3. Give it a description, set the Number of fatigue equivalent units to 1, set the Fatigue equivalent units to Repeats, and press the OK button. 4. Re-run the vibration fatigue analysis (bs_fresp_v) of a single load input using the new PSD. 5. List the results at Node 72. Note how different they are than the original analysis as reported in the table above (1.8E6 vs. 2.1E6 Seconds). Even though the total areas under each input PSD curve is identical between the two (the difference being that one has twice as many points), the underlying dominant factor is the frequency resolution of the Transfer Function in the important areas of the input PSD. Because our Transfer Function has evenly incremented frequency steps of

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two Hz, we may skip over certain peaks or valleys in the input PSD. Interpolation only occurs within the input PSD at frequency points found in the Transfer Function and not those found in the input PSD. This is illustrated below for two different cases:

Input PSD

Transfer Function

Response PSD Potentially Less Damaging

More Damaging

So for our case, with more and more points defining the input PSD and no greater resolution of the Transfer Function, the damage calculated becomes greater because the analysis is calculating more area under the curve than there really is. The opposite could also be true if a large spike occurred between two frequencies in the Transfer Function, and was missed entirely. Also do the same thing in FEFAT with the time domain solution for the same load case and node location (although you will not need to change the mean stress correction). Below are histogram plots (viewed in 2D mode) from the pseudo-static and vibration fatigue analyses for the vertical load case. Note how the time domain solution has discrete finite sequence whereas the frequency domain solution has values in all bins based on the probability of cycles occurring at that stress range. It is a statistical representation of an infinitely long sequence.

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Dynamic Transient vs. PSD Approach Now we will investigate the bracket model where the density has been adjusted to lower the natural frequencies. It is assumed that you now have a good handle on using all aspects of MSC.Fatigue. No new functionality will be introduced in this section, so step by step instructions will be minimal. The loading information for this model is identical, however we can no longer use the pseudo-static method as any means of comparison because we have introduced dynamic effects. The only means of comparison must be done against an actual time domain, transient FE analysis. First a modal analysis was performed to ensure that the modal frequencies fell within the input loading PSD range. For each of the three load cases, an FE transient analysis was performed with 5% critical damping. A fourth transient analysis was done for the combined loading where all three act simultaneously on the structure. The time variation for these analyses was taken directly from the input loading (7d_44-50.dac, 8d_44-50.dac, and 9d_44-50.dac). Hint:

The load step information for the transient FE analyses was captured, for practical purposes of this exercise, from the XY contents of these time history files which were dumped to ASCII files using the MSC.Fatigue utility module MCOE (channel editor) and then converted into TABLED1 cards for MSC.Nastran. Before doing this however, the signals were filter to remove any frequency content above 50 Hz (using MBFL) and then decimated to reduce the number of points in the signal from 6001 to 601 (using PTIME, Sample Rate Adjust).

Frequency response analyses were also performed for the three load cases and the fully correlated combined loading case. If you wish to reproduce the results of this exercise you will need the following results files (bd_ = bracket dynamic model): bd_modal.op2, bd_fresp_v.op2, bd_fresp_h.op2, bd_fresp_t.op2, bd_trans_v.op2, bd_trans_h.op2, bd_trans_t.op2, bd_trans_vth.op2

Close the old database and open a new database and call it bracket_d.db. Read in the Output2 files in the order listed above. Make sure that you read Both model data and results from the first file and Result Entities only from the rest. It is suggested that you do the following:

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Plot Mode Shapes Plot the mode shapes of the bracket. Note that the lowest mode is around 6 Hz and that there are six modes between zero and 50 Hz consisting of first bending, first twist, first lateral, and three second order modes. All major modes will be excited by the input PSD. Plotting modes is done from the Results application with the Object set to either Quick Plot or Deformation. Mode 1, 6.06 Hz

Mode 2, 12.68 Hz

Mode 4, 22.89 Hz

Mode 5, 27.24 Hz

Mode 6, 46.45 Hz

Mode 3, 20.32 Hz

Run Vibration Analyses Run the vertical load case vibration fatigue analysis using the new Transfer Function. The easiest way to do this is to read in the old job (bs_fresp_v) and then change the jobname and other appropriate parameters (Job Control...| Read Saved Job). After reading in the old job called bs_fresp_v, change only the following: 1. Jobname: bd_fresp_v 2. Frequency Resp: 3.(11-51)-2.1-2-(vertical load) Here you are assigning the proper Transfer Function for this new model, BD_FRESP_V, corresponding to the vertical load on the Loading Info... form. 3. Input PSD: 7D_44-50.PSD When you re-run the job you will see that the predicted life at Node 72 is very small (2 seconds). This shows you that the dynamic effects are quite significant. Because of this, change the material from MANTEN to the higher strength steel, RQC100 and modify it to also have no endurance limit (Cut-off =2E12) as you did with MANTEN.

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Note: In order to be able to compare these results with the Transient Analysis, you must create a group called hole with only the elements (one layer deep) around the hole AND their nodes (Node 65:72 129:135 202:214 221 222 237 263 and Element 29:34 108:117). Then you must select this group as the Region of interest on the Material Info... form. Rerun all four vibration jobs with new job names (bd_fresp_v, bd_fresp_h, bd_fresp_t, and bd_fresp_vth_c) and the appropriate Transfer Function Result Case IDs. Make sure you change the material to RQC100 from MANTEN. Note: The frequency resolution of the Transfer Functions is much higher (26 frequencies vs. 41 frequencies) to better capture the dynamic effects around each natural frequency.

Run Transient Analyses Now run the transient analysis jobs. The best way to do this is to read in the corresponding pseudo-static job, change the Jobname, set the material to RQC100, set the Region to the group you created earlier (hole), and then change the Result Type from Static to Transient on the Loading Info... form, selecting all the time steps for the corresponding load case of course. The four Jobnames should be bd_trans_v, bd_trans_h, bd_trans_t, and bd_trans_vth for the vertical, horizontal, twist and combined loading cases respectively. For the combined loading, turn on Biaxiality Analysis for later comparisons to the angle spread of the principal stress axes from the vibration analysis (on the Solution Params... form).

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The table below shows the results from all the jobs run from this second model (calculated using the smallest bin size of 32). Use PFPOST to list results at Node 72.

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Vertical Load Horizontal Load Twist Load

Shown here are the fringe plots of the log of fatigue life comparing the transient (Total Life, bd_trans*) and the vibration fatigue analyses as was done for the pseudo-static (Vibration Analysis, bd_freq*) comparisons earlier. Only the region around the hole is shown since only fatigue life was calculated in the this region due to the enormity of the transient analysis FE results.

Vibration Analysis

Transient Analysis

Combined Loading

View and Compare Results

339

340

Transient* / rms

Vibration / rms

Factor

Vertical Load

131 Seconds / 235

40 Seconds / 288

~2.2

Horizontal Load

1.5E9 Seconds / 58

1E9 Seconds / 75

~1.5

Twist Load

3.4E7 Seconds / 113

3.9E5 Seconds / 142

~88

Combined Load

0.45 Seconds / 328

0.39 Seconds / 406

~1.15

* Transient analysis results from MSC.Fatigue are initially reported in Repeats of the Analysis or time history. The numbers reported here for the transient analysis have been multiplied by 6 to reflect the fact that the time history is 6 seconds of data. The following notes are made: 1. Shown in the table above are also the rms values of the stress response time histories or PSDs. They are included in the table to illustrate that you can gauge the relative magnitude of damage that a load input may cause in relation to another by the rms value of the signals. The rms values of a time signal are reported in the stress response plot when you Output time histories using the FEFAT module. When there is a nonzero mean of a signal look at the standard deviation instead of the rms which is done in this case. The rms value of a response PSD is reported in the tabular listing using PFPOST. You can also estimate the relative magnitude of damage due to the combined loads by taking the root mean square of the rms values for the individual load cases ([2352+582+1132]1/2)=267 and ([2882+752+1422]1/2)=339. Although not 100% accurate, if the rms for one signal is less compared to another, the damage will also be less. 2. The twist load which appears to be out by a larger than acceptable factor is a special case which needs to be illustrated. When performing a transient dynamic analysis, or a pseudo-static analysis, the principal stresses are computed at every time step increment. At each increment the time history contains, in general, more than one frequency component. However, only one principal direction can exist at each time step. In contrast, for a vibration fatigue run there can, in general, be a different principal stress direction for each frequency and each load application point. The Transfer Functions in MSC.Fatigue compute principal stress PSDs where each component is a principal. The vibration fatigue approach, therefore, has the potential to give an upper bound on fatigue damage for FEA models where there is a large variation in principal stress Main Index

CHAPTER 14 Dynamic Fatigue

direction. As an analogy to this, consider the outcome from tossing dice. Make set 1 the mean value of the three dice and set 2 the highest value from the three dice. Set 2 will consistently give higher values than set 1. This will only be significant where there is a large variation in principal stress direction. This is probably the case for this model and the twist loading as the results appear to indicate. In all cases where there appears to be a large amount of mobility, the vibration fatigue calculations are conservative. 3. Care must be taken when comparing transient analysis fatigue results to PSD fatigue analysis results when the loading is correlated. The time domain analysis could impose cancelling effects if the sign of the loads are not applied correctly, which would cause the comparison to be unfavorable. 4. The lives are very low for the vertical and combined runs which means the region of the S-N curve that is being used is not really valid. Random vibration fatigue using the S-N method is valid only for high cycle fatigue problems (>1e4 cycles). However, since the curve is linear, it is still valid for comparison purposes. 5. As a continuation of the previous comment, the time domain transient analysis does not report back the exact life if less than one repeat of the signal. If you run the combined transient case, it will report 1 repeat or 6 seconds as the life. But in actuality it lasted less than one repeat. A fatigue life can be determined from a single shot analysis by placing a scale factor of 0.8 on the loading. This gives a life of around 55 seconds. In order to estimate the fatigue life equivalent to the actual stress level, we worked out the slope of the S-N curve at around 1e0 cycles to be: -b = 1 / 21.5. We know that the number of cycles to failure N, is N = S-b Therefore the fatigue life goes down by: (0.8)21.5 = 0.0083 55 x 0.0083 = 0.45 Seconds

Recreate the Transfer Function For a single load case you should be able to recreate the Transfer Function from the response PSD and the input PSD by dividing one by the other (for a multiple input analysis, this becomes a matrix operation). Use the vertical load case analysis, bd_fresp_v, to illustrate this. This is a multi-step operation and uses a number of MSC.Fatigue utilities that are explained in more details in the next chapter.

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1. In Pre&Post or MSC.Patran, use the Results application to make the plot shown here. This is the Transfer Function of Max. Principal stress for Node 72 and is created by setting the Object to Graph, selecting all the frequencies associated with the BD_FRESP_V Result Case for Node 72 (Target Entities) and plotting the Magnitude of the complex number (Plot Options). 2. Run FEVIB and use the option Output power spectrum to create the response PSD at Node 72. Give it the output name: resp_psd.psd. The final name will be resp_psd.psd72, as it appends the node number. 3. Create an ASCII dump file of the response PSD (resp_psd.psd72) using the utility module MDTA (Convert Binary .dac to ASCII). It can be invoked from the Tools + | File Conversion Utilities pulldown in Pre&Post or from the system prompt by typing mdta. This module is straight forward to use. Do not write any header information or any multi-channels. A file called resp_psd72.asc will be created containing the xy data points. 4. Read the ASCII file into PTIME using Add an entry | ASCII convert + load. Set Data y type to Power spectrum; set the Frequency Rate (sample rate) to 1.024; select X-y pairs as the Equally Spaced Data and set Take All Numbers to Yes. Call the new Power Spectrum, resp_psd. Enter a description when asked. Do not worry about the Load Type or Units. They will be wrong. Just remember that the units are MPa2/Hz. This will create the response PSD with the exact same sample rate as the input PSD.

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5. Use another utility to cut down the size of the input PSD to the same number of points as the response PSD. Use the program MLEN (File Cut and Paste) which can be accessed from the Tools | Fatigue Utilities | Advanced Loading Utilities pulldown in Pre&Post or from the Tools | MSC.Fatigue | Advanced Loading Utilities pulldown in MSC.Patran. It can also be accessed from the system prompt by typing mlen. Use the Extract Section - Single File option and select the input PSD, 7d_4450.psd. Give the output file name of input_psd.psd. Change the End Time to 50, indicating 50 Hz. The start time should be either START or 0. Now both the input PSD (input_psd.psd) and the response PSD (resp_psd.psd) are identical in length with the same sample rate. They are plotted here using MMFD. 6. Now divide the response PSD by the input PSD to recreate the Transfer Function using another utility module called MMFM (Multi-File Manipulation). The module is also invoked in the usual way. Select the Division option. Select the resp_psd.psd and input_psd.psd as the two input files. Note: Files must be selected exactly in this order. Select them both from the file browser at the same time by clicking on input_psd.psd and then holding down the Control key and selecting resp_psd.psd. The Output filename should be something like trans_func.frf. The Divide by Zero Value should be zero (0) and the Special case of 0/0 should be Zero. 7. The last thing that must be done is to take the square root to convert to stresses. Use the utility module called MART (Arithmetic Manipulation) which has an option to Raise to a power. The power to raise the entire function to should be 0.5 (square root). Change the YLabel + Units to Max. Principal and MPa. Overwrite the existing file.

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8. Use MQLD (Quick Look Display) to plot the Transfer Function which is shown here. It is almost identical to that shown in Pre&Post or MSC.Patran as we would expect.

Plot the Stationarity The stationarity plots are shown below for Node 72. Compared to the previous model, the dynamic effects of this model are much more

Transient Analysis Principal vs. Angle

vertical

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horizontal

twist

CHAPTER 14 Dynamic Fatigue

apparent. Note that the plot of Maximum Abs. Principal vs. Angle from the transient analysis shows about an 85 degree spread and the vibration analysis shows around 108 degrees total, which compares favorably. Hint:

The biaxiality plot from the time domain analysis was produced in FEFAT under the Assess multiaxiality option.

As further explanation, the error bars are showing the degree of angular spread due to class II principal stress variation (due to differential damping at each frequency) while the solid red line is showing the maximum angular variation due to class III stress variation (how the stress tensor changes with frequency). It is quite possible that the vector having the maximum departure from an arbitrary base vector (class III) is different from the one exhibiting maximum spread through class II. The plots are essentially saying then, that load case one and three, in particular, are giving us a class II and class III non-stationarity. Compare this plot with load case 2, here we see a stationary tensor. For a more in-depth discussion on multiaxiality and biaxial indicators, see the MSC.Fatigue User’s Guide on Vibration Fatigue analysis.

Random Vibration FE Results As a final exercise in this chapter, set up a vibration fatigue analysis where the response PSD has already been calculated and supplied by the FE solver. The same model was run through ABAQUS using the vertical load input PSD. The results can be found in the file abarun.fil. Either open a new database and import Both the model and results with the Analysis Preference set to ABAQUS, or read the Result Entities only into the existing database you have been using with the second dynamic model after changing the Analysis Preference to ABAQUS. The drawback to this method is that MSC.Fatigue cannot resolve the tensor to obtain principal stresses or directions. Only the real stress tensor of response PSD components is supplied from the FE analysis. Only when Transfer Functions are supplied in the form of complex stress tensors of components is this possible. Because of this, one of the components must be selected for the analysis. To set up a vibration fatigue analysis using FE response PSDs is straight forward and almost identical to that of a Transfer Function analysis. 1. Jobname: abapsdrun_y Give it a new jobname and a title if desired. 2. Solution Param... form: Use Dirlik and no mean stress correction (None). Select the Y Normal component of stress. 3. Material Info... form:

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The set up on this form is identical. Use RQC100 with a polished finish and no treatment. However, use the previous group, hole, as the region since response PSD results exist in the database for only the elements and nodes around the hole. 4. Loading Info... form: Change the Results Type to Power Spectrum. Select all the frequencies from the new Result Case, RandomResponse. Select the Stresses, Components at SECTION_POINT_1. Note that you do not have to associate and input loading PSD to your Result Case as with the Transfer Function approach. This is analogous to the pseudo-static versus transient approaches where the transient does not need any external load variations defined because the transient analysis already defines them. Thus, similarly we are using a response PSD directly from the analysis code, ABAQUS, in this case. 5. Job Control: Full Analysis Results are shown below (with the transient analysis) for the Y-component direction at Node 72.

Transient Approach Y-component Vertical Load

~300 Seconds

PSD Approach ~2150 Seconds

The FE response PSD from the Results application for the Y-component is shown next to that extracted from FEVIBs Output a power spectrum option at Node 72.

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CHAPTER

15

Temperature Corrected Fatigue Analysis

■ Temperature Corrected MSC.Fatigue Analysis

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15.1

Temperature Corrected MSC.Fatigue Analysis This section describes the temperature corrected fatigue analysis MSC.Fatigue. A simple example is used to highlight the features. Temperature corrected fatigue analysis allows the calculation of uniaxial E-N or S-N fatigue through utilization of temperature corrected materials data at non-ambient temperatures. Temperatures may be assigned to the analysis group globally, by group (region) or extracted from a Thermal case in the Patran database The temperatures are steady state (i.e. constant with time) and do not include time varying or creep effects but may vary across the FE model. Note: Temperature corrected fatigue analysis may only be performed with S-N and E-N analysis only. Optimization and Fast Analysis options are not available.

Objective • To introduce temperature corrected fatigue analysis. Table 15-1 Chapter 15 Necessary Files File P3_HOME/mscfatigue_files/examples/plate_thermal.op2 P3_HOME/mscfatigue_files/examples/plate_structural.op2

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Import the Model In a clean working directory, create a new database called Thermal. In the first exercise, we will assign the temperatures to the analysis group from an MSC.Nastran Thermal case. Set the Analysis Code to MSC.Nastran and the Analysis Type to Thermal. Click OK.

Press the Import toggle switch in Pre&Post (Analysis in MSC.Patran) on the main form. When the form appears, set the Action to Access Results, the Object to Read Output 2, and the Method to Both (model and results); then press the Select Results File button, select the file plate_thermal.op2, and press the Apply button. View the temperature results from the results menu. A fringe plot of the temperature profile is shown below:

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Click on Preferences and set the Analysis Code to MSC.Nastran and the Analysis Type to Structural. Import the results for the mechanical case by setting the Action to Access Results, the Object to Read Output2, and the Method to Results Entities. Select the plate_structural.op2 file and press the Apply button. View the results from the results menu. A fringe plot of the Von Mises stress is shown below. Note: The stress units are in Pascals since the model dimensions are in meters and the applied force is in Newtons

Set Up the Fatigue Analysis Temperature corrected Fatigue is available for S-N or E-N analysis only. Set the General Setup form as follows: 1. Analysis: S-N 2. Results Loc: Nodal 3. Nodal Ave: Global 4. F.E. Results: Stress 5. Res Units: Pascals 6. Jobname: Thermal_test 7. Title: Temperature test case

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Solution Parameters Accept all the defaults on this form.

Material Information Temperature assignments to the analysis entities are made on this form. There are 4 methods, Global, Region, Temp. Case, or ASCII Temp to assign temperatures to the analysis group(s) from the Temp Type pull down menu. Global assigns the same temperature to all the analysis groups, Region allows the user to specify temperatures on the analysis groups on the material form (an extra cell has been added to the material form to specify temperatures), Temp. Case extracts the temperatures from a Thermal analysis case from the Patran database, and ASCII Temp requests that the user assign a default global temperature and then specify the name of the TDS file to use in overwriting the temperature for certain nodes. For Global, Region, Temp. Case, or ASCII Temp, an ASCII materials database is used (nmats.htd) that is copied form the installation directory to the users run directory to facilitate editing and addition of material data. Click on the Materials Database Manger button to view the .htd file with a text editor. Just like other analysis types, the list of available materials will be displayed in the list box on the material form. The regular Material database is displayed via PFMAT if the default Temp. Type of None is selected.

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In this exercise, we will set the Temp. Type to Temp. Case. As suggested at the end of this Chapter, users may want to exercise this form by using the Global, Region, and ASCII Temp definitions.

From the Temp. Type optionmenu, select Temp. Case. A button called “Get Temperature Case” appears on the Materials Information form. Click on the Get Temperature Case button and select the temperature case as shown below:

Click on the Fill Databox button and press the OK button to accept the temperature case. Temperatures from this case will be assigned to the analysis group selected on the material form.

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Set the remaining widget values as follows: 1. Temp. Units: Celsius 2. Number of Materials: 1 3. Material: AL_PISTON 4. Finish: No Finish 5. Treatment: No Treatment 6. Region: default_group Your form should look like the one below. Ignore any warning messages that come up and press OK to accept the inputs.

