Troubleshooting Finite-Element Modeling with Abaqus (eBook)

Foreword by Dr. Sonell Shroff 6
Foreword by Gautam Puri 8
Foreword by Prof. David Bassir 10
Preface 11
Acknowledgements 13
Contents 16
Abbreviations 23
Part I Methodology to Start Debugging Model Issues 24
1 Introduction 25
1.1 Global Mindset 25
1.2 The Four Absolutes of Quality in Analysis 30
1.3 Checklist for Performing Analysis 31
1.4 A Heuristic Analysis Confidence Ratio 31
References 37
2 Analysis Convergence Guidelines 38
2.1 Symptoms of Convergence Problems 38
2.2 Causes of Convergence Problems 39
2.3 Helping Abaqus Find a Converged Solution 39
2.4 General Tools 40
2.5 Tools for Contact Stabilization 42
2.6 Tools for Contact Related Convergence Problems 42
Reference 44
3 Method to Debug a Model 45
3.1 Debugging Flowchart 45
3.2 Job Diagnostic 45
3.2.1 Making a Test Model 45
3.2.2 Output Check 49
3.2.3 Syntax Check 50
3.2.4 Data Check 51
3.2.5 Loading and Boundary Conditions Check 54
3.2.6 Materials Check 56
3.2.7 Constraints Check 58
3.2.8 Elements Check 59
3.2.9 Interference Fits Check 60
3.2.10 Contact Check 61
3.2.11 Initial Rigid Body Motion and Over Constraints Check 63
3.2.12 Static Stabilization Check 67
3.2.13 Dynamics Check 69
3.3 Causality Energy Method 74
3.3.1 Basic Energy Approaches, Assumptions and Limitations 75
3.3.2 The Energy Method 76
3.3.3 Energy Method Example to Scale Analyses 77
3.3.4 Causality and Energy Derivatives 78
References 79
4 General Prerequisites 80
4.1 Vocabularies 80
4.1.1 Interpreting Error Messages 82
4.1.2 Interpreting Warning Messages 83
4.2 An Identified Unconnected Region in the Model 84
4.3 Correction of Errors During the Data Check Phase of an Abaqus/Standard Analysis 86
4.4 Tips and Tricks for Diagnostic Error Messages 88
4.5 Trying to Recover a Corrupted Database 89
4.5.1 Procedure 1 89
4.5.2 Procedure 2 90
4.6 Kinematic Distributing Couplings in Abaqus 91
4.6.1 Nature of the Constraint Enforcement 91
4.6.2 Defining Constraints in Abaqus/CAE 94
4.7 Abaqus Geometric Nonlinearity 94
4.8 Differences Between Implicit and Explicit Schemes 97
4.8.1 Equations for Dynamic Problems 98
4.8.2 Time Integration of the Equations of Motion 98
4.8.3 Automatic Time Incrementation with Abaqus Standard 100
4.8.4 Automatic Time Incrementation with Abaqus Explicit 105
4.8.5 Dynamic Contact 107
4.8.6 Material Damping 107
4.8.7 Half-Increment Residual Tolerance 108
4.8.8 Comparing Abaqus/Standard and Abaqus/Explicit 109
4.9 Unstable Collapse and Post-buckling Analysis 110
4.10 Low-Cycle Fatigue Analysis Using the Direct Cyclic Approach 112
4.11 Steady-State Transport Analysis 113
4.11.1 Convergence Issues in a Steady-State Transport Analysis 114
4.12 Heat Transfer Analysis 116
4.12.1 Transient Analysis 117
4.13 Fluid Dynamic Analysis 121
4.13.1 Convergence Criteria and Diagnostics 121
4.13.2 Time Increment Size Control 123
4.14 Introduction to the User Subroutines 124
4.14.1 Installation of a Fortran Compiler 126
4.14.2 Run a Model Which Uses a User Subroutine 128
4.14.3 Debugging Techniques and Proper Programming Habits 128
4.14.4 Examples of User Subroutine with Abaqus Standard 131
4.14.5 Examples of User Subroutine with Abaqus Explicit 133
4.14.6 Examples of User Subroutine with Abaqus CFD 135
References 135
Part II Stop Struggling with Specific Issues 136
5 Materials 137
5.1 Generalities 137
5.2 The Current Strain Increment Exceeds the Strain to First Yield 139
5.3 Convergence Behavior of Models Using Hyperelastic Materials 140
