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Fatigue Life Analyses of Welded Structures
Fatigue Life Analyses of Welded Structures
Tom Lassen
Naman Récho
First Published in Great Britain and the United States in 2006 by ISTE Ltd
Apart from any fair dealing for the purposes of research or private study, or criticism or
review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may
only be reproduced, stored or transmitted, in any form or by any means, with the prior
permission in writing of the publishers, or in the case of reprographic reproduction in
accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction
outside these terms should be sent to the publishers at the undermentioned address
Abbreviations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
PART I. Common Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Chapter 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The importance of welded joints and their fatigue behavior . . . . . . . 3
1.2. Objectives and scope of the book. . . . . . . . . . . . . . . . . . . . . . . 4
1.3. The content of the various chapters. . . . . . . . . . . . . . . . . . . . . . 5
1.4. Other literature in the field. . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5. Why should the practicing engineer apply reliability methods? . . . . . 8
1.6. How to work with this book . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.7. About the authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Chapter 2. Basic Characterization of the Fatigue Behavior of
Welded Joints. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1. Introduction and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2. Fatigue failures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3. Basic mechanisms of metal fatigue. . . . . . . . . . . . . . . . . . . . . . 15
2.4. Parameters that are important to the fatigue damage process. . . . . . . 17
2.4.1. External loading and stresses in an item. . . . . . . . . . . . . . . . 17
2.4.2. Geometry, stress and strain concentrations . . . . . . . . . . . . . . 19
2.4.3. Material parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.4.4. Residual stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4.5. Fabrication quality and surface finish . . . . . . . . . . . . . . . . . 25
2.4.6. Influence of the environment . . . . . . . . . . . . . . . . . . . . . . 25
2.5. Important topics for welded joints . . . . . . . . . . . . . . . . . . . . . . 26
2.5.1. General overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.6. Various types of joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.1. Plated joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.2. Tubular joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.7. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Chapter 3. Experimental Methods and Data Analysis . . . . . . . . . . . . . 37
3.1. Introduction and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2. Overview of various types of tests . . . . . . . . . . . . . . . . . . . . . . 38
3.3. Stress-life testing (S-N testing) of welded joints . . . . . . . . . . . . . . 38
3.3.1. Test specimens and test setup. . . . . . . . . . . . . . . . . . . . . . 38
3.3.2. Preparations and measurements . . . . . . . . . . . . . . . . . . . . 41
3.3.3. Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.4. Testing to determine the parameters in the strain-life equation . . . . . 49
3.5. Crack growth tests – guidelines for test setup and specimen
monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.6. Elementary statistical methods . . . . . . . . . . . . . . . . . . . . . . . . 55
3.6.1. Linear regression analyses. . . . . . . . . . . . . . . . . . . . . . . . 55
3.7. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Chapter 4. Definition and Description of Fatigue Loading. . . . . . . . . . . 61
4.1. Introduction and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2. Constant amplitude loading . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.3. Variable amplitude loading . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3.2. Rain-flow cycle counting of time series. . . . . . . . . . . . . . . . 64
4.3.3. The energy spectrum approach . . . . . . . . . . . . . . . . . . . . . 69
4.4. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Chapter 5. The S-N Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.1. Introduction and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.2. Method, assumptions and important factors. . . . . . . . . . . . . . . . . 76
5.2.1. Statistics for the S-N approach, median and percentile curves . . 76
5.2.2. Discussion of S-N curves-important factors . . . . . . . . . . . . . 78
5.2.2.1. The threshold phenomenon. . . . . . . . . . . . . . . . . . . . 78
5.2.2.2. Mean stress and loading ratio. . . . . . . . . . . . . . . . . . . 79
5.2.2.3. Stress relieving . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.2.2.4. The thickness effect . . . . . . . . . . . . . . . . . . . . . . . . 80
5.2.2.5. Misalignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2.2.6. Post-weld improvement techniques . . . . . . . . . . . . . . . 82
5.2.2.7. Corrosive environment . . . . . . . . . . . . . . . . . . . . . . 83
5.3. Mathematics for damage calculations . . . . . . . . . . . . . . . . . . . . 84
5.3.1. Linear damage accumulation; load spectrum on a histogram
format. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
...
11.4.3. Measured effect of the loading angle on the crack growth rate . 329
11.4.4. Measured effect of weld on the crack growth rate . . . . . . . . . 331
11.4.5. Measured crack extension angle under mixed mode loading. . . 332
11.5. Validation of the model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
11.5.1. Verification of the models for non-welded steel specimens
under mixed-mode loading . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
11.5.2. Verification of the models for non-welded and welded steel
specimens under mode I loading. . . . . . . . . . . . . . . . . . . . . . . . 336
11.5.3. Verification of the models for welded steel specimens under
mixed-mode loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337
11.5.4. Verification of the effect of the welded residual stress on
the fatigue life. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338
11.5.5. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . 339
11.6. Extension to full test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340
11.6.1. Modeling methodology. . . . . . . . . . . . . . . . . . . . . . . . . 341
11.6.2. Global calculation scheme . . . . . . . . . . . . . . . . . . . . . . . 341
11.6.3. The crack box technique . . . . . . . . . . . . . . . . . . . . . . . . 343
11.6.4. Crack-propagation rate . . . . . . . . . . . . . . . . . . . . . . . . . 344
11.6.5. Description of experiments carried out . . . . . . . . . . . . . . . 345
11.6.6. Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345
11.6.7. Weld toe geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 346
11.6.8. Numerical calculations . . . . . . . . . . . . . . . . . . . . . . . . . 347
11.6.8.1. Crack initiation . . . . . . . . . . . . . . . . . . . . . . . . . . 347
11.6.8.2. Crack growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
11.7. References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
Chapter 12. The Effect of Overloads on the Fatigue Life. . . . . . . . . . . . 355
12.1. Introduction and objectives. . . . . . . . . . . . . . . . . . . . . . . . . . 355
12.2. Residual stress opening approach at the crack tip following
an overload during fatigue. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
12.3. Numerical modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
12.3.1. Modeling aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
12.3.2. Finite element modeling choices . . . . . . . . . . . . . . . . . . . 363
12.4. Proposed deterministic approach to fatigue crack growth following
an overload. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
12.5. Reliability modeling including the effect of an overload . . . . . . . . 370
12.6. Application of the reliability model to a fillet welded joint . . . . . . . 371
12.7. References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
Appendix A. Short Overview of the Foundations of Fracture Mechanics . 381
A1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
A2. Elementary failure modes and stress situations. . . . . . . . . . . . . . . 383
A3. Foundations of fracture mechanics . . . . . . . . . . . . . . . . . . . . . . 383
A4. Parameters characterizing the singular zone. . . . . . . . . . . . . . . . . 385
xiv Fatigue Life Analysis of Welded Structures
A4.1. The stress intensity factor (SIF), K. . . . . . . . . . . . . . . . . . . 385
A4.2. The energy release rate, G . . . . . . . . . . . . . . . . . . . . . . . . 387
A4.3. The J-integral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388
A4.4. The crack-opening displacement (COD) . . . . . . . . . . . . . . . 389
A5. Asymptotic stress field in elastic-plastic media. . . . . . . . . . . . . . . 390
A5. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
Appendix B. Spreadsheet for Fatigue Life Estimates . . . . . . . . . . . . . . 393
Appendix C. CG – Crack Growth Based on Fracture Mechanics . . . . . . 395
Appendix D. CI – Crack Initiation Based on Coffin-Manson . . . . . . . . . 399
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403
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