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Loading Information There is no change to the loading form. Set the Load Case ID to the only available value. Set the Time History to SINE01. Accept all other default values and click OK to accept the inputs. The completed from is shown with all the inputs.

Note: If SINE01 does not exist in your list of available Time Histories, then use the Time History Manager button to “Copy from Centeral” the needed DAC file.

Run the Fatigue Analysis Open the Job Control form. Set the Action to Full Analysis and press the Apply button. On job completion, open the Results form and set the Action to Read Results and press the Apply button. This will read the results into Patran.

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The log of damage result is shown below. As expected, the highest damage is at the fixed end where the maximum stresses and temperatures occur.

Verification: Run the same example with a global temperature of 20°C. Damage at the same location should be approximately three times lower.

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CHAPTER

16

Aerospace Spectrum File Support

■ Aerospace Spectrum File Support ■ File Definitions ■ Example Problem ■ Conclusion

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16.1

Aerospace Spectrum File Support This section describes the aerospace spectrum file support in MSC.Fatigue. It extends the previously cumbersome methods for generating stress spectra for the aerospace sector by making use of a spectrum file consisting of load events that reference static stress cases in a load control file. Load time histories are not required as stress spectra are generated entirely from the information in ASCII editable Spectrum (extension .spe) and Load Control files (extension .lcs). In essence, the stress spectrum at each analysis location (nodes or elements) is generated by stepping through a sequence of static load cases that are defined in the Load Control file.

Objective • To illustrate the concept of generating a stress spectrum using the Spectrum and Load Control files to create a fully reversed stress cycle.

• To show the results obtained here are identical to those obtained in the first exercise in Chapter 2 of the Quick Start Guide. Aerospace spectra are not supported for the following analysis types:

• • • • • • •

Strain data in FES file Seam weld Spot weld Vibration fatigue Multi-analysis (duty cycle analysis) Fast Analysis Critical Plane & Multiaxial Analysis

Table 16-1 Chapter 16 Necessary Files File P3_HOME/mscfatigue_files/examples/simpleSN.op2 P3_HOME/mscfatigue_files/examples/example.spe P3_HOME/mscfatigue_files/examples/example.lcs

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16.2

File Definitions Spectrum file Spectrum files are ASCII and must have a .spe extension. Format, explanation of each input, and a typical file are shown below.

Flight, , , , , ,..., , , ,..., . .

The header "Spectrum File V2" must be the first line in the .spe file. Scatter factor accounts for scatter in materials and loads data. The calculated life will be divided by this factor. The Number of Cycles and Stress Factor parameters have not been implemented. Therefore, these values should be set to 1. The "#" character is used to denote comments. All text appearing after the "#" are ignored.

Example Spectrum File Spectrum file V2 Example 1 1 2 1 Flight, 1, 3, 15 # 'Flight', Flight Number, No. of repeats Push Back, 7, 1, 2, 0, 3, 1, 2, 1 # Sequence name, repeats, cases,... Engine Run-up, 9, 0, 5, 0, 1, 5, 1 Taxi, 20, 0, 2, 0, 1, 4 Flight, 2, 2, 10 Take-off, 1, 10, 11, 0, 10, 50 Gear retract, 5, 0, 25, 0

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Load Control file Load Control files are ASCII and must have a .lcs extension. Format, explanation of each input, and a typical file are shown below.

,..., , , < scale 1>,..., ... , , ,...,

The number of FE cases has to be greater than or equal to 2. Blank lines after the 5 header lines are ignored, as is anything after the # character. The ID must be a positive integer. The divisor values are the loads applied to the model for each of the FE cases and must not be zero.

Example Load Control File Title Units 10 # Number of load cases 5 # Number of FE cases 1.0,1.0,1.0,1.0,1.0 # Divisor Values - magnitude of the unit loads for each FE Case 0,Load 0,0,0,0,0,0 1,Load 1,-1.000,.5125, 0.2,0.4,-1.8 2,Load 2,.55,-.76, 0.3,0,1.1 3,Load 3,0.5,-0.4,1.8,3.3,1.0 4,Load 4,-2.000,.5, 0.6,0.7,1.9 5,Load 5,.65,-.77,3.4,9.1,2.5 10,Load 10,1.1,2.5,6.8,9.0,-2.0 11,Load 11, 4.1,1.4,-4.3,-2.4,-1.0 25,Load 12, 4.2,2.4,-3.4,-2.4,-1.0 50,Load 50, 4.4,-0.4,-6.5,-2.4,-1.0

In the example files above, the spectrum file is made up of 2 flights that references one or more of the 10 load cases. Each load case is comprised of 5 static FE Cases that are factored to compute the stress for the given load case.

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CHAPTER 16 Aerospace Spectrum File Support

16.3

Example Problem In a clean working directory, start Pre&Post or MSC.Patran and import the model and results using the MSC.Nastran results file simpleSN.op2 into a new database called load_spec. Open the Main MSC.Fatigue form and set Analysis to S-N, Results Loc. to Node, Node Ave. to Global, F.E. Results to Stress, Res. Units to MPa, Jobname to load_spec, and Title to Load Spectrum Analysis.

Solution Parameters Open the Solution Params form and verify that the Mean Stress Correction is set to None and that the Stress Combination is set to Max. Abs. Principal. Press OK to accept the inputs.

Material Information Open the Material Info form and set Material to MANTEN_MSN, set Finish to No Finish, set Treatment to No Treatment, and set Region to default_group. Press OK to accept the inputs.

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Loading Information The Aerospace Loading capability is invoked by setting the Job Setup for widget to Load Spectrum. Note the absence of the reference to the load time history database as this is not required for handling aerospace spectrum files. Instead there are two navigation bars pointing to the location of the Spectrum and Load Case files. See the form below:

Users can browse to the directories containing these files and edit the files to create their own Spectrum and Load Control files. The example Load Control and Spectrum files are shown below. Stresses for the 2 FE- Cases are read from the Patran database.

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CHAPTER 16 Aerospace Spectrum File Support

Note that in the given example, the same FE case is selected twice. This is because at least two FE cases must be used for the Aerospace Loading capability. Since the stress due to a fully reversed load is to be modeled, the FE case is scaled first by +1.0, and then by -1.0. QSG Sample Load File N 3 2 1.0,1.0 0,Load 0, 0.0,0.0 1,load +1, 1.0, 0.0 2,load -1, 0.0, -1.0 SPECTRUM FILE V2 QSG Sample Spectrum File 1 1 1 1 Flight, 1,1,1 Block1,1, 0,1,2,0

Run Fatigue Analysis Open the Job Control form. Set the Action to Full Analysis and press Apply. When the job is completed open the Results form on the main MSC.Fatigue setup form and set the Action to Read Results. Press Apply. This will read the results into the database. The Log of damage, together with the stress time history at node 1 is identical to the example in Chapter 2 of the Quick Start Guide.

Exercise: Change the number of repeats for the flight in the spectrum file to 2 and compare the results with the above. Damage at Node 1 should double. The individual event (Block 1) may also be repeated to obtain the same result.

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16.4

Conclusion Spectrum and Load Control files may be constructed to produce realistic loading spectra for aerospace applications. Although the example used for demonstration purposes is simple, the versatility of the tool is evident as complex spectra can be constructed by utilizing the two input files and the FE Cases.

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CHAPTER

17

Multiple Fatigue Analysis (Duty Cycle Analyzer)

■ Introduction ■ Example Problem ■ Conclusion

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17.1

Introduction This is a powerful feature that allows the analyst to access damage from multiple sequences and events, through an intuitive interface that allows the analyst to set up realistic usage sequences. A typical usage sequence may be a car running at a certain gross weight and subjected to various events such as turning, braking, and traveling over potholes. It is very useful for the analyst to identify which events contribute significant damage in a usage sequence or which usage sequence (e.g., different weight configurations) cause significant damage. In either case, the Duty Cycle capability in MSC.Fatigue allows the user to simulate usage profiles (sequences) consisting of multiple events, as illustrated in the following example.

Table 17-1 Definitions Term

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Description

Sequence

One or more events that make up a given usage scenario; e.g., the events driving over a rough surface and then a smooth surface at a particular weight configuration may be one sequence.

Event

An event consists of loading conditions used to define the event. It may take any number of conditions to completely define an event. For example, a taxi event for an aircraft may require 1 loading condition at the nose gear and 2 load conditions at the main landing gears. In this particular case, the event will be defined with 3 loading conditions.

Condition

A particular combination of a unit FEM results case and an associated time history file; this may also be referred to as a loading condition.

Channel

Point on the structure where a load or acceleration data is measured; a channel is normally associated with only one coordinate direction. This is also the location where FEM loads are applied for an FE analysis.

CFG

A configuration file for specifying which conditions and events are to be used in constructing the Usage profile.

CHAPTER 17 Multiple Fatigue Analysis (Duty Cycle Analyzer)

17.2

Example Problem The multiple analysis capability is available for S-N, E-N, Spot Weld, Seam Weld, Vibration, Multiaxial, and Crack Growth analysis modules. We will demonstrate the Multiple Fatigue Analysis tool with a simple problem using our keyhole model from Chapter 2. Open a new database and call it duty_cycle.db. Now import the MSC.Nastran results file simpleSN.op2 into this database. Open the Main MSC.Fatigue form and set Analysis to S-N, Results Loc. to Node, Node Ave. to Global, F.E. Results to Stress, Res. Units to MPa, Jobname to dc_test, and Title to Duty Cycle Analysis.

Solution Parameters Open the Solution Params... form. Nothing needs to be changed here. Simply press OK to accept all the defaults.

Material Information Open the Material Info… form and set Material to MANTEN_MSN, set Finish to No Finish, set Treatment to No Treatment, and set Region to default_group. Press OK to accept the inputs.

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Loading Information Open the Loading Info… form and set Job Setup to Duty Cycle. This will cause the Loading Information form to be updated. So that it looks like the form shown below:

Now press the Get Duty Cycle Information button to open the Duty Cycle Setup form. Data for the Duty Cycle form may be filled out sequentially (i.e. complete definition of a sequence followed by the next sequence) or added randomly. However, in the latter case the user has to exercise caution to ensure that the data being entered is for a particular sequence and event; e.g. if the user has 3 sequences, and the user wishes to enter data for Sequence 3, one of the cells in the Sequence Information spreadsheet

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CHAPTER 17 Multiple Fatigue Analysis (Duty Cycle Analyzer)

for Sequence 3 must be active. The same logic applies to entering Time History data for a particular event. In the example below, we shall use the random method and point out the focus requirement as necessary.

This cell is only visible for the Static Duty Cycle case.

Import Duty Cycle Setup Data: Imports a previously saved Duty Cycle Setup file named .cfg. The first sequence and event information from this file is loaded to the form.

Sequence Information Number of Sequences: This is either entered or displayed from the imported setup file. This number also sets the number of rows to display in the spreadsheet for entering the sequence information. The maximum number of sequences allowed is 10. For this exercise, we will have 2 sequences. Enter 2 in the number of sequences and enter the sequence definitions as defined below. Sequence Fill Down OFF: Selecting this toggle will set Sequence Fill Down to ON. The values that are entered for each cell in the Sequence spreadsheet is repeated for all rows. In the case of the Sequence Name, an underscore followed by an incremented number will be appended to the name the user entered (i.e., test becomes test_1, test_2, etc.). Sequence Name Cell: Enter or display the names of the sequence in the imported file. Enter sequence names wt1 and wt2 by clicking in the respective sequence name cells

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Number of Repeats Cell: Enter or display the number of repeats of the current sequence in the imported file (Default =1). Enter 1 for sequence wt1 and 2 for sequence wt2. Number of Events Cell: Enter or display the number of events for the sequence in the imported file. This number also sets the number of rows to display in the Event Information spreadsheet. The maximum number of events per sequence is 100 and the default is 1. Enter 3 for sequence wt1 and 4 for sequence wt2. This signifies that the first sequence wt1 shall have 3 events and the second have 4 events. Delete Sequence: Deletes the sequence and associated information (event, time histories, etc.) associated with the sequence. The delete function allows single or multiple deletes. Sequences may be added by incrementing the number of remaining sequences. We shall proceed to defining the events specified for each sequence.

Event Information The event information for each sequence is added by clicking on the sequence name cell or anywhere in the row for the sequence and defining the event information for the selected sequence in the event information spreadsheet. Since we defined 3 events for the first sequence, 3 rows will be displayed for sequence wt1 and 4 for the second sequence. Event Fill Down OFF: Selecting this toggle will set Event Fill Down to ON. The values that are entered for each cell in the Event spreadsheet is repeated for all rows. In the case of the Event Name, an underscore followed by an incremented number will be appended to the name the user entered. Event Name Cell: Enter or display the names of the event in the imported file for a selected sequence. With the focus on the first sequence, enter Push_back for event 1, Taxi for event 2 and flight for event 3. For the second sequence, re-establish the focus and click on the Sequence Name wt2 and add the same information as for sequence wt1. However, since we defined an extra event for wt2, enter Land as the name for the fourth event. Number of Repeats Cell: Enter or display the number of repeats of the current event in the imported file (Default = 1). We shall assign 1 repeat for every event except the flight event in both sequences, where we will define 2 repeats. With the focus set appropriately for each sequence, enter 2 for the number of repeats for Sequence wt1, event flight and Sequence wt2, event flight. Number of Time Histories Cell: Enter or display the number of Time Histories or other load types from the imported file. (Note: This cell is only available for the Static Duty Cycle case.) This number also sets the number of rows in the load association spreadsheet. In this exercise, we will use a single time history for each event (see table 17-2 and 17-3 below). Delete Event: Deletes a selected event and associated information with the event. The delete function allows single or multiple deletes.

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CHAPTER 17 Multiple Fatigue Analysis (Duty Cycle Analyzer)

Add Event: Clears the widgets and allows information to be added for a new event after the selected event. If no selection is made, the event is added at the top.

Entering Load Data The load data needed and the look of the load data spreadsheet will change based on what kind of Duty Cycle analysis will be run. For the Static case, multiple rows are possible and we will need Time History data. For the Transient case, we only have one row and we will need Results data. For the Vibration case, we only have one row and we will need Frequency data and maybe even PSD data. This section of the documentation will discuss all three types but we will be using the Time History data for our example case. 1. Time History Data: This version of the load data section of the Duty Cycle form will be displayed when the Result Type is set to Static for most analysis types.

Fill Down OFF: Checking this will set Fill Down to ON, where a user can take advantage of filling the spreadsheet if time histories and load cases have been named in a logical fashion. This function will enable users to retain some of the functionality of the previous Multiple Analysis Tool.

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Time History Cell: Selecting this cell will display a listbox containing the list of DAC files found in the current directory. The user can change the current directory by pushing the browse button. This will cause a browser dialog box to be displayed. Pick the DAC files from any directory. Selecting a DAC file from the directory will load up the first cell and a list box shall display all the DAC files in that directory for subsequent picking (see image below).

We shall use the time histories by browsing to the central location (p3_home/mscfatigue_files/ptime) and picking up Sine01.dac for Sequence wt1, Event push_back. The list box should show all time histories in this directory from which the following shall be used for both sequences as defined below:

Table 17-2 Time History Data Sequence

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Event

Time History

Wt1

Taxi

Saetrn.dac

Wt1

Flight

Saebrkt.dac

Wt2

Push_back

Sine01.dac

Wt2

Taxi

Saetrn.dac

Wt2

Flight

Saebrkt.dac

Wt2

Land

Saetrn.dac

CHAPTER 17 Multiple Fatigue Analysis (Duty Cycle Analyzer)

Results Case Cell: Selecting this cell will display a list box and the load and results types in the Patran database. Selecting the load and results type will load the root results case identifier (i.e. the load case and results type) into the cell. Notice the absence of a layer selection – this has been automated and the correct layer information will be extracted based on the results type. Normalizing Load, Scale Factor and Offset Cells: The default values are 1.0, 1.0 and 0.0 respectively. The max/min value in the entered time history will be displayed to allow the user to normalize the time history. For this example, the following normalizing values shall be applied to every occurrence of the time histories in the table above.

Table 17-3 Normalizing Load Data Saetrn.dac

999.

Saebrkt.dac

738.

2. Frequency Response Data: This version of the load data section of the Duty Cycle form will be displayed when the Analysis is set to Vibration and the Result Type is set to Transfer Function.

This version is displayed when the Result Type is set to Power Spectrum instead.

Frequency Response Cell: Selecting this cell will display a Result Load Case listbox and a Stress Tensor listbox, just like it does for the non-duty cycle case. The only difference is that there is no layer information associated with the cell. This has been automated and the current layer information is written to the CFG file.

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Input PSD Cell: Selecting this cell displays a listbox containing a list of all the PSD files in the current directory. The user can use the Browse button to go and select a PSD from a different directory. A Create PSD from DAC button has been put on the form to allow users to create a new PSD file from an existing DAC file. The resulting file will be placed in the current working directory unless otherwise specified. 3. Result Case Data: This version of the load data section of the Duty Cycle form will be displayed when the Result Type is set to Transient for most of the analysis types.

Result Case Cell: Selecting this cell will display a Result Time Steps listbox and a Stress/Strain listbox just like it does for the non-duty cycle case. The only difference is that there is no layer information associated and the correct layer information is written to the CFG file. Scale Factor Cell: The default value for this cell is 1.0.

Managing the Duty Cycle Form Save File: Saves the current information that has been either loaded from an existing load setup file (existing_jobname*.cfg) as existing_jobname*.cfg. A warning message will be issued advising if a .cfg file exists in the directory to prevent accidental overwriting of a previous setup. If a user does not wish to overwrite an existing .cfg file, the Job name may be changed on the Main form to force a save to the changed Job name. Save your setup file and cancel out of the Duty Cycle setup form. Note: Messages will be echoed warning the user if problems are encountered in generating the .cfg file. The user should pay attention to these messages and correct the problems by making a note of the messages and ensuring that all cells in the row identified are reloaded and saved. Save As: Saves the current information that has been either loaded from an existing load setup file or that has been entered to the directory and filename choosen by the user. This gives the user the capability to store the file anywhere and not just in the current working directory under the defined jobname. Defaults: Restores the form to the default state. Cancel: Discard all inputs and exit out of the form.

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CHAPTER 17 Multiple Fatigue Analysis (Duty Cycle Analyzer)

Job Control From the Job Control form submit a Full analysis (notice the absence of the method button in the image below) and a Duty Cycle analysis will be submitted automatically.

Review Results Monitor the progress of the job and at completion, read the results from the analysis using Read results from the Main form. Notice again the absence of the Job Type button as in the previous versions of MSC.Fatigue. A Duty Cycle job is automatically detected in the run directory and results are read accordingly. With the Setup described above, the results form should display the results for the 2 sequences, wt1 and wt2 as shown below:

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The result shown above is the Duty Cycle result at the bottom surface for sequence wt1. In the example used, results for the top and bottom surface are identical and for illustration and comparison purposes, only the results for the bottom surface are shown below.

Sequence

Event

Damage (Node 1)

Wt1

Push_back

2.34e6

Wt1

Taxi

1.18e-5

Wt1

Flight

2.7e-4

Duty Cycle

2.84e-4 Wt2

Push_back

2.34e-6

Wt2

Taxi

1.18e-5

Wt2

Flight

2.7e-4

Wt2

Land

1.18e-5 5.92e-4

Your results, depending on the platform you are running on, should produce approximately the same results as above. If there are gross differences , please compare your and files with duty_cycle* files in p3_home/mscfatigue_files/examples folder or import them to setup your job. Note: The damage results for event Flight in both sequences take into account the number of repeats (2) applied to this event. The Duty Cycle results are, as expected, the sum of the damage in each sequence – note that the Duty cycle result for sequence wt2,takes into account the repeat factor specified for this sequence.

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CHAPTER 17 Multiple Fatigue Analysis (Duty Cycle Analyzer)

17.3

Conclusion The implementation of Duty Cycle analysis provides a powerful and flexible tool for setting up and analyzing complex sequences or conditions (as they were referred to in the prior development). The user has complete flexibility in setting up sequences, events consisting of uneven time histories in each event and the ability to navigate to the source of time history data.

