Fatigue Life Analyses of Welded Structures Flaws

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ISBN: 9781905209545 | 1905209541
Cover: Hardcover
Copyright: 11/3/2006

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Practical problems in structural design and service inspection are addressed in this handbook to dealing with fatigue design of welded structures. Details of how to use calculation tools on an Excel spreadsheet are included for application in realistic industrial cases.

Abbreviations	p. xv
Common Practice	p. 1
Introduction	p. 3
The importance of welded joints and their fatigue behavior	p. 3
Objectives and scope of the book	p. 4
The content of the various chapters	p. 5
Other literature in the field	p. 7
Why should the practicing engineer apply reliability methods?	p. 8
How to work with this book	p. 9
About the authors	p. 10
Basic Characterization of the Fatigue Behavior of Welded Joints	p. 11
Introduction and objectives	p. 11
Fatigue failures	p. 11
Basic mechanisms of metal fatigue	p. 15
Parameters that are important to the fatigue damage process	p. 17
External loading and stresses in an item	p. 17
Geometry, stress and strain concentrations	p. 19
Material parameters	p. 20
Residual stresses	p. 24
Fabrication quality and surface finish	p. 25
Influence of the environment	p. 25
Important topics for welded joints	p. 26
General overview	p. 26
Various types of joints	p. 30
Plated joints	p. 30
Tubular joints	p. 34
References	p. 35
Experimental Methods and Data Analysis	p. 37
Introduction and objectives	p. 37
Overview of various types of tests	p. 38
Stress-life testing (S-N testing) of welded joints	p. 38
Test specimens and test setup	p. 38
Preparations and measurements	p. 41
Test results	p. 46
Testing to determine the parameters in the strain-life equation	p. 49
Crack growth tests - guidelines for test setup and specimen monitoring	p. 50
Elementary statistical methods	p. 55
Linear regression analyses	p. 55
References	p. 60
Definition and Description of Fatigue Loading	p. 61
Introduction and objectives	p. 61
Constant amplitude loading	p. 62
Variable amplitude loading	p. 63
Overview	p. 63
Rain-flow cycle counting of time series	p. 64
The energy spectrum approach	p. 69
References	p. 73
The S-N Approach	p. 75
Introduction and objectives	p. 75
Method, assumptions and important factors	p. 76
Statistics for the S-N approach, median and percentile curves	p. 76
Discussion of S-N curves-important factors	p. 78
The threshold phenomenon	p. 78
Mean stress and loading ratio	p. 79
Stress relieving	p. 79
The thickness effect	p. 80
Misalignment	p. 81
Post-weld improvement techniques	p. 82
Corrosive environment	p. 83
Mathematics for damage calculations	p. 84
Linear damage accumulation; load spectrum on a histogram format	p. 84
Discussion of the validity of the linear damage accumulation	p. 86
Definition of the equivalent stress range	p. 88
Load spectrum on the format of a Weibull distribution	p. 88
S-N curves related to various stress definitions	p. 91
Nominal stress, geometrical stress and weld notch stresses	p. 92
Geometrical stresses in tubular joints	p. 96
Fatigue life estimate based on the weld notch stress approach	p. 98
Conclusions on the various stress approaches	p. 101
Some comments on finite element analysis	p. 104
Current rule and regulations	p. 110
General considerations	p. 110
The original fatigue classes and S-N curves from DoE	p. 112
S-N life predictions according to Eurocode 3-Air environment	p. 117
S-N life predictions according to HSE	p. 119
S-N life predictions according to NORSOK and DNV	p. 120
S-N life predictions for ship structures	p. 122
The industrial case: an offshore loading buoy	p. 130
References	p. 136
Applied Fracture Mechanics	p. 139
Introduction	p. 139
Objectives of this chapter	p. 142
Basic concepts of linear elastic fracture mechanics	p. 142
The local stress field ahead of the crack front	p. 142
Fracture criterion due to extreme load	p. 152
Mixed mode rupture	p. 153
The R6 criterion and critical crack size	p. 154