5.4 Models Using Incompressible or Nearly Incompressible Materials 141
5.5 Equivalence of Uniaxial Tension and Compression Hyperelastic Test Data 142
5.5.1 Uniaxial Compression Test Data for a Rubber Material 143
5.5.2 Specifying Tension or Compression Test Data for the Marlow Hyperelasticity Model 144
5.5.3 Using Simple Shear Experimental Data for Hyperelastic Materials 145
5.6 Path Dependence of Nonlinear Results Using an Elastic Material 147
5.7 User Material Subroutine 149
5.7.1 Guideline to Write a UMAT or a VMAT 150
5.8 UMAT Subroutine Examples 151
5.8.1 UMAT Subroutine for Isotropic Isothermal Elasticity 154
5.8.2 UMAT Subroutine for Non-isothermal Elasticity 156
5.8.3 UMAT Subroutine for Neo-Hookean Hyperelasticity 158
5.8.4 UMAT Subroutine for Kinematic Hardening Plasticity 163
5.8.5 UMAT Subroutine for Isotropic Hardening Plasticity 169
5.8.6 UMAT Subroutine for Simple Linear Viscoelastic Material 175
5.9 VUMAT Subroutine Examples 178
5.9.1 VUMAT Subroutine for Kinematic Hardening Plasticity 180
5.9.2 VUMAT Subroutine for Isotropic Hardening Plasticity 183
References 187
6 Mesher and Meshing 189
6.1 Generalities 189
6.1.1 Mesh Control Options 190
6.1.2 Mesh Controls for a 2D Structure 190
6.1.3 Mesh Controls for a 3D Structure 190
6.1.4 Understanding a Mesher 192
6.1.5 Mesh as Grid Generation 197
6.2 The Abaqus Model Meshed Has Changed into a Nonphysical Shape with a Regular Pattern 208
6.3 Excessive Element Distortion Warnings 209
6.4 Compatibility Errors Printed to the Message File for a Model with Hybrid Elements 209
6.5 User Element Subroutine 210
6.5.1 Guideline to Write a UEL 211
6.6 UEL Subroutine Examples 219
6.6.1 UEL Subroutine for Planar Beam with Nonlinear Cross Section 220
6.6.2 Generalized Constitutive Behavior 225
6.6.3 UEL Subroutine for a Horizontal Truss and Heat Transfer Element 227
6.6.4 UELMAT Subroutine for 4 Nodes in Plane Strain 232
6.7 Using Nonlinear User Elements in Various Analysis Procedures 240
References 244
7 Contact 245
7.1 Generalities 245
7.1.1 Understandings 248
7.1.2 Define Contact Pairs 252
7.1.3 Define General Contact 252
7.1.4 Representation of Curved Surfaces 254
7.1.5 Contact Formulation Aspects 255
7.2 Friction 280
7.2.1 Static and Kinetic Friction 281
7.2.2 Change Friction Properties During an Analysis 284
7.2.3 Classic Friction Values 284
7.3 Hard or Soft Contact 285
7.3.1 Identification of the Mathematical Stiffness Function 288
7.3.2 Exponential Contact Stiffness 292
7.3.3 From Hard Contact to Exponential 294
7.4 Obtain a Converged Contact Solution 296
7.5 Convergence Difficulty in the First Increment 298
7.6 Causes and Resolutions of Contact Chattering 299
7.7 Understand Finite Sliding with Surface-to-Surface Contact 301
7.8 Using Penalty Contact 304
7.9 Using Augmented Lagrangian Contact 308
7.10 Using Stiffness-Based Contact Stabilization 310
7.11 Modeling Contact with Second-Order Tetrahedral Elements 312
References 313
Part III A Toolbox to Do the Job 314
8 Troubleshooting in Job Diagnostics 315
8.1 Guidelines with Abaqus Standard 315
8.2 Job with Abaqus Standard Completes, But the Results Look Suspicious 317
8.3 Model a Structure Undergoing a Global Instability 320
8.4 Correct Convergence Difficulties Caused by Local Instabilities 321
8.5 Correcting Errors During the Data-Check Phase of an Analysis 322
8.6 Analysis Ends Prematurely, Even Though All the Increments Have Converged 324
8.7 Debugging Divergence with Too Many Cutbacks in the Last Attempted Increment 325
8.8 Using Follower Loads in Nonlinear Analyses 326