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MSC.Fatigue QuickStart Guide

CHAPTER

18

Fatigue Utilities

■ Problem Description ■ Fatigue Preprocessing ■ Material Management ■ Advanced Loading Utilities ■ Advanced Fatigue Utilities ■ Graphical Display Utilities ■ File Conversion Utilities ■ Other Utilities

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18.1

Problem Description A number of utility modules exist in MSC.Fatigue to help in the proper set up and interpretation of fatigue analysis. These utilities are a subset of the test-based fatigue software, nSoft, (created by nCode International, MSC’s fatigue technology partner) packaged for the FE-fatigue analyst. They are broken into six main categories:

• • • • • •

Fatigue Preprocessing Material Management Advanced Loading Utilities Advanced Fatigue Utilities Graphical Display Utilities File Conversion Utilities

A brief description is given of each of these categories in this chapter with examples where appropriate. For full details of a module’s operation, see the MSC.Fatigue User’s Guide. Most of the modules described here can be accessed by typing their program name at the system prompt. For example, to invoke MASD, type masd. They can also be accessed from Pre&Post under one of the five pulldown menus found in the Tools |Fatigue Utilities pulldown menu. Or from MSC.Patran, under one of the five pulldown menus found in the Tools | MSC.Fatigue pulldown menu.

Table 18-1 Chapter 17 Necessary Files File P3_HOME/mscfatigue_files/examples/1pk.asc P3_HOME/mscfatigue_files/examples/2pk.asc P3_HOME/mscfatigue_files/examples/3pk.asc

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CHAPTER 18 Fatigue Utilities

18.2

Fatigue Preprocessing This is the first of the planned preprocessing utilities for MSC.Fatigue. Other utilities will be added under the Preprocessing option.

Low Damage Removal The Low Damage Removal utility is designed to extract areas of the model that are likely to sustain high damage. These areas are then grouped based on User selection of existing User created groups or automatically extracted material groups resident in the Patran database. In the example shown on the right the User may select all or pick one or more of the automatically extracted material groups. The Von mises stress or strain results from all selected loadcases are scanned and for each material group, a group is created that contains the top User selectable percentage of loaded entities in that group. The solution sequences that utilize stress or strain results can benefit from this extraction. The only exception to this is Seam weld and although this utility can be used to identify the critical areas, Users will have to exercise caution in using the extracted groups as the Seam weld module performs an extraction of the seam off the weld group and it is possible that the weld may not be a part of the extracted group. For the force based Spotweld solution sequence critical Spot weld elements are extracted based on the magnitudes of the translational and rotational components. Either the material group associated with the Spot weld elements or a group containing only the Spot weld elements (recommended) is used for the extraction. The groups extracted by this utility may then be used in a subsequent fatigue analysis for a quick assessment of the high damage areas of the model. Note:

This extraction performed by this Utility does not take into account the phasing from Superpositioning of the results for multiple channel loading. FASTAN should be used in this case as proper account of the phasing is taken into account to extract high damage areas.

This Low Damage Removal utility can be accessed by selecting the option from the Fatigue Preprocessing pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post).

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From the low damage removal form: 1. Select the Type (either Group or Material). 2. Select either the material names or group names that are of interest. 3. Select the loadcases. 4. Select the percentage of loaded entities desired. The default value is 10 but you can select a value anywhere from 1 to 20. (i.e., If you select 15 then the top 15 percent of damaged entities will be added to the new group) 5. Hit Apply. The results data for each selected material name or group name is processed and the top requested percent of elements with the worst damage are put into a group using the following naming convention: HD_material name or HD_group name. (ie. If you selected mat1.1 and mat1.2 then two new groups would be created. One called HD_mat1.1 and the other called HD_mat1.2) 6. These new groups can then be used in the Materials Information form to quickly identify the “Fatigue hot-spots” in the Model.

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18.3

Material Management This module makes use of the PFMAT utility by either calling it directly or by processing a predefined file.

Materials Database Manager - PFMAT PFMAT allows the user to access the materials database to add, edit, or view the data. The data stored in the database define the monotonic and cyclic properties for materials. The cyclic properties include stress-life, strain-life, cyclic stress-strain and crack growth rate curves.

ASCII Materials File Reader This selection brings up a form that allows the user to create and edit MAT files directly from the GUI. Selecting the Apply button on the form reads the specified MAT file into the materials database creating the material defined in the file.

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18.4

Advanced Loading Utilities The main module delivered with any basic MSC.Fatigue system is PTIME. This basic module has been used extensively throughout these example problems and allows for the following:

• • • • • • • •

ASCII file load xy point entry graphical edit waveform creation (including white noise) block definition matrix creation polynomial and lookup table transformation unit and sample rate conversion

From PTIME it is possible to access certain display and manipulation modules that have already been described in the various exercise problems in this manual such as:

• • • •

Multi-File Display (MMFD) Quick Look Display (MQLD) Three Dimensional Display (MP3D) Two Parameter Display (MTPD)

In addition to PTIME, the following utility modules exist to help in the definition of loading information:

Arithmetic Manipulation - MART MART allows you to take any time signal or even a histogram and apply arithmetic operations such as adding, subtracting, multiplying, or dividing by a constant. You can normalize a signal to a new mean, raise it to a power, apply trigonometric or logarithmic functions, take the absolute value or use the linear equation Y=mX+c. You can apply these arithmetic operations to the entire signal or only a portion thereof. For example, use PTIME to Copy to central the signal SAETRN. Use MART to raise the signal from 1000 seconds to the end of the signal by the power 1.1.

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1. Invoke MART by typing mart from the system prompt or by selecting the Arithmetic Manipulation option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Select the Raise to a power option. 3. Select saetrn.dac as the Input Filename and press OK. 4. Set the Output Filename to saetrn2.dac. 5. Change the Raise to Power databox to 1.1. 6. Change the From databox to 1000 and press OK. This will raise all Y values to the power 1.1 starting at 1000 seconds to the end of the signal.

Above are the results, before and after as displayed by MMFD.

Multi-Channel Editor - MCOE MCOE allows you to tabularly view, edit, or create multiple time signals (files/channels) simultaneously. As an example: 1. Invoke MCOE by typing mcoe at the system prompt or select the Multi-Channel Editor option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post).

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2. Select Edit as the mode of operation. You will be presented with a screen to select files. The operation of selecting files is identical whether you are browsing, editing, or creating. When creating you simply enter a file name and press OK for each column of data. 3. Using the List button select the two files from the previous exercise, saetrn.dac and saetrn2.dac. Use the Shift key to select both files. Press the OK button two times. A spreadsheet with four columns will appear. The first two columns are fixed data corresponding to the point number and the time. Only the right two columns of data are editable. 4. Edit any cell in these two right columns by selecting it with the cursor or using the arrow keys and typing the new value. There are a number commands at the top of the spreadsheet that are useful for editing and viewing data. These are: File: This has only two options, Back or OK. Both will end the editing session and quit from MCOE. Only OK will actually save any changes. View: These commands under this pulldown are simple. They allow you to scroll up or down, right or left, or to the beginning or end of the spreadsheet. Goto: This allows you to specify which row to go to based on the X value (time). finD: This will find the next row in the current column with the value specified based on the criteria of greater than, less than, between, or closest to. Opts: This is perhaps the most useful of the commands for editing the tabular data. These options are discussed below. Pref: This sets preferences such as how you would like to select cells, or view number formats in the columns. You can even scale the X-axis (time) and define the format of exported files. Next: This finds the next number based on the previously set finD command.

Editing Options Under Opts The following options are available for editing the spreadsheet (see the MSC.Fatigue User’s Guide for options not mentioned here). Experiment with any of these as you see fit.

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Gap: This command appends the specified duration of time onto the end of a signal. You type a number into the automatically selected cell and a linear interpolation occurs filling the cells in-between. The next time duration gap is determined from you entering another data value in the selected cell with interpolation occurring again. This cycle continues until you select another cell or invoke some other command. Delete: This deletes all cells within the specified time duration. Cells below the deleted cells move up closing the gap. Copy: Copies the data values in the specified time duration. paSte: Pastes values that have been copied into the specified time duration overwriting any existing values. cYcle: Appends a saw tooth type signal to the end of a column by specifying the maximum/minimum, range, number of cycles and mean. Insert: Inserts the number of points or the time duration specified at the current row for the specified columns. A start and end data value are requested and intermediate values are linearly interpolated. Append: Appends to the end of the signal the length of time or the number of points specified. It also requests a beginning and ending value and all other values inbetween are linearly interpolated. Join: This command asks for a start time and an end time and changes all values inbetween based on a linear interpolation for the specified data columns. Export: Creates a file with default extension .txt of the columns of data in the spreadsheet. Plot: Plots the specified columns using the MMFD multi-file display program. Rescale and offset: Specifies a start time and an end time, the column(s) to apply the scaling and offset, and the scale factor and the offset values. Format columns: This allows you to hide or unhide columns from the spreadsheet, fix (or unfix) them to protect columns from data entry if desired, and to group columns for multiple column operations in other options. eXit: This is the same as OK from the File pulldown to save and exit. Quit: This is the same as Back from the File pulldown exit without saving. Note: A back up file of each file specified is created with extension .bak. So you can always retrieve the original data if you make a mistake.

Rainflow Cycle Counter - MCYC The rainflow cycle counter, mCYC, processes a time series signal, by extracting fatigue cycles according to the rainflow cycle counting algorithm. The utility is useful as it allows the user to count cycles using the same parameters (gate, range, bin width) for Main Index

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comparing and assessing various time signals. The results are presented in the form of a range-mean or a max-min matrix which can be displayed or used as input to mCLF or MSLF. In addition, a file containing a description of each cycle can be generated. If the time of each cycle can be stored, this file may also be used in crack growth analysis.

For example, use PTIME to Copy from central the time history, SAETRN. 1. Invoke MCYC by typing mcyc from the system prompt or select the Rainflow Cycle Counter option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Select input file. By default, mCYC expects the input data to be a standard .dac file but files with the correct internal format but different file extensions must have their names entered in full e.g. filename.pvx 3. Select Output Type - Histogram, Cycles Files, or Both. The layout of the lower part of the above screen and next screen (shown here) will depend on the selection made here. If Histogram or Both is selected the Gate, Histogram Filename, Range Parameters and Mean Parameters are prompted for in the screen as shown on page 416. 4. Enter Gate to filter cycles (e.g. 75 which is approximately 10% of the max indicated value 747). The value entered here must be in physical units (usually microstrain) and greater than zero. If the gate value is more than half the size of the largest cycle in the input file, an error message will be issued. All cycles bigger than the gate will be counted. 5. Enter Window Type - Time or Points. Selecting one or the other changes the next input to time or points

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6. Specify Start Time and End Time or Start Points and End Points. (e.g. start+6 -start 6 seconds or points from start and end-100 - end 100 seconds or points from end). Default is start and end 7. Specify histogram filename if the Output Type selected is Histogram or Both. 8. Store Cycle Time and Cycles Filename are only activated if Cycles File or Both are selected above. If yes is selected, a time based cycles file (.tcy) is generated that can be used in crack growth analysis. If no is selected, a .cyc file is generated that stores the ranges and means from the largest cycle onwards. 9. Specify Cycles Filename if the Output Type selected is Cycles File or Both. The .tcy file can be re-ordered if desired using the Sort Cycles. If no is selected the cycles are sorted in order of size. Note that if sorting is not carried out, it is possible that a crack growth analysis will be wrong since the order of the cycles is important in crack growth analysis. If yes is selected the cycles are sorted in order of time. Slow selects an old method for sorting which requires less disk space than the current faster method. If disk space is not a concern, do not use this option. 10. The WSR component field is for the exponent on which to base the weighted stress range (range 2-10). The value of 2 would give a rms of stress ranges, the default of 3 gives a root mean cube used typically with welded joint S-N curves. For crack growth, the Paris Law exponent, m, should be used. 11. Pressing OK takes you to the next screen or if only Cycles files was selected a results summary is displayed as shown below.

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Histogram Limits The following form is displayed if both or Histogram is selected above.

The fields that are activated depend upon whether the environment keyword HISTFORM is set to MINMAX (use the full range from minimum to maximum values) or BINSIZ (specify on the minimum and the bin width). 1. Min (Range) - For the purposes of scaling the histogram, the range of the smallest cycle to be represented in the histogram must be entered in physical units. If there are any cycles smaller than the minimum range specified, then those cycles will be excluded from the histogram. 2. Max (Range) - For the purposes of scaling the histogram, the range of the largest cycle to be represented in the histogram must be entered in physical units. If there are any cycles larger than the maximum range specified, then those cycles will be excluded from the histogram. 3. No. of Bins - To scale the histogram, specify the number of bins into which to classify the cycle ranges. Any integer up to a maximum of 128 may be entered. 4. Min (mean) - For the purposes of scaling the histogram, the smallest mean value to be represented must be entered in physical units. If there are any cycles whose mean values are smaller than the value specified, then those cycles will be excluded from the histogram. 5. Max (mean) - For the purposes of scaling the histogram, the largest mean value to be represented must be entered in physical units. If there are any cycles whose mean values are greater than the value specified, then those cycles will be excluded from the histogram. 6. No. of Bins - To scale the histogram, specify the number of bins into which to classify the cycle means. Any integer up to a maximum of 128 may be entered.

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7. If the environment variable HISTFORM=BINSIZ (set in mENM) then this field is displayed. The format (size and shape) of the histogram can be set by specifying the bin width. By default the program calculates the bin width needed to include the maximum values in the input file but the user can enter a smaller or larger width. 8. The max-min toggle will plot a histogram based on a count of cycles between the between the maximum and minimum cycle. To scale the histogram, specify the number of bins into which to classify these cycles. Any integer up to a maximum of 128 may be entered. The output histogram using the range-mean option is shown below.

Formula Processor - MFRM MFRM is a formula processor for time series data or histogram files. It is a very powerful tool that includes its own programming language. Virtually any formula can be applied to any number of time history or histogram files. The formulas are defined via an ASCII template file that is created manually using a text editor. MFRM then reads the template file and executes the commands. After execution the resultant files can be graphically displayed.

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As a quick example, define a file called example.frm using any text editor with the following content: ; --- Define all the files --%QYFIL /FILE=F1 /PROMPT="Enter filename" /TYPE=INPUT %DFFIL /FILE=F2 /NAME=positive /TYPE=OUTPUT /OV=y %DFFIL /FILE=F3 /NAME=negative /TYPE=OUTPUT /OV=y %DFFIL /FILE=F4 /NAME=positive2 /TYPE=OUTPUT /OV=y %DFFIL /FILE=F5 /NAME=negative2 /TYPE=OUTPUT /OV=y ; ; --- Carry out the calculations --; %BEGIN ; %IF(F1 > 0) F2=F1 %ELSE F2=0 %ENDIF %IF(F1 < 0) F3=F1 %ELSE F3=0 %ENDIF F4=MAX(F1,0) F5=MIN(F1,0) ; %END

Next invoke MFRM from the system prompt or by selecting the Formula Processor option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 1. Select Run the formula template. 2. Enter example.frm as the template file that you just created. Press OK. 3. You will be prompted for an input file. This can be any .dac formatted type of time series file. Select saetrn.dac as the input file from the previous exercises in this section. The processing will begin once this input file has been selected. If you look carefully at the template file you can see what operations are being executed. The first line prompts for the input file to process. Internally the input file will be known as a variable called F1. The next four lines define output files that will be internally knows as variables F2, F3, F4, and F5 respectively. They are given the names positive, negative, positive2, and negative2. The default file extension is .dac. The next section of the template file contains the operations to be performed. These operations take F1 and break it up into its positive component and its negative component. This can be accomplished in two ways. The first way is done in the first IF-ELSE-ENDIF command by setting any points greater than zero in file F1 to file F2 and anything less than zero to zero. The second IF-ELSE-ENDIF command then does

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the negative side. The final two lines accomplish the exact same task by using the intrinsic math function MAX and MIN by taking the maximum or minimum of each point in the signal by comparing to zero. 4. Plot the results by selecting Plot results files. A plot of the resulting F2, F3, F4, and F5 files is shown to the right. MFRM has the ability to perform very complex operations using IF statements, LOOPs, arithmetic, logarithmic and trigonometric functions as well as addition, subtraction, multiplication, division, and raising to a power. See the MSC.Fatigue User’s Guide for a full description of the MFRM commands and language syntax.

File Cut and Paste - MLEN MLEN allows you to adjust the length of time signals by extracting or deleting sections, and concatenating files. Signals can be reversed and the starting and ending points of each signal can be smoothed. These operations can be on a single file or multiple files simultaneously. For example, use PTIME to Copy from central the three SAE histories, SAETRN, SAESUS, and SAEBRAKT. Then use MLEN to extract a section from each simultaneously such that each signal is the same length. 1. Invoke MLEN by typing mlen from the system prompt or select the File Cut and Paste option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Select option 2. Extract Section - Multiple File. 3. Select Separate filenames as the Entry Method. 4. Use the file List mechanism/browser to select the three signals, saetrn.dac, saesus.dac, and saebrakt.dac and press the OK button. 5. Select Modify extension and input a new extension such as mod. 6. We want to extract a portion of each signal such that all three become the same length. So set the Window Selection to Time[X-axis] and leave the Start Time at START and change the End Time to 1898 seconds, corresponding to the maximum time of the shortest signal. Press the OK button.

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The first 1898 seconds of each signal is extracted and new files called saetrn.mod, saesus.mod, and saebrakt.mod are created. The before and after displays using MMFD are shown below.

Multi-File Manipulation - MMFM MMFM is a module that allows for addition, subtraction, multiplication, division and vector addition of multiple time signals. Addition of individual files is as such: file1 + file2 + ... + fileN. Subtraction of individual files is as such: file1 - (file2 + file3 + ... + fineN). Multiplication is as such: file1 * file2 * ... * fileN. Division is as such: file1 / (file2 * file3 * ... * fileN). Vector addition is performed as follows: (file12 + file22)1/2 or (file12 + file22 + file32)1/2. Experiment by taking the three signals from the previous exercise and performing the various arithmetic operations. For example, here is a plot where the three signals were added together. Note that the original signals were of varying length.

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Peak-Valley Extraction - MPVXMUL MPVXMUL extracts turning points (maxima and minima or “peaks” and “valleys”) from single parameter files such as .dac and RPC multiple data - channel files. The peak valley extraction process maintains synchronous phase by writing corresponding data values to all the output files whenever a turning point is found in any channel. Facilities for gating out small peak valley pairs by absolute value or by percentage of range, on each channel, are available. 1. Invoke MPVXMUL by typing mpvxmul at the system prompt or select the Peak-Valley Extraction option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Select the Input File type of DAC or RPC. Press OK. 3. Select the generic Input Filename, the Channels, the Output Filename, and whether or not to write a time file. Press OK. See the MSC.Fatigue User’s Guide for file naming conventions and other information. 4. Either the range of cycles from rainflow analysis can be used as the gate in the completion of your analysis. This information along with other relative data is entered on the analysis setup form that is displayed. See the MSC.Fatigue User’s Guide for more information. Note:

In Input .dac files exist as families of files with a common generic name but with different numbers appended to the name which denotes the channel number (i.e., test01.dac, test02.dac, etc., where test is the generic name).

Simultaneous Values Analysis DAC/RPC - MSIMMAX MSIMMAX performs simultaneous values analysis on either multi-channels in a single RPC file or multiple DAC files from the same test. Two analysis methods are available. The first uses a “control” channel, from which turning points are extracted and scanned for the highest peaks, the lowest valleys or the highest absolute maxima. Up to 50 of these events may be saved, with their positions in the data. The simultaneous values of all the other channels at these positions are saved into the output files. The second method scans each of the input channels for the single largest maximum, minimum or absolute maximum. For each channel, the simultaneous values of all other channels at the position of the largest event is saved into the output files. The output file created is a tab separated ASCII file suitable for input to a spreadsheet or word processing package.

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1. Invoke MSIMMAX by typing msimmax at the system prompt or select the Simultaneous Values Analysis DAC/RCP option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Select the Input File type of DAC or RPC. Press OK. Note: Input .dac files exist as families of files with a common generic name but with different numbers appended to the name which denotes the channel number (i.e., vib01.dac, vib02.dac, etc., where vib is the generic name).

3. The next form is the Filename Input form which allows the names of the input and output files to be specified. For an input file type of RPC the RPC Filename field appears and for an input file type of DAC the Generic Filename field appears. Select the RPC Filename or the Generic Filename, the Channels, the Output Filename, and whether or not to recalculate the statistics. Press OK. See the MSC.Fatigue User’s Guide for file naming conventions and other information.

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4. The final form for MSIMMAX is the Analysis Definition form. This form allows you to choose between Sort Channel analysis and All Channels analysis. It is here that the limits are set and the event type is selected. This information along with other relative data is entered on the form that is shown below. See the MSC.Fatigue User’s Guide for more information.

Amplitude Distribution - MADA MADA, amplitude distribution analysis, calculates the probability density distribution (which defines the probability of finding a value of a particular magnitude within the population of measured values) and other function of a time signal. For example if you use saetrn.dac as input to MADA and set the Analysis Type to Prob. Distribution, it will output a file saetrn.ada shown here which is the probability density function of Y-values.