Fatigue threshold and fatigue crack growth	p. 156
Crack growth models	p. 156
Parameters C and m	p. 159
Residual stresses	p. 160
Some notes on the size of the initial cracks	p. 161
Geometry function and growth parameters given in BS7910	p. 161
The geometry function	p. 162
Parameters C and m	p. 163
Fracture mechanics model for a fillet welded plate joint	p. 165
Basic assumptions and criteria for the model	p. 165
Data for crack growth measurements (database 1)	p. 166
Data for fatigue lives at low stress levels (database 2)	p. 167
Procedure and curve fitting	p. 167
Growth parameters C and m	p. 169
The initial crack depth a[subscript 0]	p. 172
Prediction of crack growth histories and construction of S-N curves	p. 173
Conclusions for fillet joints with cracks at the weld toe	p. 175
Fatigue crack growth in tubular joints	p. 176
Discussion of current models	p. 179
Conclusion on the empirical fracture mechanics model	p. 183
Proposal for model improvements	p. 183
A brief overview of stiffened panels	p. 184
Units and conversion for fracture mechanics parameters	p. 186
Industrial case: fatigue re-assessment of a welded pipe	p. 186
Introduction	p. 186
Description of the loading buoy with steel pipe	p. 187
Replacement and inspection strategy	p. 189
Re-assessment based on the S-N approach	p. 190
Re-assessment based on fracture mechanics	p. 191
References	p. 193
Stochastic Modeling	p. 197
Stochastic Modeling	p. 199
Introduction and objectives	p. 199
Overview of models and methodology	p. 200
Sources of uncertainty	p. 200
Introduction to the random variable model and related methods	p. 201
Requirements for a stochastic model	p. 203
The concept of the limit state function and the safety margin	p. 204
The first and second order reliability methods (FORM/SORM)	p. 206
Elementary reliability models	p. 207
General considerations	p. 207
The Lognormal distribution	p. 208
The Weibull distribution	p. 209
The random variable model using simulation methods	p. 212
General considerations	p. 212
The realization of a random variable by the Monte Carlo method	p. 213
Random variable models based on the S-N approach	p. 215
The lognormal format for the S-N fatigue life	p. 215
Example: full-penetration butt joint in an offshore structure	p. 217
Monte Carlo Simulation of the S-N fatigue life	p. 219
Random variable models based on fracture mechanics	p. 220
General considerations	p. 220
Taking account for future inspections and inspection results	p. 221
Characterization of the performance of the non-destructive inspection technique	p. 223
Simulation with account for future planned inspections	p. 225
A first approximation to the inspection problem	p. 225
Full stochastic simulation	p. 226
Simulation of planned inspections for a fillet welded joint	p. 229
Updating based on inspections results	p. 231
The Markov chain model	p. 235
Basic concepts	p. 235
Simple illustration on how the model works	p. 235
Elaboration of the model	p. 242
Influence of scheduled inspection and repair	p. 244
Parameter estimation	p. 246
Hybrid model to account for additional scatter	p. 248
Analysis of a fillet welded joint	p. 249
Short review and elaboration of database 1	p. 250
Determination of parameters in the Markov model	p. 251
Reliability results and discussion	p. 253
A damage tolerance supplement to rules and regulation	p. 255
Introduction	p. 255
An industrial case study: single anchor loading system	p. 260
Example 1: butt weld in upper pipeline	p. 262
Example 2: welded brackets on the main plates	p. 263
Conclusions for the damage tolerance supplement	p. 263
Risk assessments and cost benefit analysis	p. 264
Reliability and risk assessment for the riser steel pipe	p. 267
References	p. 268
Recent Advances	p. 271
Proposal for a New Type of S-N Curve	p. 273
Introduction and objectives	p. 273
General considerations for the conventional S-N approach	p. 275
Basic assumptions	p. 275
The S-N approach based on BS5400 and Eurocode 3	p. 275
S-N curves based on a random fatigue limit model	p. 277
Experimental data for model calibration	p. 278