8.9 Understanding Negative Eigenvalue Messages 327
8.10 Divergence with Numerical Singularity Warnings 329
8.11 Zero Pivot Warnings in the Message File 330
8.12 Convergence Difficulty in the First Increment of a Contact Analysis 331
8.13 Explicit Stable Time Increments When Using the Marlow Model with Noisy Test Data 333
8.14 Cause of an Analysis Ending in a Core Dump 334
8.15 Debugging User Subroutines and Post Processing Programs 334
8.16 No Free Memory Available on Linux at the End of an Analysis 339
Reference 342
9 Numerical Acceptance Criteria 343
9.1 Generalities 343
9.1.1 Commonly Used Control Parameters 343
9.1.2 Controlling the Time Incrementation Scheme 345
9.1.3 Activate the Line Search Algorithm 347
9.1.4 Controlling the Solution Accuracy in Direct Cyclic Analysis 347
9.1.5 Controlling the Solution Accuracy and Mesh Quality in a Deforming Mesh Analysis with Abaqus CFD 348
9.1.6 Convergence Criteria for Nonlinear Problems 350
9.1.7 Time Integration Accuracy in Transient Problems 359
9.1.8 Avoid Small Changes to the Time Increment Size During Implicit Integration Procedures 360
9.2 How Much Hourglass Energy Is Acceptable 361
9.2.1 Enhanced Hourglass Control and Elastic Bending Moment 362
9.2.2 Enhanced Hourglass Control and Plastic Bending Moment 362
9.2.3 Kelvin Viscoelastic Hourglass Control 362
9.3 Errors Printed to the Message File for a Model with Hybrid Elements 363
Reference 364
10 Need Some Help? 365
10.1 Retrieving Files Referred to Examples in the Abaqus Documentation 365
10.2 Using the Abaqus Verification, Benchmarks, and Example Problems Guides 365
10.3 Excessive Memory Usage with Cavity Radiation Problems 373
10.4 Perform a Sub-model Analysis 374
10.4.1 Implementation 375
10.4.2 Loading Conditions 376
10.4.3 Sub-model Boundary Conditions 376
10.4.4 Interpolation 377
10.4.5 Step-by-Step Procedure for a Sub-model 377
10.4.6 Setting Options 380
10.4.7 Shell to Solid 381
10.4.8 Changing Procedures 383
10.4.9 Frequency Domain 383
10.4.10 Thermal and Stress Analysis 384
10.4.11 Dynamic Analysis 385
10.4.12 Limitations of Sub-modeling 386
10.5 Perform a Restart Analysis 387
10.5.1 Step-by-Step Procedure for a Restart 389
10.6 Generate a Shell Part from a Solid Part 392
10.6.1 Benefits for Using Shell Structures 392
10.6.2 Applications to Model Shell Structures 393
10.6.3 Step-by-Step Procedure to Convert Solid Model to Shell Model 394
10.7 Compile and Link a Post-processing Program Using the Standalone Abaqus ODB API 401
10.8 Create Executables Using the C++ ODB API Libraries Outside of Abaqus/Make 403
11 Hardware or Software Issues 407
11.1 Solving File System Error 1073741819 407
11.2 Interpreting Error Codes 407
11.3 Obtaining a Traceback from a UNIX/Linux Core Dump 409
11.4 Windows HPC Compute Clusters 413
11.4.1 Classics Troubleshooting with HPC Cluster 418
Reference 421
Appendix Guidelines and Good Practices Examples 422
A.1 Using *COUPLING to Simulate Pure Bending of Thin Walled Pipes 422
A.2 Available Degrees of Freedom with Kinematic Relation at Coupled Nodes 423
A.3 Stability and Accuracy of the Trapezoidal Rule 424
A.4 Accuracy Control in Highly Nonlinear Problems with a Half-Increment Residual Tolerance 431
A.5 The Art of Meshing 434
A.5.1 Free Meshing Technique 435
A.5.2 Model Partitioning with a Strategy Based on Design Symmetry 437
A.5.3 Model Partitioning with a Strategy Based on the Dominant Geometry 440
A.5.4 Small Edges and Consequences for the Mesher 444
A.5.5 Incompatible Mesh 448
Index 451