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Auto Spectral Density - MASD MASD performs a frequency analysis of a time signal to determine frequency content. Various output types are available which are beyond the scope of this text. Perhaps the best use of this module comes in vibration fatigue problems for converting time signals into power spectral density functions (PSDFs). As an example, let us convert the time signal SAETRN, used in many of the previous sections in this chapter into a PSDF, which will quickly show us the frequency content of the signal. 1. Invoke MASD by typing masd at the system prompt. It can also be invoked directly from PTIME under Add an entry | creaTe psd from time or select the Auto Spectral Density option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Select the file saetrn.dac as the Input Filename. 3. Make sure the Output Type is Power Spectral Density and accept the defaults for all other inputs. 4. Press the OK button until the conversion takes place.

A display of the resulting PSDF is shown above. Note that only one predominate frequency is present in the signal at around 1/2 Hz plus a DC component at zero Hz. This PSDF could be used as input to a vibration fatigue analysis.

Fast Fourier Filtering - MFFF MFFF uses filtering techniques to remove frequency content from a signal. As an example, invoke PTIME and create a white noise signal: 1. Invoke PTIME and select Add an entry... | white Noise. Main Index

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2. Call the file noise, enter a description, and press OK. Set the Total Time of Signal to 100 and press OK. This will create a signal with frequency content at all frequencies. To see this: 3. Invoke MASD (Add an entry... | creaTe psd from time from within PTIME).

Time Domain

4. Accept NOISE.DAC as the file name and all other defaults.

Frequency Domain The time signal will be converted to a PSDF and displayed (if Plot Output was turned ON). Since it has frequency content across all frequencies it appears very random looking in the frequency domain just as it does in the time domain.

5. Now invoke MFFF and accept NOISE.DAC as input. 6. Type NOISE2 as the Output Filename. 7. Change the Filter Type to 3 band pass. 8. Set the Lower Edge Cutoff Freq. to 20 and the Upper Edge Cutoff Freq. to 30 and press OK. 9. Again invoke MASD to convert the new signal, noise2.dac to a PSDF and plot it noticing that all frequency content has above 30 Hz and below 20 Hz was removed from the signal.

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Butterworth Filtration - MBFL MBFL also uses filtering techniques to remove frequency content from a signal. If you perform the same steps as the previous section on the MFFF module and perform a Band pass filter between 20 and 30 Hz on the NOISE.DAC file with the new Output Filename set to NOISE3 you will get the top plot to the right. Notice the difference in the filtering between MBFL and MFFF. MBFL allows for some leakage and gives a smoother transition instead of completely eliminating frequency content.

MBFL Filtering

MFFF Filtering

Frequency Response Analysis - MFRA MFRA performs frequency response analysis and calculates the transfer function of a single input, single response system. Perhaps the most useful application of this module is to compute the cross correlation function between the two supplied response signals. These correlation terms can then be used as input to a vibration fatigue analysis. As an example: 1. Copy over the three files 1pk.asc, 2pk.asc, 3pk.asc. Using PTIME, select Add an entry... | ASCII convert+load to load these three signals which can be thought of as three input of a multiple load case problem acting simultaneously but in separate locations of an FE model. The Sample Rate must be 50 and X-y pairs must be specified as the Equally Spaced Data option. Quit from PTIME when you are done converting the files. These are large ASCII files and take some time to load. 2. Invoke MFRA from the system prompt by typing mfra or select the Frequency Response Analysis option from the Advanced Loading Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 3. Choose Transfer Function Analysis. 4. Set the Input Filename to 1pk.dac and the Response Filename to 2pk.dac. Press OK and continue to press OK accepting the defaults for all other screens except change the Generic Output Filename to pk1_2.

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5. When the analysis is done select Results Display and plot All result files. 6. Repeat this for 1pk.dac vs. 3pk.dac and 2pk.dac vs. 3pk.dac using the Generic Output Filename, pk1_3 and pk2_3 respectively. When the analysis is done you can then use these PSDF and their cross correlation terms as loading input to a vibration fatigue analysis. By using PTIME you can set up the PSD matrix information for these three input loadings from Add an entry... | Psd matrix, the size of the matrix being 3x3. All the files from the transfer function analysis (*.sxx, *.syy, *.sxy) must be loaded into PTIME first. The matrix of PSDF and cross correlation files would look like this: pk1_2.sxx or pk1_2.sxy pk1_3.sxx

pk1_3.sxy

pk1_2.sxy

pk1_2.syy or pk2_3.sxy pk2_3.sxx

pk1_3.sxy

pk2_3.sxy

pk2_3.syy or pk1_3.syy

Note: The diagonal terms in the PSD matrix of PSD and cross term files could also be created using MASD directly.

Statistical Analysis - MRSTATS MRSTATS analyzes a time signal and produces a number of running statistics about the signal, each of which can be plotted by the standard plotting routines such as MQLD (quick look display), MTPD (two parameter display), or MMFD (multi-file display).

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The statistics that are determined are running: RMS (.rms), Standard Deviation (.rsd), Mean (.mea), Maximum Value (.max), Minimum Value (.min), Absolute Maximum Value (.abs), Area Under Data (.are). Each new signal is defined by its extension. The statistics are determined by breaking the signal into segments and calculating the statistics on each segment. The length of a segment and the overlap of each adjacent segment is user definable. Try running saetrn.dac through MRSTATS. Shown here are the statistical signals produced from running MRSTATS against the SAE signal saetrn.dac.

Header/Footer Manipulation - MFILMNP MFILMNP allows you to view and manipulate/change header and extra detail information in any signal. It also can be used to validate the integrity of a file. Use this module if you wish to quickly and easily change axis labels or units and title information or you wish to validate the signal which will try and correct or flag any problems with the file.

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18.5

Advanced Fatigue Utilities Aside from the FE based fatigue analyzers described in detail in these exercises, the following fatigue analysis utilities are also available, some of which have been described in earlier exercises. Most accept stress or strain response data as input. These response signals can be measured or simulated from FE analysis. For instance, FEFAT has the ability to output the stress or strain response time signal at any desired location on the FE model. FEVIB also has the same ability to output a response PSDF at any location. These can be used as input to the single location fatigue analyzers described below.

Single Location S-N Analysis - MSLF MSLF is a Total Life or S-N analyzer. It accepts a stress response time signal as input in the form of a .dac file. It also can accept rainflow histograms or simple constant amplitude or maximum/minimum input. Operation is simple and very similar to that of the FE equivalent fatigue analyzer FEFAT. When invoked the first time, you are lead through a series of setup screens to define the job. Once the job is defined you are then placed in a Post Processing Options menu where you can modify any aspect of the job setup and recalculate the results. As an example, use PTIME to copy from central the three SAE time histories SAETRN, SAESUS, SAEBRAKT, if they have not already been copied over from an earlier exercise. We will assume they are stress responses this time. 1. Invoke MSLF from the system prompt by typing mslf or choose the Single Location S-N Analysis option from the Advanced Fatigue Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Enter a new job name such as “slf_example.” It is new, so answer Yes to the ensuing question. 3. Accept all defaults except for these on the setup screens as you are presented with each one: Filename: saetrn.dac; Scale Factor: 0.5; Material Name: MANTEN; Cycles File:Yes The analysis will proceed, the results will be presented and eventually you will be placed in the Post Processing Options. Answer Yes to any overwrite permission questions. 4. Select multiple File from the Post Processing Options screen. 5. Use the List/File Browser button to select the three Input Filename(s) saetrn.dac, saesus.dac, and saebrakt.dac. Use the Shift key to select all three.

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6. Press the OK button and the analysis will take place again except this time all three time signals (which are being treated as stress time histories) will be processed and the fatigue lives reported for each. Answer Yes to any overwrite permission questions. Exit from MSLF when you are finished.

Single Location e-N Analysis - MCLF MCLF is a single location Crack Initiation fatigue analyzer. It accepts a strain response time signal as input in the form of a .dac file. It also can accept rainflow histograms or simple constant amplitude or maximum/minimum strain input. Operation is simple and very similar to that of the FE equivalent fatigue analyzer FEFAT. When invoked the first time, you are lead through a series of setup screens to define the job. Once the job is defined you are then placed in a Post Processing Options menu where you can modify any aspect of the job setup and recalculate the results. As an exercise, use PTIME to Copy from central the three SAE time histories SAETRN, SAESUS, SAEBRAKT, if they have not been copied already from an earlier exercise. 1. Invoke MCLF from the system prompt by typing mclf or choose the Single Location e-N Analysis option from the Advanced Fatigue Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Enter a new job name such as “clf_example.” It is new, so answer Yes to the ensuing question. 3. Accept all defaults except for these on the setup screens as you are presented with each one: Filename: saetrn.dac; Scale Factor: 2; Material Name: MANTEN The analysis will proceed, the results will be presented and eventually you will be placed in the Post Processing Options. Answer Yes to any overwrite permission questions. 4. Select multiple File from the Post Processing Options screen. 5. Use the List/File Browser button to select the three Input Filename(s) saetrn.dac, saesus.dac, and saebrakt.dac. Use the Shift key to select all three. 6. Press the OK button and the analysis will take place again except this time all three time signals (which are being treated as strain time histories) will be processed and the fatigue lives reported for each. Answer Yes to any overwrite permission questions.

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7. Close the form with the multiple file results and then press Recalculate on the main form. Now under the Display results pick, you can display cycle and damage histogram plots or you can go back and change any of the inputs. 8. As one last exercise, select Output definition. 9. Set Hysteresis loops to Yes and press OK. 10. Press Recalculate and then close the summary page. 11. Go to Display results | Hysteresis loops. The five largest hysteresis loops will be displayed. Exit from MCLF when you are finished. Note: MCLF can accept either measured or purely elastic signals such as those from FE. If purely elastic signals are fed to MCLF, it will perform elasticplastic correction. Measured data is assumed to be the true strains and therefore undergoes no correction. You must specify this on the Service Loading Environment form.

Cycle and Damage Analysis MCDA MCDA is a 2D cycle and damage histogram display program. It allows you to look at cycles vs. range or cycles vs. mean of cycle or damage data or both simultaneously. You may specify up to two different cycle histograms with default extensions of .cyo for comparison purposes. It automatically looks for a corresponding .dhh, damage histograms, if they exist. As an example, in the last exercise using MCLF you analyzed three different time signals, saetrn.dac, saesus.dac, and saebrakt.dac. Corresponding .cyo and .dhh files should exist in your directory. 1. Invoke MCDA by typing mcda at the system prompt or select the Cycle and Damage Analysis option from the Advanced Fatigue Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post.)

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2. Specify saetrn.cyo and saesus.cyo as the Name of Cycles Histogram and Second Cycles Histogram respectively. 3. From the Plot Options Menu, select any option. Use the File | Return command to return to the Plot Options Menu. Specifically look at the Plot Damage/Cycles - File 1/2 options to see both damage and cycles superimposed on each other which clearly shows the cycles that cause the most damage.

Note: MCDA can be spawned directly from MCLF (and MSLF) from the Display results | damage Analysis menu pick if you request a Cycles file as output.

Cycles File Lister - MCYL MCYL is a convenient utility to list a cycles file to the screen or a file. Cycle files are produced by various MSC.Fatigue programs and have the extensions .cyc, .clf, .slf, or .tcy. For example in the previous exercise a cycles file called saetrn.slf was produced. Chose list a Cycles file and select saetrn.slf as the Input Filename to list the cycles file. A cycles and/or damage matrix can also be created from a cycles file or listed to the screen or a file. A cycles or damage matrix can also be written to a file that is formatted for import to a spreadsheet program. Note: You can spawn MCYL from both MCLF and MSLF to list cycle files and matrices directly from the Display Results | List cycles menu pick if you request a Cycles file as output.

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Time Correlated Damage - MTCD MTCD is a time correlated damage analyzer. It is similar in nature to MCLF in that it is strain based and calculates damage based on Crack Initiation, the difference being that damage is summed over time and in the sequence that the cycles are seen. To see this do the following: Use the SAE time histories SAETRN as in the previous MCLF example. Again we are assuming this signal is a strain response. 1. Invoke MTCD from the system prompt by typing mtcd or choose the Time Correlated Damage option from the Advanced Fatigue Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Enter a new job name such as “tcd_example.” It is new, so answer Yes to the ensuing question. 3. Accept all defaults on each setup screen as you are presented with each one except for these: Filename: saetrn.dac; Scale Factor: 2; Material Name: MANTEN The analysis will proceed, the results will be presented and eventually you will be placed in the Post Processing Options. Answer Yes to any overwrite permission questions. 4. Select Display Results | Time-Damage plot. Note that this plots the strain time signal above a damage file. The damage is shown vs. time and distinctly shows where in the time signal most of the damaging events are occurring.

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5. Close the graphical plot and then select Output Definition. 6. Change the Output Filetype to Cumulative and press OK. 7. Press Recalculate and then OK on the summary page. 8. Select Display Results | TimeDamage plot. Note that this time the plot of damage is cumulative over time. Exit from MTCD when you are finished.

Single Location Vibration Fatigue - MFLF MFLF is a single location, stress-based fatigue analysis module that accepts stress response PSDFs as input. This module has also been mentioned in an earlier chapter. As an example of usage copy over the original SAE history saetrn.dac to your working directory. This signal is assumed to contain a stress time response. Use MASD to convert the time signal into the frequency domain by converting it to a PSDF. See the section on MASD in this chapter for instruction on how to do this. Use all the default settings. The output file name should be saetrn.psd. 1. Invoke MFLF from the system prompt by typing mflf or choose the Single Location Vibration Fatigue option from the Advanced Fatigue Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Accept all defaults for all setup screens except for these: Input Filename: saetrn.psd; Dataset Name: MANTEN The analysis will proceed, the results will be presented and eventually you will be placed in the Post Processing Options. Answer Yes to any overwrite permission questions. 3. Go to Display results... | Cycles histogram.

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Exit from MFLF when you are finished. Note: This example is for illustration purposes only. The signal used in this example is not actually an appropriate signal to use in that it is not truly random or gaussian as required by a random vibration fatigue analysis.

Stress-Strain Analysis - mSSA Stress-Strain Analysis processes rosetta data and finite element data from MSC.Fatigue, including software strain gauges. It creates outputs suitable for use by either the stress or strain-life fatigue analyzers. It also provides an indication of the state of multiaxiality present, suggests possible processing routines through the fatigue analyzers and has a multiaxial fatigue analyzer that works by using a MSC.Fatigue .fes file. In addition to this, the module can be used to convert elasticplastic strain records, measured on one material, to that of another material. It can also convert elastic-plastic strain records to equivalent fully elastic ones and visa-versa.

Multi-Axial Life Analysis - MMLF MMLF is a single location multiaxial fatigue analyzer based on Crack Initiation and has been briefly referred to in a previous chapter. It requires three strain input signals which typically come from strain gauge rosettes. For rectangular rosettes the signals are separated by 45 degrees. For delta rosettes the signals are separated by 60 degrees. As an example, take the three SAE histories that we have been using thus far (saetrn.dac, saesus.dac, saebrakt.dac), except run them through MLEN and chop them all to 1800 seconds. (See the previous section on MLEN to learn how to do this.) We will assume that these new signals are from a rectangular rosette.

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1. Invoke MMLF from the system prompt by typing mmlf or choose the MultiAxial Life Analysis option from the Advanced Fatigue Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post). 2. Enter a new job name such as “mlf_example.” It is new, so answer Yes to the ensuing question. 3. Accept all defaults for all setup screens except for these: Gauge 1: saetrn.dac; Gauge 2: saesus.dac; Gauge3:saebrakt.dac;Material Name: MANTEN The analysis will proceed, the results will be presented and eventually you will be placed in the Post Processing Options. Answer Yes to any overwrite permission questions. 4. Go to Display results | Stress and Strain. Plot this result and any of the others you wish in this menu selection. Exit from MMLF when you are finished.

Note: Strain signals input to MMLF are assumed to be elastic-plastic. No elasticplastic corrections are performed in MMLF. Use MSSA and/or SSG to do this if necessary from FE data.

Crack Growth Data Analysis MFCG MFCG calculates the Paris Law coefficient, C, and exponent, m, in the expression da/dN = C(∆K)m from actual raw test data obtained under constant amplitude loading conditions.

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CHAPTER 18 Fatigue Utilities

Kt/Kf Evaluation - MKTAN MKTAN is the database library for calculating the stress concentration factors, Kt, of various geometries. In a sense it is very similar to the PKSOL utility function for calculating compliance functions for Crack Growth analysis. The value of Kt calculated can be used as the Kt or Kf input to the single location analyzers, MSLF and MCLF. Both a secured database of standard geometries and a user database for user defined geometries is available. The elastic stress concentration factor, Kt, is the ratio of the maximum stress at a stress raiser to the nominal stress computed by the ordinary strength- of-material formulae, using the dimensions of the net section. It can be used to account for the presence of a notch within a component or structure. The magnitude of the Kt required depends on the nature of the notch and its geometry. It is well known that small notches have less effect in fatigue than is indicated by Kt. This has led to the idea of a fatigue concentration factor, Kf, which is normally less than Kt, being introduced and being used to replace Kt within Neuber's rule. Kf is related to Kt according to: Kf = 1 + (Kt - 1) / {1 +

( p' ⁄ r ) }

where: p' is a material constant dependent on grain size and strength and r is the notch root radius. As an example of calculating a Kt value: 1. Select Calculate | Secure Database from the MKTAN Main Menu. 2. Select Holes. 3. Select Elliptical hole in an infinite plate (the first selection) and press OK. 4. Press Calculate. 5. Enter 2 for b and 1 for a. The Kt calculated will be displayed. Exit from the program when done. Note: This utility is mostly useful for measured responses where the measurement is a nominal value away from the actual failure location or stress concentration. With FE based fatigue calculation, the stresses and strains are all local, therefore Kt is always unity. Naturally, additional Kt or Kf values may be entered in these cases if the FE is not capturing some stress concentration.

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18.6

Graphical Display Utilities Several display routines and three plotting/printing routines exists in MSC.Fatigue. Before using the plotting/printing modules to plot or display a graphical screen dump, you must capture the image to a file. This is done from any graphical display in MSC.Fatigue using the File | Hardcopy command. You will be prompted to supply a file name and a plot title. The file name can be anything, and you do not need to supply a file extension. The extension of .plt will automatically be appended to the file name.

Graphical Editing-mGED This module is the multi-channel interactive graphical editor for time series data allowing online manipulation of a signal. Tasks such as cleaning up bad data, creating data, extending a signal, spike removal, etc., are all easy and quick to carry out. This module can also operate in batch. For multi-channel edits it creates it’s own NCL macro so that operations defined for one channel can be applied to all others, without the need to do them interactively (on DOS platforms a BTP module is created). The assumption is that the other signals are from the same test or at least exhibit the same sample rate, etc.

Multi-File Display - mMFD This module displays single parameter data files. The files may contain any type of sequential data including time series, power spectra, time at level distributions, etc. Files may be displayed across four screen pages, with a maximum of eight files per page. Thus, allowing up to 32 files to be presented. Three modes are offered for displaying the files on each page. They are: separate plots, overlaid plots, and crossplots. Separate plots are those where each file is plotted independently of the others.

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Overlaid plots are where all of the files are plotted using common axes. Cross-plots are where one file nominally forms the X-values against which the other files are plotted on common axes.

Quick Look Display - mQLD This module displays single channel data file. The file must be in the .dac format, which includes time histories, ASD results, ADA results, and any other results that have a constant X-axis increment. Use mTPD for pared (X-Y) data and mP3D for histogram and waterfall data.

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Two Parameter Display - mTPD The two parameter display module displays pared (X-Y) data files. Displays may be scaled in various ways. Functions for windowing specific fields and picking off coordinate pairs are also available. After the data has been displayed, a menu will appear. Select your options and give the name of a data file to display. The file is assumed to be in the local directory and have an extension .mdf. If you wish to access another directory or use a file with a different extension, you will need to type in a fuller file specification.

Polar Display - mPOD The polar display module displays pared (X-Y) data files. Displays may be scaled in various ways. Functions for windowing specific fields and picking off coordinate pairs are also available. After the data has been displayed, a menu will appear. Select your options and give the name of a data file to display. The file is assumed to be in the local directory and have an extension .pod. If you wish to access another directory or use a file with a different extension you will need to type in a fuller file specification.

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Three Dimensional Display - mP3D This module is the histogram and waterfall display module. It accesses a standard nSoft histogram or waterfall file and provides a 3D graphical representation in the form of a tower, surface, or waterfall plot. The display can then be zoomed into, and positioning using rotation, tilt, and quadrant operations may expose hidden areas. For histogram files originating from fatigue analysis damage/cycles files may be plotted directly. In addition, mP3D will display the sum total occurrences of values along the X or Y-axis and display the result as a 2D plot. For waterfall files, 2D plots of X-slice and Y-slice may be produced.

