Cumulative damage of welded joints >> By T R Gurney, formerly TWI, UK

cordoba123 · 12-11-2011, 03:49 PM

- covers the wealth of research in the field of fatigue strength and its role in the design and manufacture of welded components
- invaluable reference source for welding engineers, supervisors, inspection personnel and designers

Fatigue is a mechanism of failure which involves the formation and growth of cracks under the action of repeated stresses. Ultimately, a crack may propagate to such an extent that total fracture of the member may occur. To avoid fatigue it is essential to design the structure with inherent fatigue strength. However, fatigue strength for variable amplitude loading is not a constant material property and any calculations are necessarily built on a number of assumptions. Cumulative damage of welded joints explores the wealth of research in this important field and its implications for the design and manufacture of welded components.

After an Introduction, chapter two introduces the constant amplitude database, which contains results obtained in test conditions and which forms the basis of the basic S-N curves for various types of joint. Chapter three discusses the influence of residual stresses which can have a marked effect on fatigue behaviour. Chapter four explores variable amplitude loading and the problem of how information from laboratory tests, obtained under constant amplitude conditions, can be applied to the design of structures for service conditions. This problem is further investigated in the next chapter which is devoted to two and three level load testing. Chapters six, seven and eight look at the influence that the variety of variable loading spectra can have on fatigue strength, whether narrow or wide band loading or cycles of small stress range. Taking all of this knowledge, chapter nine discusses structure designs.

Cumulative damage of welded joints is a comprehensive source of invaluable information for welding engineers, supervisors, inspection personnel and designers. It will also be of great interest for academics working in the fields of structural and mechanical engineering.

About the author

Dr Gurney worked at TWI (formerly the British Welding Research Association) for over thirty years, during which time his primary interest was in the fatigue of welded structures. He has written or co-authored over fifty papers throughout his career.

Contents

Introduction
- Background
- Characteristics of fatigue cracking
- Fatigue testing
- The S-N curve and fatigue strength
- Fracture mechanics assessment of constant amplitude fatigue behaviour

The constant amplitude database
- Introduction
- Method of analysis and joint design classification: Continuous longitudinal welds, intermittent longitudinal fillet welds
- Transverse butt welds: Transverse butt welds on a permanent backing bar, cruciform joints, stud shear connectors, fillet welded joints, beams with welded flange cover plates, longitudinal gusset on a plate edge
- Influence of plate thickness
- Influence of mean stress: Transverse butt welds, Fillet welded joints

Residual stresses
- Introduction
- The formation of residual stresses
- Comparison between static and fatigue conditions
- Approximate theoretical analysis
- Tests on welded specimens under constant amplitude loading
- Prior overloading

Variable amplitude loading and testing
- Introduction
- Variable amplitude loading
- Rainflow counting
- Reservoir counting
- Level-crossing counting
- Statistical interpretation of count data
- Miner’s rule
- Variable amplitude fatigue testing; a brief history: block programme loading, random order loading, narrow band loading, wide band loading, superimposed sine waves, summary of early test results

Tests under two and three level loading
- Introduction
- Theoretical analysis: stress sequences with a single size of excursion, stress sequences with excursions of two sizes, general case; the area rule
- Fatigue tests using stress sequences with excursions of two sizes
- Influence of stress ratio and residual stresses
- Summary of findings

The influence of spectrum shape and block length
- Introduction
- Fatigue tests under concave upwards spectra
- Fatigue tests under Rayleigh and Laplace loading spectra
- Tests under Weibull stress spectra
- Influence of spectrum shape and clipping ratio combined
- Influence of block length and clipping ratio combined
- Influence of block length and spectrum shape combined
- Summary

The influence of narrow and wide band loading
- Introduction
- Comparing loading types
- Tests under narrow band loading
- Tests under wide band loading
- Tests under service loading spectra
- Summary

The influence of cycles of small stress range
- Introduction
- Block testing of low stresses
- Comparative tests on stress relieved joints
- Predicting fatigue life
- Summary

Design for variable amplitude loading
- Introduction
- Testing for different types of stress
- The area rule
- Possible modifications to Miner’s rule
- The fracture mechanics approach

More on the fracture mechanics approach – the effect of stress interaction
- Introduction
- Summary of experimental evidence about stress interaction effects: Type 1 loading, type 2 loading, type 3 loading, type 4 loading, type 5 and 6 loading, type 7 and 8 loading, type 9 loading, types 10 and 11 loading
- Summary of loading effects
- Discussion
- Concluding remarks

Appendices
- A) Statistical analysis of constant amplitude test data, References used in compiling the British fatigue design S-N curves
- B) Fatigue loading spectra
- C) Summary of test results obtained under variable amplitude loading
- D) Probability density curves

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