ACCOUNTING FOR RESIDUAL STRESS IN FATIGUE CRACK GROWTH RATE TESTS: VALIDATION OF RESIDUAL STRESS INTENSITY FACTOR MEASUREMENTS

Fatigue crack growth rate (FCGR) test data are a key to ensuring structural safety by design and inspection, but residual stress in test specimens can lead to significant (and unknown) bias in FCGR data. In turn, biased FCGR data confound estimates of structural capability for fielded systems. The paper will describe an experimental method for measuring the residual stress intensity factor as a function of crack size, Kres(a), during FCGR tests and provide data for validation. Further test data show that simultaneous measurements of FCGR and Kres(a) enable residual stress bias to be removed from FCGR test data.
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RELAXATION OF RESIDUAL STRESS IN WELDED PLATES DURING LONG LIFE FATIGUE LOADING

The presence of residual stresses affects the fatigue response of welded components. In the present study of a thick welded cantilever specimen, residual stresses were measured in an as-welded A36 steel sample and in a sample subjected to a long history of bending loads where minimal local plasticity is expected at the fatigue hot-spot weld toe. Extensive XRD measurements describe the residual stress state in a large region in front of the weld toe both in an untested as-welded sample and in a sample subjected to a long load history that generated an estimated 0.001 strain amplitude at the stress concentration zone at the weld toe. The results show that such a test will moderately alter the welding induced residual stresses. Fatigue life prediction methods need to be aware that such alterations are possible and incorporate the effects of such cyclic stress relaxation in life computations.
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