Plot File (.plt) Display - MQPLOT (for UNIX) MQPLOT is a UNIX based plotting utility where once you have created a .plt file you can load them into MQPLOT for easy plotting and printing. You can load multiple files into MQPLOT and use it as a slide show program also. To print the currently displayed plot, simply use the File | Print menu command. For this to work you must have defined a printer using the MPLTSYS module explained next.

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Plot File (.plt) Display - MWNPLOT (for Windows) MWNPLOT is a Windows based plotting utility where once you have created a .plt file you can load them into MWNPLOT for easy plotting and printing. You can load multiple files into MWNPLOT and use it as a slide show program also. To print the currently displayed plot, simply use the File | Print menu command. Any compatible printer on the network can be accessed if you have added it to your list of printers using the standard Windows Add Printer command.

Printer and Device Setup - MPLTSYS MPLTSYS defines and sets up the plotter or printer definitions. The best way to describe its usage is through a couple of examples. First let us suppose that we want to view all of our .plt files in the current directory using MQPLOT and when the Print command is selected, convert the current plot to a postscript file. Then using a UNIX command the postscript file can be sent to a color postscript printer. 1. Invoke MPLTSYS by typing mpltsys from the system prompt or choose the Printer and Device Setup option from the Graphical Display Utilities pulldown menu under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post.) 2. The easiest thing to do is to modify an existing default printer definition. Select CPOST and press the Modify button. CPOST is the name of an already defined printer/plotter definition. 3. You are given the opportunity to change the name and description if you wish. Press OK to continue accepting all the defaults. 4. Next you are asked whether this is to be modified locally (in the local directory) or whether it should be a central change. Select Local.

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CHAPTER 18 Fatigue Utilities

When the modifications are done a new printer definition file will be left in the local working directory. As long as this file is in this directory, any printing using MQPLOT will see this definition file (cpost.de1). 5. This then puts you in the Modify Settings menu where you can change any necessary items. Under Size Settings change the Units of Size to Inches and press OK. 6. Under Output Settings change the Output Destination to File. 7. Change the Filename Method to Plot Filename. This will tack on the .spl extension for a color postscript file. 8. Now invoke MQPLOT and open one or more or the .plt file that you have created from any of the MSC.Fatigue graphical modules. 9. You have two Output choices after selecting the files: Device or Screen. If you select Device and the Device name, CPOST, the color postscript will automatically be created in your directory and the program will end. If you select Screen, they will be graphically displayed and you will then have to use the Print command to create the postscript files.

10. Once the files have been created you can send them to the printer. For example if your printer name is qmc_1081 and you have the UNIX lpr command set up properly you should be able to issue the following: lpr -Pqmc_1081 filename.spl

Next let us set up a printer/plotter name that automatically does the print submission for us. Follow the previous steps up to the point where you modify the Output Settings: 11. Under Output Settings change the Output Destination to Queue.

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12. Change the Queue Text String to Precede Filename to lpr -Pqmc_1081. Include a space after the printer name. This essentially defines the command line to use when sending the file to the printer. The file name will be appended to the command line. You can also define any other string that needs to be appended after the file name also. 13. Now invoke MQPLOT and open any .plt file. If the Output choice is Device or if a Print command is chosen, the file will be directly sent to the printer.

The other Output device not mentioned yet is an actual Output Device Name such as when you have a printer directly connected to your UNIX computer. Since these vary dramatically from computer to computer it is not convenient to describe this setup here. Note: A variety of different printers and plotters are available when you create a new printer/plotter definition from scratch such as HPGL, Calcomp, Canon, DEC, Graphtec, Epson, HP, OKI, etc.

Plot/Pen Colors Utility - MNCPENS MNCPENS is a simple utility to allow you to change graphic colors on any plot. It is accessible through most all graphic utilities such as MQLD, MMFD, MGED, MPOD, etc. from the Preference pulldown menu. Select the Pen Setup option from this pulldown menu or invoke the program from a DOS or UNIX window with the symbol mncpens. You can customize colors of any plot attribute such as background, curve data, grid lines, titles, text annotation, etc. You save the settings to a file in either the local working directory, your home directory, or the global installation area (if you have privileges). Once this has been set, you can select whether the local, home, or global

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color environment is used, or some other custom color file is selected which remains in effect until you change it. The best way to learn this program is to experiment and see what it does.

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18.7

File Conversion Utilities Six file translation utilities exist in MSC.Fatigue. These utilities can be invoked from the system prompt or can be selected from the Tools | MSC.Fatigue | File Conversion Utilities pulldown menu (for Patran) or from the Tools | Fatigue Utilities | File Conversion Utilities pulldown menu (for Pre&Post).

Convert Binary .dac to ASCII - MDTA and Convert ASCII to Binary .dac - MATD MDTA converts standard MSC.Fatigue .dac files into ASCII form. This can also be done using the MCOE. The difference is that MDTA can be run in batch mode and MCOE cannot. MATD converts ASCII files into standard MSC.Fatigue .dac files. This can also be done using PTIME. The difference is that MATD can also process multi-channel ASCII files, thus creating multiple .dac files from a single ASCII input file. Both run in batch mode for easy processing of multiple ASCII files.

Signal Regeneration - MREGEN MREGEN will take a three dimensional (three parameter) histogram matrix and will regenerate the time signal from it. The resulting time signal is statistically equivalent to the original in that it will give the same cycle count. As an example of this, take the SAE signal saetrn.dac and run it through MSLF as in the section on MSLF in this chapter. This will create a cycles histogram file called saetrn.cyo. Use the Range-Mean Rainflow Matrix option to convert it to a time signal. Be sure to give it another output file name so as not to overwrite the original signal. Plot the new signal vs. the old signal using MMFD (multifile display).

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As a final exercise you might want to run the new signal through MSLF and then compare the new rainflow cycle count matrix to the old. They should look almost identical in nature. Use MP3D to plot each .cyo file. Note: The life estimate may not be the same because the scale factor is not applied to the cycles file. Scale the time history (using MART or PTIME) in order to use a scale factor of 1.0 to create the cycles file in MSLF. Then run this through MREGEN.

Convert RPC File to .dac - MREMDAC and Convert .dac to RPC file - MDACREM MREMDAC is used to convert an MTS RPC (remote parameter file - .rsp) into a set of MSC.Fatigue compatible .dac files. MDACREM is used to take a set of .dac files and convert them back into an MTS RPC file. These files come from data accumulated in

MTS test simulation machines.

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Cross Platform Conversion MCONFIL MCONFIL is a utility to convert standard MSC.Fatigue type binary files for crossplatform compatibility. For example, if you created or acquired a number of time signals as .dac files on a PC and needed to transfer them to a UNIX workstation you would need to run all the .dac files through CONFIL to byte swap them. Other files that are necessary to convert using MCONFIL are ptime.tdb, the time history database file, nmats.mdb, the materials database file, and any compliance functions with extension .ksn as well as any other one, two or three parameter files.

Waterfall File Create MWFLCRE MWFLCRE allows creation of waterfall plots from multiple single parameter files. For example, say you had a PSD plot for each RPM of a motor from 1100 RPM to 4500 RMP in 50 RPM increments. You could combine all RPM PSD plots into a single three parameter waterfall plot with this utility. Conversely you can break waterfall plots up into individual single parameter history plots. Three parameter plots can be displayed using the MP3D graphic utility.

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18.8

Other Utilities Environment Settings - MENM This module allows the user access to the nSoft environment files: the local environment (ENVI.USR), the global or central environment (ENVI.SYS), and the home environment (ENVI.HOM). Each of these three environment files is made up of a keyword/value string pair where the keyword can be up to 8 characters in length and the value up to 120 characters in length. They are used to define the default settings (e.g. pen colors) and to pass information from program to program (e.g. last used file). Local environment - Only programs running in the directory where the file resides can access the local environment file. Global environment - The global environment file resides in the NSSYS directory and may be accessed by many users. Home environment - The home environment file resides in the home directory as is defined in the system registry on Windows or it resides in the same directory as the normal logon home directory for UNIX. Only the owner of the home directory can access this file. This utility can be invoked by selecting the Environment Settings option from under Tools | MSC.Fatigue (for Patran) or under Tools | Fatigue Utilities (for Pre&Post).

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For each of the three environment settings the user can do the following: 1. List environment to screen - this option lists all the defined keyword/value pairs to the screen. 2. Output environment to file - this option outputs all the defined keyword/value pairs to an ASCII file. It allows the file to be overwritten or to have the pairs appended to it. The character separating the keyword and value may be specified, along with a filter to allow a subset of keywords to be written. 3. Add or alter keyword - this option allows the addition of new keyword/value pairs to the selected environment, and also allows existing keyword values to be modified. If altering a keyword, the keyword may be selected from a list using the F3 key or ‘List’ button. 4. Remove keywords - this option allows keywords to be deleted from the environment. A single keyword may be typed in, or a ‘wildcard’ may be used to define a set of keywords to be deleted. Keywords may be selected from a list using the F3 key or ‘List’ button. 5. Clear environment - this option allows the environment to be deleted. All keywords will be destroyed. If the environment file is to be deleted, answer YES. Environment Copy – Allows the user to specify the source and target environments to use in the copy process. Individual entries, selected entries or all entries may be copied from the source environment to the target. An example of an environment variable that the user might want to create is PFCONAMP. This environment variable is used for constant amplitude zero mean time histories. It allows life to be calculated directly from the true max-min cycle, which is stored along with the matrix. In order to make use of this environment variable the user would need to invoke MENM and set the environment keyword PFCONAMP to ON.

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MSC.Fatigue QuickStart Guide

CHAPTER

19

Miscellaneous Features

■ Problem Description ■ Element Centroidal Calculations ■ Group Averaging ■ Extracting Time Histories ■ Identify Critical Location ■ Defining Histogram Matrices ■ Constant Amplitude Zero Mean Time Histories

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19.1

Problem Description This section discusses a few of the other features of MSC.Fatigue not discussed thus far. No in-depth discussions or exercises are given for them, except for the Multiple Fatigue Analysis. For the other features you can take any of the models and job setups used in this chapter and experiment as you see fit. The following files will be required to demonstrate some of the features:

Table 19-1 Chapter 18 Necessary Files File P3_HOME/mscfatigue_files/examples/ patran_els.fin P3_HOME/mscfatigue_files/examples/ key_tran.op2 P3_HOME/mscfatigue_files/examples/ key_stat.op2 P3_HOME/mscfatigue_files/examples/ transient.fin P3_HOME/mscfatigue_files/examples/ static.fin P3_HOME/mscfatigue_files/examples/ key_tran.asc P3HOME/mscfatigue_files/examples/ simpleSN.op2

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19.2

Element Centroidal Calculations In the General Setup Parameters on the main MSC.Fatigue setup form you can select Element for the Results Loc. This simply means that calculated fatigue lives will be determined at the element centroids. The following comments are made with respect to this feature: 1. It only makes sense to use this with 1D or 2D elements since fatigue cracks tend to initiate on the surface.

Element Fill Plot

2. FE results in the database that exist at nodes or integration points will be averaged to the element centroid. 3. External nodal PATRAN Results files cannot be used if Element is specified. And the converse is true also. 4. When postprocessing element centroidal results, it is best to color code the elements as opposed to making a fringe contour plot. This is done in the Results application. To specify that a fringe plot be element filled, use the Display Attributes mode after selecting the appropriate result to plot and change the Style to Element Fill. An example of this type of plot using the results from the patran_els.fin setup file from one of the earlier mini-exercises is shown here.

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19.3

Group Averaging In the General Setup Parameters on the main MSC.Fatigue setup form you can select Group as the Nodal Averaging method. The default is Global and when set, element nodal results or results at gauss (integration) points are extrapolated to and averaged at the nodes from all element contributions. When this feature is set to Group, the averaging is done only for those elements in the current group. The following comments are made with respect to this feature: 1. This applies to nodal fatigue calculations only. 2. You must make sure you have the proper group set to be the current group. The current group is always displayed in the title of the graphics window. Use Group | Set Current... to set the current group. 3. Only one material and surface finish/treatment combination can be set with this feature. 4. This feature is convenient to use when you want to exclude the contributions of adjacent elements that may not be appropriate to include in the averaging such as when different materials or properties butt up against each other or you have some geometric features or element types. As a simple explanation of this feature consider the four nodes to the right. For element nodal results, each of them has contributions from the surrounding elements shown as number in magenta (small font). These stresses are an indication of model quality. If they are all identical the element quality is perfect. This is rarely the case however.

5.1

5.325

5.3

5.5

6.1

6.8

5.4

5.8

6.3

5.4

5.8

4.53

3.7

6.25

3.2 3.3 3.6 When Global averaging is set, all element contributions 4.3 3.6 4.5 4.6 are considered in the averaging. Thus the stress values used will be as shown in blue (large font). If only the center element exists in the current group and Group averaging is set, the stress values at the nodes used in the analysis will be the contributions from the center element only. If the center element and the bottom three elements are in the current group then the averaged stresses will only contain the contribution from those elements as shown in green.

4.475

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4.5

CHAPTER 19 Miscellaneous Features

19.4

Extracting Time Histories This is an extremely useful utility for a number of reasons. First it gives you a sanity check in case you are having trouble understanding the results. It also gives you access to the single location analyzers MSLF and MCLF which will be discussed and illustrated later in this guide. This feature is accessed from the main MSC.Fatigue setup form in the Results... form by setting the Action to Extract Time History. Simply select the node or element of the model graphically and press Apply (or you can type the node or element number manually with “Node” or “Elem” supplied in front of the actual entity ID). This will invoke FEFAT’s Time History Creation mode. Note:

You do not have to supply a node or element number; but if you do not, you will have to supply one within FEFAT before a successful operation. When a node or element is supplied, a file called pfatigue.ent is created from which FEFAT extracts the ID. This file will be empty if you do not supply a number, in which case, you have to manually supply the ID in FEFAT.

When FEFAT begins you will be presented with a simple setup screen which contains a few items that can be modified if so desired. Press the OK button to extract the time history. For multiple load cases, this will create the actual combined stress or strain time history before (and used for) rainflow cycle extraction. Simple statistic of the signal are also given, such as maximum, minimum and mean values.

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19.5

Identify Critical Location This is also a useful utility if you wish to quickly identify on the graphics screen which node (or element) has the most damage or the shortest life. This feature is accessible from the main MSC.Fatigue setup form in the Results... form by setting the Action to Identify Location. There are several options depending on which type of analysis you have performed and what results are available. For nodal results, the node with the shortest life is highlighted, circled and pointed to with a vector arrow which also reports the life value. The life is also reported in the invoking form with the node number indicated in the databox. For elemental results, the element is highlighted.

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19.6

Defining Histogram Matrices It is possible to start with a rainflow matrix as the load input (for a single load case only) as opposed to a time history file. There are two ways to do this using PTIME: 1. Convert a load history into a rainflow matrix. Do this from Add an entry | rainflow Matrix. Select the time history file and press OK. The file will undergo rainflow cycle counting and a new matrix file will be created. This matrix file can then be selected in the spreadsheet on the Loading Info... form instead of a time history. The file will have a .cyh extension instead of a .dac extension. Hint:

A Fast Analysis (on the Job Control... form) does exactly this. It converts the load history to a rainflow matrix and then simply scales the histogram according to the stress or strain instead of doing a rainflow cycle count for each location for a single load case analysis.

2. Import an ASCII definition of a matrix. This is also done under Add an entry | rainflow Matrix, in a very similar fashion as reading an ASCII time history. The format of the ASCII file takes on a specific form however. It is possible to define the matrices in two forms:

• Range Bin - Mean Bin - Number of Cycles • Range Value - Mean Value - Number of Cycles As an example let us say we want to define a load to have the following spectrum:

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Number of Cycles

Range

Mean

5

2.0

0.0

10

1.0

0.5

10

1.0

-0.5

20

0.5

1.0

20

0.5

-1.0

431

432

Create a file with the following format and numbers: #V6.0 # Example using range_mean_data BINS=32 MEAN_MIN=-1.1 MEAN_MAX=1.1 RANGE_MIN=0 RANGE_MAX=2.1 RANGE_MEAN_DATA: 2 0 5 1 0.5 10 1 -0.5 10 0.5 1 20 0.5 -1 20 END_DATA

Note that the file defines the number of bins (32, 64, or 128), the maximum and minimum values on the x and y axes (range and mean) and then specifies the range and mean data that will be placed in the bins. The values in the RANGE_MEAN_DATA are , and . The maximum and minimum values on the x and y axes must be slightly larger than the largest specified range and/or mean value for any bin. This file results in the matrix shown to the side. Note:

The maximum and minimum values of the range and mean axes will determine the accuracy of the matrix. Each entry in the ASCII file must be able to fall into a unique bin or an error will occur on import.

Run PTIME and select Add an entry | rainflow Matrix. Then specify to input the matrix via an ASCII file and select the file that you created. Give the matrix the name “range” and import the file as Force in Newtons. Be sure to give it a description also. Then plot the entry. If you know what bins to place cycles in you can define the ASCII file in terms of #V6.0 # Example using bin_data BINS=32 MEAN_MIN=-1.1 MEAN_MAX=1.1 RANGE_MIN=0 RANGE_MAX=2.1 BIN_DATA: 31 15 5 16 9 10 16 23 10 8 2 20 8 32 20

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where BIN_DATA now replaces RANGE_MEAN_DATA and the data to be entered is , , and . If you import this file it should give you a matrix that looks equivalent to the one shown for our RANGE_MEAN_DATA values.

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19.7

Constant Amplitude Zero Mean Time Histories For constant amplitude zero mean time histories, there is an environment keyword that allows life to be calculated directly from the true max-min cycle, which is stored along with the matrix. Set the environment variable PFCONAMP to ON. Use the MENM utility to do this. Refer to Section 16.7 for details.

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MSC.Fatigue QuickStart Guide

APPENDIX

A

Glossary of Terms

■ Glossary Terms

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A.1

Glossary Terms Note: The terms and definitions in this appendix may have multiple meanings to different people. The definitions give here are as used in the context of this guide.

Amplitude Amplitude is half of the range of a cycle. It is the maximum less the minimum divided by two. σ σa σm

∆σ t

σ ma x – σ m i n σ a = ------------------------------2

August Woehler This German gentleman is probably the most famous of all fatigue researchers being the “Father of Fatigue” as many know him. He is responsible for the invention of the “stress-life” or “S-N” method of fatigue life prediction. See Stress-Life (S-N) (p. 453).

β-Solution See Compliance Function (p. 437).

Biaxial - Biaxiality Ratio For surface resolved stresses the two major principal August Woehler (1819-1914) stresses lie in the plane of the surface with the third principal stress being zero (normal to the surface). The principal stresses therefore, correspond to the X, Y, and Z=0 component stresses. The ratio of the minimum inplane stress divided by the maximum in-plane stress defines the biaxiality ratio. This ratio can take on any number between -1 and 1. Zero indicates a uniaxial state of stress with only one principal stress being non-zero.

Broad Band See Wide Band (p. 454).

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APPENDIX A Glossary of Terms

Compliance Function For crack growth analysis, a compliance function needs to be defined. This is also known as a β-function or a K-solution. It is defined based on the crack geometry and the dimension of the specimen. In physical terms it is simply a measure of how the compliance (stiffness or flexibility) of the structure changes as the crack propagates. These functions take on the form of look up tables in the software and can be defined in terms of a polynomial equation or selected from a standard library set. They are then used in the crack growth rate (Paris) equation to determine the stress intensity for any given stress cycle and, in turn, used to determine a incremental crack size. da m ------- = C ( ∆K ) Paris Equation dN ∆K = Yσ πa Stress Intensity Compliance Function

Component S-N This is an S-N curve which is tied to a specific component geometry and relates nominal stress (S) to life. The stress that is looked up on the S-N curve is not the actual stress at the failure location (in general) but the stress as measured in a location away from the failure. This is usually because of the impracticality of placing a strain gauge at the failure location. The S-N curve can only be used for components with the same geometry (and material) as that used to create the S-N curve. The components themselves are used as test specimens to create the S-N curves (such as a weld class). These types of S-N curves are not used to identify the location of a possible fatigue failure since this location is already identified by the nature of the S-N curve. Instead they are used to evaluate resistance to variation of the loading. These S-N curves are generally used when the failure mechanism is not well modeled with material S-N curves or other methods. They represent a more macro way of characterizing the failure mechanism by building into the curve all influences on the fatigue life, i.e., plasticity, geometry, residual stress, etc. The challenge when using these S-N curves with FE analysis is to know where the measured or reference location is, which is the stress that should be used to look up damage using these S-N curves.