Data for fatigue life at high stress levels (database 1)	p. 278
Data for fatigue lives at low stress levels (database 2)	p. 279
Comparison between the F-class curve, the RFLM-based curve and the data	p. 279
Conclusions	p. 284
References	p. 284
Physical Modeling of the Entire Fatigue Process	p. 287
Introduction and objectives	p. 287
Modeling the fatigue crack initiation period	p. 289
Basic concept and equations for the local stress-strain approach	p. 289
Definition of the initiation phase and determination of parameters	p. 292
Local toe geometry and stress concentration factor	p. 292
Transition depth	p. 294
Cyclic mechanical properties and parameters in Coffin-Manson equation	p. 295
Constructing the S-N curve from the two-phase model	p. 297
Damage accumulation using the TPM	p. 301
The practical consequences of the TPM	p. 302
General considerations	p. 302
Life predictions and dimensions	p. 302
Predicted crack evolution and inspection planning	p. 303
Conclusions	p. 306
Suggestions for future work	p. 307
References	p. 308
A Notch Stress Field Approach to the Prediction of Fatigue Life	p. 309
A modified S-N approach	p. 309
General considerations	p. 309
The basic theory for the notch stress intensity factor	p. 311
S-N data analysis for fillet welded joints	p. 313
A modified crack growth approach	p. 315
References	p. 317
Multi-Axial Fatigue of Welded Joints	p. 319
Introduction and objectives	p. 319
Overview of theory and crack-extension criteria	p. 321
The crack box technique	p. 322
General considerations for finite element analysis and element mesh	p. 322
Methodology	p. 322
Examples	p. 324
Tentative mixed-mode model to crack propagation in welded joints	p. 325
Modeling the effect of the loading mode on the crack growth rate	p. 327
Modeling the effect of the residual stress due to the weld on the crack growth rate	p. 328
Measured effect of the loading angle on the crack growth rate	p. 329
Measured effect of weld on the crack growth rate	p. 331
Measured crack extension angle under mixed mode loading	p. 332
Validation of the model	p. 333
Verification of the models for non-welded steel specimens under mixed-mode loading	p. 334
Verification of the models for non-welded and welded steel specimens under mode I loading	p. 336
Verification of the models for welded steel specimens under mixed-mode loading	p. 337
Verification of the effect of the welded residual stress on the fatigue life	p. 338
Discussion and conclusions	p. 339
Extension to full test	p. 340
Modeling methodology	p. 341
Global calculation scheme	p. 341
The crack box technique	p. 343
Crack-propagation rate	p. 344
Description of experiments carried out	p. 345
Results	p. 345
Weld toe geometry	p. 346
Numerical calculations	p. 347
Crack initiation	p. 347
Crack growth	p. 349
References	p. 351
The Effect of Overloads on the Fatigue Life	p. 355
Introduction and objectives	p. 355
Residual stress opening approach at the crack tip following an overload during fatigue	p. 359
Numerical modeling	p. 362
Modeling aspects	p. 362
Finite element modeling choices	p. 363
Proposed deterministic approach to fatigue crack growth following an overload	p. 366
Reliability modeling including the effect of an overload	p. 370
Application of the reliability model to a fillet welded joint	p. 371
References	p. 375
Short Overview of the Foundations of Fracture Mechanics	p. 381
Introduction	p. 381
Elementary failure modes and stress situations	p. 383
Foundations of fracture mechanics	p. 383
Parameters characterizing the singular zone	p. 385
The stress intensity factor (SIF), K	p. 385
The energy release rate, G	p. 387
The J-integral	p. 388
The crack-opening displacement (COD)	p. 389
Asymptotic stress field in elastic-plastic media	p. 390
References	p. 391
Spreadsheet for Fatigue Life Estimates	p. 393
CG - Crack Growth Based on Fracture Mechanics	p. 395
CI - Crack Initiation Based on Coffin-Manson	p. 399
Index	p. 403
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Fatigue Life Analyses of Welded Structures Flaws

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