Crack Initiation See Strain-Life (e-N) (p. 452).

Crack Growth (Propagation) See LEFM (p. 445).

Main Index

437

438

Critical Plane Analysis The principal stress tensor axis can rotate from time step to time step when subject to multiple load inputs. A fatigue analysis done at various angles is said to be a critical plane analysis. The plane that exhibits the most damage is said to be the critical plane. These types of analyses are typically illustrated using a polar plot of damage versus angle.

Cycles (cycle counting) A stress or strain cycle is one event that may or may not cause damage. A single constant A B C D amplitude, fully reversed sine or triangle wave E F that passes upwards positively through its mean G will register a stress cycle each time. Many H methods of counting cycles in a randomly D B varying signal have been adopted and F abandoned over the years. The rainflow cycle C counting method is the most widely accepted. H Another way to understand cycle counting is E A G illustrated by the diagram where a portion of a time signal is stood on end and then each point is mapped onto stress-strain space. This simulates loading and unloading where the material may yield producing a hysteresis loop. Each loop represents one stress cycle. Cycles with noticeable inner area are damaging and cycles which appear as straight lines are purely elastic and cause no damage. For any time varying load, all cycles will fall with in one outer, large cycle representative of the maximum and minimum of the signal. So in short, cycle counting counts the number of hysteresis loops and keeps track of their range/mean or maximum/minimum values.

Cyclic Properties Cyclic material properties are those that are obtained from a material test similar to a monotonic test with one big difference. The loading is reversed and cycled at various load levels. From these tests are derived the cyclic stress-strain curve and the strain-

Main Index

APPENDIX A Glossary of Terms

life curve used in crack initiation analysis. At each load level the stabilized hysteresis loop is extracted, its maximum stress/strain value extracted and plotted onto a single plot to create the cyclic stress-strain curve.

σ1 ε1 Test 1

σ3

σ3

σ2 ε2 Test 2

σ2 ε3

Test 3

σ1 ε1 ε2 ε3 Cyclic σ-ε Curve

Cyclic Hardening This is a behavior Cyclic Hardening σ exhibited by σ ε materials that, when subject to cyclic loading, actually strengthen with time. This is Stress Response Constant Amplitude Stress-Strain Strain Signal Mapping illustrated by the σ hysteresis loops becoming taller and skinnier on stress-strain space. The yield strength become greater.

Cyclic Softening This is a behavior Constant Softening exhibited by σ materials that, ε when subject to cyclic loading, weaken with time. This is illustrated Stress Response Constant Amplitude by the hysteresis Strain Signal loops becoming shorter and fatter on stress-strain space. The yield strength lessens.

Main Index

σ

Stress-Strain Mapping

439

440

da/dN Curve This is the crack growth rate (da/dN). It is a material characteristic and as such, is treated Region 3 as a material data set and is obtained by experiments. It relates the growth rate of a crack to stress intensity (∆K), or in other words, the driving force of the crack. There da/dN are three regions on a da/dN curve. Region Region 2 1 is the threshold region where the driving force is not great enough to grow a crack Region 1 (like a fatigue limit). Region 2 is the linear region of the curve where behavior is ∆K (stress intensity) described by the Paris equation. Region 3 is where static or fast fracture occurs as the driving force reaches or becomes very near to the fracture toughness of the material.

Damage (Log of Damage) Damage is the reciprocal of Life. Sometimes it is reported in log base(10) units mainly for convenient contour plotting.

Damage Summation Stress This is the mechanism of summing the damage from the various stress ∆D1 = N1 / Nf1 cycles. All cycles are identified using ∆D2 = N2 / Nf2 rainflow cycle counting. Damage due to each is determined from the appropriate damage curve such as Total Damage = Σ Ni / Nfi an S-N or strain-Life curve. Damage is then summed using the linear damage summation law as defined Cycles to Failure by Palmgen and Miner which simply states that each cycle causes a damage which is equal to 1/(number of cycles to failure at that load level). When you apply a series of cycles, damage is added up linearly until the total is unity (1) when failure is predicted. Fatigue damage is a non-linear process, but we find that if we have a fairly random repeated sequence it works satisfactorily.

Damage Tolerant This is a fatigue life design philosophy which adopts the crack growth method and is used in conjunction with the fail safe philosophy. A crack or flaw is assumed to exist and its growth rate determined to set up specific inspection periods to ensure that the flaw will not grow to any critical size between these inspections.

Main Index

APPENDIX A Glossary of Terms

Deterministic

Response

This is a loading event which can be determined at any point in time, such as a constant amplitude sine wave. Repeatable loading falls into this category. This is in opposition to a random load where no events can be determined at any given point in time or more specifically, where the next sequence of events cannot be determined from any previous events.

Time

Durability Durability is the conglomeration of all aspects that effect the life of a product and usually concerns itself with much more than just fatigue and fracture, but also loading conditions, environmental concerns, material characterizations, and testing simulations to name a few. A true product durability program in an organization that takes all of these aspects and more into consideration.

Elastic Elastic behavior refers to a component or material, which when subject to loading conditions that cause structural deformation, if removed, returns to its original state. No permanent state of deformation is left when the loads are removed. Linear elastic analysis denotes that as the loads vary, the responses vary in a linear and elastic manner relative to the loads. For instance if you double or triple the load, the responses will double or triple respectively. Whether the stresses exceed the yield or even the ultimate stress is not taken into account.

Elastic-plastic Correction See Neuber’s Rule (p. 447).

Endurance Limit This is similar to the fatigue limit and is an imposed limit of reversals on strain-Life curves above which the component is said to have infinite life. This limit, referred to as the material cut-off, is set to 2e8 reversals but can be changed by the user.

Failure Criterion The criterion that defines failure such as catastrophic failure into two or more pieces, until an engineering crack of 2mm appears, or until a crack reaches a critical size to be deemed unsafe. Understanding the failure criterion in a fatigue analysis is very important. The material properties used in any fatigue analysis, be it an S-N curve, strain-Life curve, or crack propagation da/dN curves, define the failure criterion.

Main Index

441

442

Fail Safe This is a fatigue life design philosophy which adopts the total life (S-N) method generally where failure cannot be tolerated. Therefore built in redundancy is generally used such that if a failure were to occur, the structure would fall into a state that it would survive until repair can be accommodated.

Fatigue This is a failure under a repeated or otherwise varying load which never reaches a level sufficient to cause failure in a single application. The initiation and growth of a crack, or growth from a pre-existing defect, until it reaches a critical size, such as separation into two or more parts

Fatigue Concentration Factor, Kf This is similar to the stress concentration, Kt, except it accounts for the fact that small notches have less effect on fatigue than is indicated by Kt. This has led to the idea of a fatigue concentration factor, Kf, which is normally less than Kt, being introduced and being used to replace Kt within Neuber’s rule. Kf is related to Kt according to Kf = ( 1 + ( Kt – 1 ) ) ⁄ ( 1 +

p' ⁄ r )

where p’ is a material constant dependent on grain size and strength and r is the notch root radius.

Fatigue Limit

Log Stress Range

This is a stress level below which no fatigue failures will occur. See Stress-Life (S-N) (p. 453).

b1 1

Transition Life

Fourier Analysis

b2 1

In simplistic terms, Fourier analysis is the ability to represent a finite length of time signal by the sum of a series of sine waves with varying amplitudes, frequencies, and phases.

Fatigue Limit Log Life

Fracture A fracture is the growth or propagation of a crack once it has been initiated. Fracture also denotes sudden breakage of a component or structure in two. However for the purposes of this manual it refers to the life prediction method of crack growth as implemented using LEFM. See LEFM (p. 445).

Main Index

APPENDIX A Glossary of Terms

Fracture Mechanics Triangle

crack length

The fracture mechanics triangle states that if any two of the three variables are known, through fracture mechanics and their relationships to one another, the other can be determined. stress intensity

Frequency Domain

stress

FFT Force

Force

The time domain relates a Frequency Domain variable (stress) to time Time Domain and describes how the variable changes with time. Time signals can also be represented in the time frequency frequency domain which relates the variable to frequency, describing how that variable changes with or is affected by frequency. The time and frequency domains present the same information in different ways, helping the engineer understand the effect a signal or response may have on a structure. Consider, for example, a random signal which when converted to the frequency domain shows only content at 10 Hz. This signal when applied to a structure with natural frequencies well above 10 Hz may not be damaging at sufficiently low enough levels. However if the structure has natural frequencies in the 10 Hz range, the signal would be far more damaging. Being able to view a signal in the frequency domain can alert an engineer to this potential danger.

Gaussian For a random signal and for most engineering purposes the amplitude Probability Density Function (PDF) will be approximately Gaussian. This means that the density distribution will take on a bell like curve as shown here where the highest levels of the signal are the least probable of occurring. See also Power Spectral Density (PSD) (p. 448).

Gerber Mean Stress This is a mechanism to correct for a non-zero mean stress range for the S-N method. See Mean Stress Correction (p. 446).

Goodman Mean Stress This is a mechanism to correct for a non-zero mean stress for the S-N method. This is a more conservative method than the Berber one. See Mean Stress Correction (p. 446).

Main Index

443

444

High Cycle Fatigue (HCF)

Strain

This is the ability of a component or structure to withstand or survive many stress cycles. S-N high cycle fatigue analysis applications deal completely in the high cycle regime and are not valid in the low cycle regime. The technical definition of high cycle versus low cycle fatigue is where the low cycle elastic and plastic strain-Life curves cross each other on the strain-Life plot. This is known as the transition life above which is high cycle Reversals (2N) fatigue and below which is low cycle fatigue. It is clear that above the transition life elastic events dominate and below it, plastic events dominate. S-N analysis does not compensate for plastic events in an adequate way as the strain-life method does and for this reason is not a good choice for low cycle fatigue problems. The strain-life method can handle both high and low cycle fatigue problems. The transition life is generally around 104 or 105 cycles and is material dependent.

Hysteresis This is a material behavior that is illustrated by loading a material beyond its yield point and then unloading it and perhaps reversing the load until it yields in compression and cycling. When the stress and strain are cross plotted, they create plots such as the one shown here. Each loop is a hysteresis loop. This phenomenon is know as the Baushinger effect after the German engineer that first documented this behavior of most metallic materials.

σ

ε

Inverse Fourier Transform The ability or methodology of converting a frequency domain signal back into the time domain by recreating the time signal from a power spectral density (PSD) function is called the Inverse Fourier Transformation. Because no phase information is kept with a PSD, random phases are created. The regenerated time signal will not be exactly the same as the original but will be statistically equivalent.

Main Index

APPENDIX A Glossary of Terms

Irregularity Factor

Stress (MPa)

Time History This is a parameter for describing or characterizing a process such as a time signal or a power spectral density function. For a time signal, it is defined as the ratio of time the number of times a signal passes upward in a positive manner through the mean of a signal divided by the number of 1 second peaks. In the frequency domain, the irregularity factor is determined from its = upward zero crossing moments. The irregularity factor takes on = peak values between zero and one, one describing a narrow band process and nonzero values describing wide or broad band processes. A value of unity describes a process whose peaks and adjacent valleys are roughly the same order of magnitude but of opposite sign whereas a value of near zero represents a signal that has an infinite number of peaks versus upward mean crossings, e.g., a dominate sine wave with noise superimposed on top of it.

K-Solution See Compliance Function (p. 437).

LEFM Linear Elastic Fracture Mechanics. This is the art of crack growth prediction as determined from linear elastic stresses. It assumes only a localized plastic zone around the crack tip and uses the stress intensity or driving force of the crack to determine crack growth rates according to the Paris equation.

Life (Log of Life) The Life (Log of Life) is the result reported as to how long a component or structure will last. This life can be reported in terms of stress cycles or reversals survived, however this is usually not a convenient way of reporting it. Time series are generally given some sort of fatigue equivalent units such as laps, miles, hours, missions, etc., which are more descriptive to a user in describing the life. A repeat of a time history may have many stress cycles but can be described as representative of, say, 30 times around a cobblestone test track. The life is then reported as laps. Because the computed life of a component can vary dramatically from location to location on the component itself, the life is often reported in log (base 10) units. This is convenient because the spread can be from some small finite number (1000) to infinite life (the cutoff being around 1018). This helps spread out contour bands on graphical plots for better visualization and for xy plots.

Local Strain See Strain-Life (e-N) (p. 452). Main Index

445

446

Low Cycle Fatigue (LCF) Low cycle fatigue is the inability of a component or structure to withstand or survive many stress cycles. See High Cycle Fatigue (HCF) (p. 444).

Material Cut-off See Endurance Limit (p. 441).

Material S-N This is an S-N curve that relates local stress (σ) to life. These types of curves are generally obtained through material tests of highly polished test coupons where the monitored stress is the stress experienced at the failure location. These type of S-N curves are geometry independent; that is, the S-N curves are valid for any geometry and are only dependent on the actual material that they represent. All plasticity modeling is built into the curve.

Mean Mean is the average value of a cycle or signal. It is the maximum plus the minimum divided by two for a simple constant amplitude oscillating signal as shown here. Note that the two small cycles in the stress-strain plot have the same strain range but have different mean stress. σ ma x + σ m in σ m = -------------------------------2

σ σa σm

∆σ t

Mean Stress Correction This is a technique for correcting or compensating for non-zero mean signals when looking up damage on damage curves that have been created with zero mean (R=0) signals in a test laboratory. Various methods exist for both the S-N and strain-life methods. Fracture mechanics uses different da/dN curves for different R-ratios.

Miner’s Constant Miner’s constant is the damage summation constant that defines failure, usually set to unity (1). See Damage Summation (p. 440).

Main Index

APPENDIX A Glossary of Terms

Monotonic Properties Monotonic material properties are those that are obtained from a material test. Test coupons are placed in servo-hydraulic machines and loaded in a single application of steady load increase through the yield point of the material and to ultimate fracture of material. From these tests come various material parameters such as Young’s Modulus (E), the yield strength (σy), and the ultimate tensile (UTS). The load is not reversed, nor is it cycled to obtain these properties.

σ−ε Curve

σ

Elastic Line

Stress

Ultimate Tensile Strength Yield

ε

Strain

Morrow Mean Stress A mechanism to correct for non-zero mean stress for the strain-life method. See Mean Stress Correction (p. 446).

Multiaxial Multiaxial means that the stress state is not uniaxial. More than one principal stress exists. The biaxiality ratio, ae, defined as the minimum in-plane stress divided by the maximum in-plane stress (for surface stresses), is non-zero. There are two different degrees of multiaxial stress states: proportional and non-proportional. Proportional multi-axial or proportional loading refers to the principal stresses always being in proportion to one another in magnitude and are stationary. Stationary means that the principal stress axes do not rotate significantly with time or in other words, the maximum and minimum principal stresses are always in the same direction. Nonproportional loading is the opposite of this where the two principal in-plane stresses are not proportional to each other at any given time, nor is the principal stress axis always in the same direction.

Narrow Band Narrow Band

This is a signal which contains frequency content predominantly at or around one frequency which when converted to the frequency domain appears as single peak spanning only a portion of the frequencies.

PSD Time History

frequency Hz

Neuber’s Rule This is one of a few mechanisms to correct for plasticity given only elastic stresses and strains. The Neuber method enables us to predict elastic-plastic stress and strain by providing a way of estimating the amount of stress and strain redistribution. You should remember that Main Index

∆σ

∆ε = σ +

Ε∆εe

2E

2

∆σ∆ε = E∆ε ∆εe2

σ, ε ∆εe

∆ε

1/n’

[ ∆σ ] 2K’

447

448

this is an approximation! Basically the elastic strain excursion is calculated from the FE model, and the stress is assumed to be ε*E. Then the elastic-plastic stress and strain excursions is estimated by drawing a rectangular hyperbola through this point and seeing where it intersects the hysteresis curve.

Non-proportional Loading See Multiaxial (p. 447).

Notch Correction This is a term that is adopted in the FE-fatigue world to signify the correction from purely elastic stresses and strains to elastic-plastic stresses and strains. See Elasticplastic Correction (p. 441). Historically the term comes from determination of stress at a notch while taking measurements away from the notch and using a stress concentration factor, Kt, knowing that the material has yielded in the notch area and an additional correction needs to be made to determine the true stress (and/or strain).

Paris Equation This is the main equation that governs the LEFM (crack growth) method and relates the crack growth rate (da/dN) to stress intensity (∆K). C and m are material constants.

da m ------- = C ( ∆K ) dN

Plasticity Fatigue does not generally involve major changes to the properties of the bulk of the material in a component. In most components that have failed by fatigue, the processes that lead to the fatigue failure are confined to the region around the crack tip. Fatigue is always caused by plastic deformation. Plasticity is an irreversible process of deforming the material beyond its yield point. Some who have experienced fatigue failures may say “there is no plastic deformation in my component,” or “the FEA results showed that all stresses were below yield.” If there is a fatigue failure, then there must be plastic deformation, even if it is confined to only a few grains, or to a very small region around the tip of a crack or a notch.

value2/Hz

Power Spectral Density (PSD) The term originated with electronic engineers in the 1940’s trying to characterize equipment noise. The PSD is a way of describing a random time signal. A random signal is random because there is no way of predicting a future section of the signal from previous sections. Therefore some sort of statistical method of describing these signals was devised. By taking a time signal, Frequency (Hz) squaring it, and taking its average you get what is called the mean square value. If the squared signal is passed through a low pass filter at various cut-off frequencies, the mean value Main Index

APPENDIX A Glossary of Terms

can be plotted as a function of frequency. The slope of this curve describes the density of the mean square with respect to frequency and is called a “spectrum.” The term “spectral density” comes from the fact that it is a property with respect to frequency such as a rainbow which is the variations of frequency in the colors of visible light. The term “power” dates back to the electrical engineers who used power as the key parameter. Dynamicists have simply adopted the term. In simple summary, a PSD is nothing other than an equivalent representation of a random time signal in a different domain, which has certain advantages over the time domain. In terms of Fourier analysis, the area under any infinitesimal strips of the PSD represents the mean square of the sine wave at that frequency where a time signal is made up of a number of sine waves summed together.

Probability Density Function (PDF) Two important Probability Density Functions (PDF) can be computed from a stress or strain time history. These are the amplitude and peak PDFs as shown. The best way to visualize these parameters is to draw tram lines horizontally through the time history and then count either the number of times the signal crosses the tram lines or the number of times a peak occurs in-between the tram lines. The complete PDFs are obtained by repeating this process for all horizontal levels in the signal. For most engineering purposes the amplitude PDF will be approximately Gaussian. Furthermore, for a narrow band process the peak PDF will be approximately equivalent to the Rayleigh PDF. A PDF, therefore, is the probability of a certain stress or strain level occurring and is represented as a density distribution.

Proportional Loading See Multiaxial (p. 447).

R-Ratio This is a measure of the mean stress or the mean of constant amplitude signal or the mean of a stress cycle. R = -1 is a fully reversed signal or a cycle with zero mean. R = 0 is a signal which goes from zero to a maximum value and returns to zero. R = infinity is the reverse where the signal goes from zero to a negative maximum value and back to zero.

Rainflow Cycle Counting See Cycles (cycle counting) (p. 438).

Main Index

449

450

Random Vibration

Response

This is excitation due to loading which is random in nature. That is to say that at any particular point in time it is impossible to determine anything specific about the loading. It can only be described by its statistics such a mean level, rms, standard Time deviation, etc. This is in opposition to a loading event which can be determined at any point in time, such as a constant amplitude sine wave. Random vibration is usually dealt with in the frequency domain by converting signals to power spectral density functions (PSDs).

Range Range is the total absolute magnitude between the maximum and minimum values of a cycle. Note that ∆σ = σ m ax – σ mi n the two small cycles in the stress-strain plot have the same stress and strain range but have different mean stress. σ σa σm

∆σ t

Reference Location When dealing with component S-N curves, this is the location on the test specimens used to create the S-N curve. The nominal stress axis of the S-N curve relates stress levels at this location to failure. When using a component S-N curve in conjunction with finite element models you must know the equivalent location (reference location) as only stress from this location relates to the S-N curve.

Regression Analysis Regression analysis is the art of taking measured data such as that for an S-N curve and determining an equation to describe the curve from the raw data, also called curve fitting.

Residual Stress This is a permanent stress that is left behind in a component or structure after unloading. Residual stress can be caused or induced in a number of ways such as shot peening, overloads, and manufacturing processes to name a few. Residual stresses can be tensile (positive) or compressive (negative) in nature and can be beneficial to bettering fatigue life (compressive) or detrimental (tensile).

Main Index

APPENDIX A Glossary of Terms

Root Mean Square (rms) By taking a time signal, squaring it, then taking the average, you get the mean square of the signal. If you take the square root of the mean square of the signal you get the root mean square (rms). The rms is a parameter that allows you to gauge the overall intensity of a signal relative to another random signal.

Safe Life This is a fatigue design philosophy which adopts the crack initiation method. In general it is applied to relatively inexpensive components which can easily be thrown away and replaced. In addition it is applied to structures or parts where the initiation of a crack takes up the majority of the life relative to the growth of the crack or where it is intolerable to have a crack in the structure. This philosophy generally produces fairly optimized structures and is used heavily in the ground vehicle industry. A failure of a component designed with this philosophy should not have catastrophic consequences.

Sample Rate When measuring a signal, the sample rate is the number of times you take a sample in a given period of time, usually one second. It is the frequency of samples in number/second. Sampling too slowly can cause important loading events to be missed.

Spectral Moments Spectral moments are used to obtain other statistical properties of the PSD. The n-th spectral moment m n of a PSD is defined by ∞

mn ( S ) =

n

S ( f ) ⋅ f df

–∞

Stress Concentration Factor, Kt This is a factor which relates stress at one point in a structure to stress at another point. For example, the stress concentration factor for a large plate with a hole is three (3). This relates the nominal stress (P/A) at an area away from the hole to Kt=3 Kt=1 + 2a/b σmax = Ktσ b =0; Kt=infinity the stress at the radius of the hole. Concentration factors have come about due to the fact that it is difficult to place a measurement device

Main Index

451

452

directly on the highest stressed area. Therefore some factor had to be established to convert measured response to actual responses at critical locations. In FE fatigue based analysis, Kt is generally taken as unity (1), since in this case we do know the stress at the critical area. In fact we know the local stress at all locations.

Stress Intensity K Controls the In simplistic terms, this is the stress around the tip driving force that causes a crack to propagate forward. It controls the stress around a crack tip and is Fracture Zone know as K (not to be confused with Kt or Kf, the stress and fatigue Plastic concentration factors). When the Zone magnitude of K reaches the fracture toughness of a material, failure occurs. K is a function of the crack length, a, the nominal or far field stress away from the crack, and other geometric dimensions of the component or structure and has units of stress-length1/2.

Strain Hardening See Cyclic Hardening (p. 439).

Strain-Life (ε-N) Strain (log units)

This is a fatigue life prediction method commonly referred to as “crack initiation,” or “local strain.” It only concerns itself with the initiation of a crack. A Typical The method is called “strain-life” because it relates ε-N Curve local strain to life. It is a fairly recent and well accepted method of fatigue life prediction brought about by the work of many but principally the Americans, Manson and Coffin in the mid 1950s. Reversals (log units) This work would not have been possible without the invention of the servo-hydraulic test machine. These machines allowed strains to be precisely controlled as opposed to stresses which are near impossible to control. Because of this the scatter in material data for the strain-life method is much less than that of the S-N method and a more accurate prediction of fatigue life can be made.

Strain Softening See Cyclic Softening (p. 439).

Main Index

APPENDIX A Glossary of Terms

Stress (log units)

Stress-Life (S-N)

This is a fatigue life prediction method commonly A Typical referred to as “total life” because it does not make a S-N Curve distinction between initiating or propagating a crack but instead considers only the total life of the component until failure into two or more pieces. The method is called “stress-life” because it relates nominal or local stress to life. It was the first method Life (log units) of fatigue life prediction conceived by the German, August Woehler in the late 1800’s due to his work in the railway industry. His famous rotating-bending tests gave rise to the concept of the S-N curve. These curves are generally denoted in log units and some materials exhibit a “fatigue limit,” a stress level below which no fatigue failures will occur.

Woehler’s Rotating-Bending Test Machine

STW Mean Stress This is a mechanism to correct for non-zero mean stress for the strain-life method. See Mean Stress Correction (p. 446).

Surface Resolved Stresses Surface resolved stresses are the stress on the surface of a structure or component which is said to be in a state of plane stress. The two principal stresses are in the plane of the surface while the third principal which is normal to the surface is zero. Finite element shell element models produce surface resolved stresses by default. However many solid element models produce stress results in elemental coordinate systems and need to be transformed into surface resolved stresses. Surface resolved stresses are needed to correctly calculate biaxiality ratios and perform multiaxial assessments

Total Life See Stress-Life (S-N) (p. 453).

Main Index

453

454

Transfer Function A Transfer Function is a way of relating one PSD in p u t × TF = PSD r es po n se quantity to another. In the frequency domain the structure is modeled by a linear Transfer Function relating input loads to output responses. The output from the model is expressed as a PSD. In frequency response analysis these Transfer Functions are determined by subjecting the model at the input load point to a series of sine waves with unit amplitude over the frequency range of interest. Multiplying the input PSD of load by the Transfer Function then gives the response PSD.

Uniaxial This is the stress state of a component or location in a component where only one principal stress exists, all others being zero. The biaxiality ratio, ae, defined as the minimum in-plane stress divided by the maximum in-plane stress (for surface stresses), is zero in this case. The principal stress is also stationary; that is, the principal stress is always in the same direction and not rotating such as a rod in tension.

White Noise White noise is a signal which contains frequency content from all frequencies and when converted to the frequency domain, is a constant line. A sharp sudden impact is also a form of signal which contains content at all frequencies.

White Noise PSD

Time History

frequency Hz

Wide Band Wide band is a signal which contains frequency content at more than one frequency which when converted to the frequency domain can appear as multiple spikes or as a broad curve spanning multiple frequencies.

Wide Band PSD

Time History

Main Index

frequency Hz

MSC.Fatigue QuickStart Guide

APPENDIX

B

Material Listing

■ Material Types ■ Material Listing ■ Alternative Names

Main Index

456

B.1

Material Types This table shows PFMAT material classes.

Table B-1 Material Type Numbers and Descriptions Number

Main Index

Description

Type undefined

1

Flake cast iron (FCI)

2

Ferritic cast iron with compacted graphite (FCICG)

3

Pearlitic cast iron with compacted graphite (PCICG)

4

Bainitic cast iron with compacted graphite (BCICG)

5

Ferritic cast iron with spheroidal graphite (FCISG)

6

Ferrite/pearlite cast iron with spheroidal graphite (FPCISG)

7

Pearlitic cast iron with spheroidal graphite (PCISG)

8

Bainitic cast iron with spheroidal graphite (BCISG)

9

Cast steel with less than 0.2% carbon (CSL2C)

10

Normalized cast steel with 0.2-0.4% carbon (NCS24C)

11

Quenched & tempered cast steel with 0.2-0.4% carbon (QTCS24)

12

Normalized cast steel with 0.4-0.7% carbon (NCS47)

13

Plain carbon wrought steel with < 0.2% carbon (PCWS)

14

Hot rolled/normalized plain carbon wrought steel, 0.2-0.4% carbon (HNPCWS24)

15

Quenched & tempered cast steel with 0.4-0.7% carbon (QTCS47)

16

Quenched & tempered plain carbon wrought steel, 0.2-0.4% carbon (QTPCWS24)

17

Hot rolled/normalized plain carbon wrought steel, 0.4-0.7% carbon (HNPCWS47)

18

Quenched & tempered plain carbon wrought steel, 0.4-0.7% carbon (QTPCWS47)

19

Normalized low alloy wrought steel (NLAWS)

20

Quenched & tempered low alloy wrought steel (QTHSLAWS)

21

Normalized Ni/Cr/Mo wrought steel (NNCMWS)

APPENDIX B Material Listing

Table B-1 Material Type Numbers and Descriptions Number

Main Index

Description

22

Quenched & tempered Ni/Cr/Mo wrought steel (QTNCMWS)

23

Austenitic stainless steel (ASS)

24

Ferritic stainless steel (FSS)

25

Martensitic stainless steel (MSS)

26

Annealed plain carbon wrought steel, 0.2-0.4% carbon (APCWS24)

27

Annealed plain carbon wrought steel, 0.4-0.7% carbon (APCWS47)

28

Normalized carbon/manganese steel (MCMS)

29

Quenched and tempered carbon/manganese steel (QTCMS)

30

Hardened chromium steel (HCS)

31

Quenched and tempered chromium steel (QTCS)

99

Steel of unknown heat treatment (STEEL)

100

Wrought aluminium (WA)

101

Wrought aluminium-copper alloy (WACA)

102

Wrought aluminium-manganese alloy (WAMNA)

103

Wrought aluminium-magnesium alloy (WAMGA)

104

Wrought aluminium-magnesium-silicon alloy (WAMGSA)

105

Wrought aluminium-zinc alloy (WAZA)

106

Cast aluminium alloy (CAA)

107

Wrought complex special purpose aluminum alloys (WCSPAA)

200

Wrought copper (WCU)

201

Wrought brass (WBR)

202

Wrought aluminium bronze (WABR)

203

Cupronickel (CUPNI)

204

Nickel silver (NIAG)

205

Wrought phosphor bronze (WPHBR)

206

Wrought copper beryllium (WCUBE)

207

Cast copper alloys (CCUA)

457

458

Table B-1 Material Type Numbers and Descriptions Number

Main Index

Description

300

Titanium alloy (TA)

400

Wrought magnesium alloys (WMGA)

401

Cast magnesium alloys (CMGA)

500

Fusible alloys, solders (FUSSOL)

600

Cast zinc alloys (CZINCA)

700

Wrought nickel alloys (WNIA)

701

Cast nickel alloys (CNIA)

800

Precious metals (PRECMET)

900

Clad materials (CLADMAT)

1000

Thermoplastics (THERPLAS)

1001

Thermosetting plastics (TSETPLAS)

APPENDIX B Material Listing

B.2

Material Listing This table lists all materials that are delivered with the MSC.Fatigue system and available datasets.

Table B-2 MSC.Fatigue Material Listing (MPa) Name

Main Index

E

UTS

Data types

150M19

2.07E5

682

E-N

2.25Cr1Mo

2.3E5

603

2014-T6_125_HF

7.27E4

483

2014_HV_0

7.17E4

200

M.S-N

2014_HV_T4

7.17E4

410

M.S-N

2014_HV_T6

7.17E4

470

M.S-N

2017_HV_T31

7.17E4

300

C.S-N

2024-T3

7.25E4

460

2024_HV_O

7.17E4

200

M.S-N

2024_HV_T3

7.17E4

450

M.S-N

2024_HV_T4

7.17E4

410

M.S-N

2024_HV_T851

7.17E4

410

M.S-N

2024_HV_T86

7.17E4

410

M.S-N

2219-T851

7E4

448

LEFM

2219_HV_T62

7.17E4

320

M.S-N

2219_HV_T81

7.17E4

410

M.S-N

2219_HV_T87

7.17E4

470

M.S-N

2789_370

1.628E5

436

E-N

2789_420

1.724e%

468

E-N

2789_600

1.737E5

591

E-N

2789_700

1.615E5

885

E-N

2789_800

1.62E5

890

E-N

2TA11

1.171E5

1233

E-N

3.5NCMV

2E5

1320

LEFM E-N

LEFM

LEFM

459

460

Table B-2 MSC.Fatigue Material Listing (MPa) Name

Main Index

E

UTS

Data types

3003_HV_H14

7.17E4

200

M.S-N

3003_HV_H16

7.17E4

200

M.S-N

3003_HV_H18

7.17E4

220

M.S-N

3004_HV_H34

7.17E4

215

M.S-N

3004_HV_H38

7.17E4

295

M.S-N

3004_HV_O

7.17E4

200

M.S-N

300M

2.07E5

1900

LEFM

316

1.9E5

590

LEFM

349S52

1.9E5

991

E-N

352S52

1.735E5

1027

E-N

5052-H32

6.96E4

231

E-N

5052_HV_H34

7.17E4

215

M.S-N

5052_HV_H38

7.17E4

295

M.S-N

5052_HV_O

7.17E4

200

M.S-N

5056_HV_CON

7.17E4

260

C.S-N

5083_114_CF

6.9E4

414

5083_87_CF

6.9E4

385

E-N

526M60

2.02E5

939

E-N

5454_NONE_CF

6.9E4

334

E-N

605M30

2E5

705

E-N

605M36

2.07E5

835

E-N

6061-T6 80 HF

7.27E4

340

E-N

6061-T6_NONE_CF

6.9E4

389

E-N

6061-T6_NONE_SHEET

6.96E4

314

E-N

6061_HV_O

7.17E4

150

M.S-N

6061_HV_T4

7.17E4

215

M.S-N

6061_HV_T6

7.17E4

305

M.S-N

APPENDIX B Material Listing

Table B-2 MSC.Fatigue Material Listing (MPa) Name

Main Index

E

UTS

Data types

7075-T6

7.09E4

558

LEFM

7075_HV_O

7.17E4

220

M.S-N

7075_HV_T6

7.17E4

570

M.S-N

709M40

2.1E5

781

E-N

7175-T73_NONE_HF

7.13E4

524

E-N

722M24

2.05E5

976

E-N

817M40

2E5

1277

E-N

826M31

2E5

1209

E-N

835M30

2E5

1550

835M30_V

1.943E5

1034

A533B

2E5

552

AISI1012

2E5

333

E-N

AISI1020

2E5

416

E-N

AISI_4340

2E5

1700

alphafe

2.1E5

1700

E-N

ASTMA536

1.447E5

480

E-N

B40PK

2E5

394

E-N

B40PO

2E5

438

E-N

B50XF

2E5

486

E-N

B50XK-CR

2E5

461

E-N

B50XK-HR

2E5

450

E-N

B55XF

2E5

488

E-N

B60RO

2E5

503

E-N

B80RK

2E5

610

E-N

B80XF

2E5

645

E-N

Beryllium

2.894E5

323

E-N

bs1452-260

1.253E5

277

E-N

LEFM

LEFM E-N LEFM

LEFM LEFM

461

462

Table B-2 MSC.Fatigue Material Listing (MPa) Name

Main Index

E

UTS

Data types

BS376_Nickel

2.068E5

366

E-N

BS4360-43A

2.07E5

486

E-N

BS4360-43C

2.07E5

478

E-N

BS4360-43D

2.07E5

490

E-N

BS4360-50D

1.914E5

480

E-N

classB

2.07E5

500

C.S-N

classC

2.07E5

500

C.S-N

classD

2.07E5

500

C.S-N

classE

2.07E5

500

C.S-N

classF

2.07E5

500

C.S-N

classF2

2.07E5

500

C.S-N

classG

2.07E5

500

C.S-N

classW

2.07E5

500

C.S-N

Cold_rolled_sheet

2E5

303

E-N

Copper

1.136E5

206

E-N

DP1

2E5

659

E-N

DP2

2E5

753

E-N

EIBSG1400

1.75E5

1407

E-N

EICG315

1.51E5

315

E-N

EICG400

1.5E5

404

E-N

EICG493

1.63E5

493

E-N

EN24V

1.902E5

1047

E-N

EZ33A_HV_T5

4.4E4

140

FeE255TM

2E5

475

E-N

FeE37D

2E5

388

E-N

FeE420TM

2E5

490

E-N

FeE52D

2E5

550

E-N

M.S-N

LEFM

APPENDIX B Material Listing

Table B-2 MSC.Fatigue Material Listing (MPa) Name

Main Index

E

UTS

Data types

HSLA4

2E5

486

E-N

HT-30

7.1E4

355

E-N

HY130

2E5

1010

LEFM

HY80

2E5

735

LEFM

HYBRID_CASTIRON

1.51E5

296

E-N

hypress20

2E5

445

E-N

hypress23

2E5

437

E-N

hypress26

2E5

523

E-N

hypress29-4

2E5

544

E-N

hypress29-8

2E5

539

E-N

IMI685

1.2E5

955

INC718

2.041E5

1304

E-N

MANTEN

2.034E5

552

E-N

MANTEN_MSN

2.034E5

600

M.S-N

MANTEN_SN

2.034E5

600

C.S-N

Mild_Steel

2E5

462

E-N

Nitro

2E5

483

E-N

Nitro-sa

2E5

648

E-N

Rephos

2E5

421

E-N

RQC100

2.034E5

863

E-N

LEFM

RQC100_MSN

2.034E5

800

M.S-N

RQC100_SN

2.034E5

800

C.S-N

RQT501

2E5

590

E-N

LEFM

RQT701

2E5

825

E-N

LEFM

RR58

7.5E4

450

SAE1006_85A_HR

2.07E5

318

E-N

SAE1006_85B_HR

2.07E5

318

E-N

LEFM

LEFM

LEFM

463

464

Table B-2 MSC.Fatigue Material Listing (MPa) Name

Main Index

E

UTS

Data types

SAE1006_85_HR

2.07E5

318

E-N

SAE1008_91_HR

2.07E5

363

E-N

SAE1015_80_NORM

2.07E5

415

E-N

SAE1018_106_HR

2.07E5

354

E-N

SAE1018_118_QT

2.07E5

496

E-N

SAE1018_209_QT

2.07E5

696

E-N

SAE1020_107_HR

2.07E5

441

E-N

SAE1020_108_ANLD

2.07E5

392

E-N

SAE1030_128A_HR

2.07E5

454

E-N

SAE1030_128_HR

2.07E5

454

E-N

SAE1035_169_CON

2.1E5

550

SAE1045_225_ANLD

2.07E5

751

E-N

SAE1045_390_QT

2.07E5

1343

E-N

SAE1045_450_QT

2.07E5

1584

E-N

SAE1045_500_QT

2.07E5

1956

E-N

SAE1045_595_QT

2.07E5

2239

E-N

SAE1045_705_QT

2.07E5

2067

E-N

SAE1045_HV_HR

2.07E5

671

E-N

SAE1050_189_CON

2.1E5

637

M.S-N

SAE1055_251_CON

2.1E5

860

M.S-N

SAE1080_371_QT

2.07E5

1298

E-N

SAE1080_410_QT

2.07E5

1432

E-N

SAE1080_421_AUST

2.07E5

1349

E-N

SAE1315_155_CON

2.1E5

530

SAE1522_289_HR

2.07E5

1005

E-N

SAE1522_304_HR

2.07E5

1088

E-N

SAE1541_362_QT

2.07E5

1200

E-N

M.S-N

M.S-N

APPENDIX B Material Listing

Table B-2 MSC.Fatigue Material Listing (MPa) Name

Main Index

E

UTS

Data types

SAE1561_234_HR

2.07E5

836

E-N

SAE2310_138_CON

2.1E5

480

M.S-N

SAE2335_217_CON

2.1E5

745

M.S-N

SAE30304

1.87E5

670

SAE4130_259_QT

2.07E5

895

SAE4130_267_CON

2.1E5

912

SAE4130_366_QT

2.07E5

1426

E-N

SAE4142_380_QT

2.07E5

1412

E-N

SAE4142_400_QT

2.07E5

1550

E-N

SAE4142_450A_QT

2.07E5

1929

E-N

SAE4142_450_QT

2.07E5

1757

E-N

SAE4142_475A_QT

2.07E5

2032

E-N

SAE4142_475_QT

2.07E5

1929

E-N

SAE4142_560_QT

2.07E5

2239

E-N

SAE4142_670_QT

2.07E5

2446

E-N

SAE4340_242_HR

2.07E5

826

E-N

SAE4340_350A_QT

2.07E5

1171

E-N

SAE4340_350B_QT

2.07E5

1171

E-N

SAE4340_350C_QT

2.07E5

1240

E-N

SAE4340_409_QT

2.07E5

1467

E-N

SAE5160_434_QT

2.07E5

1584

E-N

SAE52100_517_H

2.07E5

2011

E-N

SAE8630_254_NORM

2.07E5

785

E-N

SAE8640_361_QT

2.07E5

1373

E-N

SAE9262_260_NORM

2.07E5

923

E-N

SAE9262_271_QT

2.07E5

999

E-N

sra_60

2E5

531

E-N

LEFM E-N M.S-N

465

466

Table B-2 MSC.Fatigue Material Listing (MPa) Name

Main Index

E

UTS

Data types

sra_70

2E5

570

E-N

st00

2.1E5

347

E-N

Ti-6Al-4V

1.2E5

986

unsg10200

2E5

393

1.400...

2E5

496

Sp.Wld

1.4301_III...

1.875E5

670

Sp.Wld

1.4301_IIIC

2E5

670

Sp.Wld

1.4589

2E5

523

Sp.Wld

AlMg5Mn

7E4

300

Sp.Wld

FePo4

2E5

313

Sp.Wld

spot_nugget_generic

2.1E5

500

Sp.Wld

spot_sheet_generic

2.1E5

500

Sp.Wld

ZSTE380

2E5

484

Sp.Wld.

LEFM E-N

APPENDIX B Material Listing

B.3

Alternative Names This table lists all materials delivered with the MSC.Fatigue system and any alternative names by which they may be known.

Table B-3 MSC.Fatigue Material Alternative Names SAE (USA)

DIN(German)

W.Nr.(German)

British

Steels

Main Index

SAE1006_85A_HR

D8-2

1.0313

040A04,En2A

SAE1006_85B_HR

D8-2

1.0313

040A04,En2A

SAE1006_85_HR

D8-2

1.0313

040A04,En2A

SAE1008_91_HR

St13

1.0333

050A04

SAE1015_80_NORM

C15

1.0401

050A15

SAE1018_106_HR

-

-

080A17

SAE1018_118_QT

-

-

080A17

SAE1018_209_QT

-

-

080A17

SAE1020_107_HR

C22

1.0402

070M20,En3

SAE1020_108_ANLD

C22

1.0402

070M20,En3

SAE1030_128A_HR

-

-

080A30,En5B

SAE1030_128_HR

-

-

080A30,En5B

SAE1035_169_CON

Cm35

1.1180

060A35

SAE1045_225_ANLD

Ck45

1.1191

060A45

SAE1045_390_QT

Ck45

1.1191

060A45

SAE1045_450_QT

Ck45

1.1191

060A45

SAE1045_500_QT

Ck45

1.1191

060A45

SAE1045_595_QT

Ck45

1.1191

060A45

SAE1045_705_QT

Ck45

1.1191

060A45

SAE1045_HV_HR

Ck45

1.1191

060A45

SAE1050_189_CON

C53

1.210

060A52

SAE1055_251_CON

C55

1.0535

070M55,En9

SAE1080_371_QT

-

-

060A81

467

468

Table B-3 MSC.Fatigue Material Alternative Names SAE (USA)

Main Index

DIN(German)

W.Nr.(German)

British

SAE1080_410_QT

-

-

060A81

SAE1080_421_AUST

-

-

060A81

SAE1315_155_CON

-

-

-

SAE1522_289_HR

20Mn5

1.1133

120M19

SAE1522_304_HR

20Mn5

1.1133

120M19

SAE1541_362_QT

36Mn5

1.1167

150M36,En15B

SAE1561_234_HR

-

-

-

SAE2310_138_CON

-

-

708A30

SAE2335_217_CON

-

-

-

SAE30304

X5CrNi18_9

1.4301

304S16,En58E

SAE4130_259_QT

-

-

708A30

SAE4130_366_QT

-

-

708A30

SAE4130_267_CON

-

-

-

SAE4142_380_QT

42CrMo4

1.7225

708A42,En19C

SAE4120_400_QT

42CrMo4

1.7225

708A42,En19C

SAE4142_450A_QT

42CrMo4

1.7225

708A42,En19C

SAE4142_450_QT

42CrMo4

1.7225

708A42,En19C

SAE4142_475A_QT

42CrMo4

1.7225

708A42,En19C

SAE4142_475_QT

42CrMo4

1.7225

708A42,En19C

SAE4142_560_QT

42CrMo4

1.7225

708A42,En19C

SAE4340_HV_NONE

-

-

-

SAE4340_242_HR

40CrNiMo6

1.6565

817M40,En24

SAE4340_350A_QT

40CrNiMo6

1.6565

817M40,En24

SAE4340_350B_QT

40CrNiMo6

1.6565

817M40,En24

SAE4340_350C_QT

40CrNiMo6

1.6565

817M40,En24

SAE4340_409_QT

40CrNiMo6

1.6565

817M40,En24

AISI4340M_HV-NONE

-

-

-

APPENDIX B Material Listing

Table B-3 MSC.Fatigue Material Alternative Names SAE (USA)

DIN(German)

W.Nr.(German)

British

SAE5160_434_QT

55Cr3

1.7176

527A60,En48

SAE52100_517_H

100Cr6

1.3505

539A99,En31

SAE8630_254_NORM

30NiCrMo2_2

1.6545

-

SAE8640_361_QT

40NiCrMo2_2

1.6546

-

SAE9262_260_NORM

60SiCr7

1.7108

-

SAE9262_271_QT

60SiCr7

1.7108

-

-

-

-

835M30,En30B

-

St52

-

BS4360-50D

ASTM A542 Class 2,3

2.25Cr1Mo

-

-

-

3.5NiCrMoV

-

-

316_S/S

-

-

-

A553B

-

-

-

HY80

-

-

-

HY130

-

-

-

MANTEN

-

-

-

MANTEN_SN

-

-

-

NAMTEN_MSN

-

-

-

RQC100

-

-

-

RQC100_SN

-

-

-

RQC100_MSN

-

-

-

RQT501

-

-

-

RQT701

-

-

-

Aluminum Alloys and Other Light Alloys

Main Index

2025_T3

-

-

-

2219_T851

-

-

-

5056_HV_CON

Al_Mg5

3.3555

-

469

470

Table B-3 MSC.Fatigue Material Alternative Names SAE (USA)

Main Index

DIN(German)

W.Nr.(German)

British

2014-T6_125_HF

-

-

-

6061-T6_80_HF

-

-

-

5052-H32

-

-

-

6061-T6_NONE_SHEET

-

-

-

6061-T6-NONE_CF

-

-

-

2014_HV_O

Al_Cu4_Si_Mg

3.1255

-

2014_HV_T4

Al_Cu4_Si_Mg

3.1255

-

2014_HV_T6

Al_Cu4_Si_Mg

3.1255

-

2017_HV_T31

Al_Cu4_Si_Mg

3.1355

-

2024_HV_O

Al_Cu_Mg2

3.1355

-

2024_HV_T3

Al_Cu_Mg2

3.1355

-

2024_HV_T851

Al_Cu_Mg2

3.1355

-

2024_HV_T4

Al_Cu_Mg2

3.1355

-

2024_HV_T86

Al_Cu_Mg2

3.1355

-

2219_HV_T87

-

-

-

2219_HV_T81

-

-

-

2219_HV_T62

-

-

-

3003_HV_H18

-

-

-

3003_HV_H16

-

-

-

3003_HV_H14

-

-

-

3004_HV_H38

-

-

-

3004_HV_H34

-

-

-

3004_HV_O

-

-

-

5052_HV_H38

-

-

-

5052_HV_H34

-

-

-

5052_HV_O

-

-

-

5083_114_CF

-

-

-

APPENDIX B Material Listing

Table B-3 MSC.Fatigue Material Alternative Names SAE (USA)

W.Nr.(German)

British

5083_87_CF

-

-

-

5454_NONE_CF

-

-

-

6061_HV_O

-

-

-

6061_HV_T4

-

-

-

6061_HV_T6

-

-

-

7075_HV_O

Al_Zn_Mg_Cu 1.5

-

-

7075_HV_T6

Al_Zn_Mg_Cu 1.5

-

-

7175-T73_NONE_HF

-

-

-

HT30

-

-

-

RR58

-

-

-

EZ33A_HV_T5

-

-

-

TI-6Al-4V

-

-

-

IMI685

-

-

-

Weld Geometries BSB5400:CLASS B BSB5400:CLASS C BSB5400:CLASS D BSB5400:CLASS E BSB5400:CLASS F BSB5400:CLASS F2 BSB5400:CLASS G BSB5400:CLASS W

Main Index

DIN(German)

471

472

Main Index

MSC.Fatigue QuickStart Guide

APPENDIX

C

Support

■ Where to Get Help ■ Technical Support Centers ■ MSC Offices

Main Index

474

C.1

Where to Get Help If you have a question about MSC.Fatigue, try finding the solution in our online help. It’s easy to find with our context-sensitive and topical help system. Once in the help system, you can find any related topic using the hypertext (indicated by red-colored text); or you can access any document in the system using the Navigation Menu, which appears at the top of every page. The on-line help system consists of a large group of view-only documents containing hypertext commands that link MSC.Fatigue to the help system, and link the documents to each other to display related information. These hypertext documents allow you to explore information nonsequentially, using the paths provided via hypertext. The hypertext is indicated with red-colored text. This system was designed to make it easy to obtain quick-access to the documentation while using MSC.Fatigue.

Accessing Help from a Form If you need help interpreting the buttons on a form or figuring out which step to take first, move the cursor onto the form and press the F1 key. The help system will display the appropriate page of the on-line help, describing the form and indicating what you need to do to continue.The F1 key works in MSC.Fatigue Pre&Post. Other MSC.Fatigue modules also have on-line help which is accessed by pressing the help button on the form or from a pulldown menu. Some of these use Adobe Acrobat PDF files.

Main Index

APPENDIX C Support

C.2

Technical Support Centers If your questions cannot be answered in the extensive on-line help system, please call the technical support center nearest you. We are ready to help you. To better answer your questions, we request that you provide us with the information outlined in Preparing to Call the Hotline (p. 476). Contact Support Services using any of the following options.

Telephone and Fax: United States

Surrey, England

Phone: (800) 732-7284

Phone: +44 (1276) 601911 FAX: +44 (1276) 601909

Munich, Germany

Tokyo, Japan

Phone: +49 (89) 43 19 87-277 FAX: +420 (5) 4517 6107

Phone: +81 (3) 3505 02 66 FAX: +81 (3) 3505 09 14

Rome, Italy

Toulouse, France

Phone: +39 06 52. 79.931 FAX: +39 06 52. 27.32.32

Phone: +33 (5) 34 60 44 80 FAX: +33 (5) 34 60 46 81

Moscow, Russia

Gouda, The Netherlands

Phone: +7095 -236 -61 -77 FAX: +7095 -232 -3575

Phone: +31 (18) 2543700 FAX: +31 (18) 2543707

Madrid, Spain Phone: +34 -915 -560919 FAX: +34 -915 -567280

Email: [emailprotected]

World Wide Web: www.mechsolutions.com, and click on Support In addition to our technical support centers, MSC has developed a broad network of local offices, staffed by a knowledgeable team, who can provide product assistance of any kind. For the location of the office nearest you, call (800) 732-7284 or refer to the MSC Offices.

Main Index

475

476

Preparing to Call the Hotline When you call the Support Hotline (1-800-732-7284), the phone will be answered by an auto-attendant. If you have previously called the Support Hotline, you may have been assigned a PIN number. Please have it handy. If you have an open log with a support engineer, please have the Log ID number available also. If this is your first time contacting Support, you will be assigned a PIN number. Please be prepared to provide the following information: Name Phone number Fax number E-mail address Company name and address Name of product you are using Version number of the product

Category of call System information Manufacturer (Sun, SGI, IBM, HP, NT, etc.) Model or chip (e.g., r10000 for SGI) OS Version (Solaris 2.5, IRIX 6.2, AIX...) Graphics board (for graphics problems) RAM (for hardware problems) Available disk space (for hardware problems) Description of the problem

If all engineers are busy, you will have the option of waiting on hold for the next available engineer, or you may leave a message for a callback from an engineer. We recommend that you wait on hold whenever possible.

Main Index

APPENDIX C Support

C.3

MSC Offices MSC.Software Corporation is the industry leader for engineering analysis solutions. For more detailed information on any of our advanced analysis programs, contact your local MSC representative. MSC.Fatigue Support North America

Corporate

2 MacArthur Place Santa Ana, CA 92707 (800) 732-7284

MSC.Software Corporation 2 MacArthur Place Santa Ana, CA 92707 USA Telephone: (800) 345-2078 Fax: (714) 784-4056

Email [emailprotected]

Europe MSC.Software GmbH Am Moosfeld 13 81829 Munich, Germany Telephone: (49) (89) 43 19 87 0 Fax: (49) (89) 43 61 71 6

Asia Pacific MSC.Software Japan Ltd. Shinjuku First West 8F 23-7 Nishi Shinjuku 1-Chome, Shinjuku-Ku Tokyo 160-0023, JAPAN Telephone: (81) (3)-6911-1200 Fax: (81) (3)-6911-1201

Worldwide Web www.mscsoftware.com

Main Index

477

478

Main Index

MSC.Fatigue Quick Start Guide

I

N

D

E

X

MSC.Fatigue QuickStart Guide

I N D E X MSC.Fatigue QuickStart Guide,

A ABAQUS results file import, 11 add load signals, 394 addition, 384 advanced fatigue analysis and display, 403 advanced loading manipulation, 384 alternative material names, 467 amplitude, 436 amplitude distribution analysis, 397 analysis modules, 22 Analysis Using Transient Results, 296 angle distribution, 203 angle spread, 201, 321 ANSYS results file import, 12 arithmetic manipulation, 384 ASCII to binary conversion, 420 assumptions, 2, 312 August Woehler, 436 auto spectral density, 398 averaging, 428

B band pass filter, 400 batch entry, 77 batch operations, 88 Baushinger effect, 444 beta-solution, 129, 436 biaxiality analysis, 189, 198 biaxiality indicators, 200 biaxiality plots, 202 biaxiality ratio, 199, 200, 436 broad band, 436 Butterworth filtration, 400

Main Index

C calculate normals, 187 changing colors, 418 colors, 418 compliance, 22, 129, 178, 436, 437 compliance function library, 130 component S-N, 70 component S-N curve, 74, 437 correlated loading, 314 correlation techniques, 283 CPU times, 194 crack growth, 8, 437 crack growth analysis, 128, 159, 177 crack growth data analysis, 410 crack growth rate, 136, 440 crack initiation, 3, 8, 437 crack initiation analysis, 90, 112, 157, 182, 404 crack lengths, 134 crack propagation rectangle, 141 critical location analysis, 404 critical location identification, 430 critical plane analysis, 208, 438 cross correlation terms, 400 cross platform file conversion, 422 cross plots, 201 cross-correlation terms, 314 customer support, 474 cut and paste signals, 393 cycle by cycle growth, 140 cycle counting, 39, 54, 101, 104, 438 cycles and damage analysis, 405 cycles file lister, 406 cyclic hardening, 96, 439 cyclic material properties, 4, 438 cyclic softening, 96, 439 cyclic stress-strain curve, 95

480 INDEX

D da/dN curve, 136, 179, 440 DAC to RPC translation, 421 damage, 440 damage histograms, 175 damage summation, 54, 60, 440 damage tolerant, 6, 440 data management, 22 definitions, 3 design optimization, 85, 120, 319 design philosophies, 6, 150 deterministic, 441 divide load signals, 394 division, 384 durability, 3, 441

E elastic, 441 elastic-plastic correction, 104, 441 element centroidal calculations, 427 Endo, 60 endurance limit, 156, 441 environments, 136 executables, 22 external results files, 18 extract response PSD, 320 extract time history, 429

F factor-of-safety analysis, 154 fail safe, 6, 442 failure criterion, 90, 441 fast analysis, 61, 194 fast Fourier filtering, 398 fast fourier transform, 302 fatigue, 3, 442 fatigue concentration factor, 442 fatigue equivalent units, 80, 124, 445 fatigue limit, 442 file conversion, 24 file import, 31 file translation utilities, 420 files, 147

Main Index

filters, 398, 400 finite element methods, 5 finite element results, 31 finite element results import, 9 five box trick, 4 formula processor, 391 Fourier analysis, 442 fracture, 3, 128, 159, 177, 442 fracture mechanics triangle, 130, 443 frequency domain, 443 frequency domain life estimation, 303 frequency resolution, 334 frequency response analysis, 314, 400 frequency response FE analysis, 302, 324

G gauge definition file, 272 gauge group, 273 gauge tool, 269 Gaussian, 312, 443 geometry information, 4 Gerber mean stress, 443 glossary, 436 Goodman mean stress, 443 graphic colors, 418 graphical display, 23, 36 graphical picking, 169 groups, 94, 153, 169, 179, 185

H header/footer manipulation, 402 help, 474 high cycle fatigue, 99, 121, 444 high pass filter, 400 histogram import, 431 histogram matrix, 62 histogram plots, 175 Hoffman-Seeger, 205 hotline, 476 hysteresis, 444

I I-DEAS Master Series universal file import,

INDEX

19 import histogram, 431 input load PSDs, 307, 314 interpolate crack sizes, 142 introduction, 2 inverse Fourier transform, 302, 444 irregularity factor, 445

J job submittal, 42

K k-solution, 129, 436, 445

L LEFM, 128 life, 445 life contour plots, 43, 84 life estimation process, 7 life prediction methods, 5 linear elastic fracture mechanics, 128, 445 linear superposition, 173 load histories, 39 load manipulation, 23 loading database, 39, 56 loading information, 4 load-strain relation, 7 local strain, 3 local strain analysis, 90, 112, 182 local strain method, 445 low cycle fatigue, 99, 121, 446 low pass filter, 400

M MARC results file import, 12 marker plots, 212, 223 Masing’s hypothesis, 105 material cut-off, 156, 446 material information, 4 material listing, 459 material names, 467

Main Index

material parameter, 205 material scatter, 65 material S-N curve, 446 material tests, 97 material types, 456 materials database, 35 Matsuishi, 60 mean stress, 446 mean stress correction, 34, 54, 65, 120, 446 Miner’s constant, 446 Miner’s rule, 440 modal analysis, 337 modal superposition method, 300 modules, 22 monotonic material properties, 4, 76, 447 Morrow mean stress, 121, 447 MSC offices, 477 MSC.Nastran model import, 10 MSC.Nastran results import, 9 MSC.Patran Advanced FEA file import, 11 MSC.Patran FEA result file access/import, 20 multiaxial assessment, 182, 198 multiaxial fatigue analysis, 409 multiaxial fatigue analyzer, 289 multiaxial loading, 199 multiaxial stress state, 447 multiaxiality, 204 multi-channel creator/editor, 385 multi-file cut and paste, 393 multi-file display, 171, 192 multi-file manipulation, 394 multiple loads, 166, 182, 313 multiplication, 384 multiply load signals, 394

N narrow band signal, 447 Neuber’s rule, 105, 447 nitriding, 123 non-linear static FE analysis, 113 non-proportional loading, 199, 205 notch correction, 7, 105, 108, 448 notches, 134

481

482 INDEX

O offices, 477 optimization, 44, 144 output a response PSD, 320 OUTPUT2 file import, 9

P Palmgren-Miner rule, 60 Paris equation, 448 PATRAN neutral and results file import, 13 peak-valley slicing, 195 plasticity, 448 plotting signals, 57 polar plots, 207, 224 power spectral density (PSD), 448 power spectrum, 302 principal stress axis, 201, 321 probabilistic nature of fatigue, 65 probability density function (PDF), 449 proportional loading, 199, 205 PSD matrix file, 315

R rainflow cycle counting, 54, 60, 104, 449 random, 312 random vibration, 450 random vibration FE analysis, 345 range, 450 reference location, 74, 153, 450 regions, 94 regression analysis, 34, 450 residual stress, 112, 118, 125, 450 response PSDs, 320, 341, 345 result listings, 44 results extraction, 274 results tabular listing, 85 root mean square (rms), 451 rosette analysis, 285 r-ratio, 449

S safe life, 6, 451

Main Index

sample rate, 39, 451 sensitivity analysis, 44, 86, 176, 225 sequence effects, 140, 407 shot peening, 124 signal reconstruction, 420 signal statistics, 284 S-N analysis, 30, 54, 70, 152, 168, 403 S-N curve, 453 S-N method, 3 software strain gauges, 268, 282 spot weld analysis, 211 spot weld definitions, 212 spot weld S-N curves, 216 static analysis, 32 static FE results, 299 stationarity check, 320, 344 stationary, 312 statistical analysis of signals, 401 strain hardening, 96, 452 strain softening, 96, 452 strain-life, 3 strain-life analysis, 90, 112, 157, 182 strain-life curve, 98, 452 strain-life relation, 7 stress concentration, 442, 451 stress concentration library, 411 stress intensity, 452 stress range, 450 stress results, 32 stress vs. strain, 108 stress-life, 3 stress-life analysis, 152, 168 stress-life curve, 453 stress-life vs. strain-life, 100 stress-strain curve, 95 stress-strain tracking, 105 STW mean stress, 121, 453 subtract load signals, 394 subtraction, 384 superposition of loads, 173 surface conditions, 94, 123 surface nodes, 185 surface resolved stress, 186, 453

INDEX

T tabular result listing, 224 technical support, 475 time correlated damage analysis, 407 total life, 3, 8, 453 total life analysis, 30, 54, 70, 152, 168 transfer function, 341, 454 transient FE analysis, 336

U uncorrelated loading, 314 uniaxial fatigue analyzer, 288 uniaxial loading, 199 uniaxial stress, 454 units, 191 universal file import, 19 using strains, 108 utilities, 24, 380

V vibration fatigue analysis, 324, 336, 408

W waterfall plots, 422 welds, 152 what if analysis, 44 white noise, 454 wide band, 454

X XDB file import, 10

Y Young’s modulus, 108

Main Index

483

484 INDEX

Main Index

MSC.Fatigue Quick Start Guide - PDFCOFFEE.COM (2024)

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