Residual stress effects on ductile tearing

Residual stresses are internal stresses generated during the fabrication and/or operation of engineering structures. Such stresses can provide the major element of the driving force for crack initiation and growth. Structural integrity assessment procedures, provide guidance for the assessment of defects located within regions of high residual stress. However, such guidance may be conservative where the defect develops progressively during service. This paper describes recent experimental and numerical work aimed at quantifying such conservatisms and providing improved guidance for undertaking more realistic analyses. The results demonstrate that pre-loaded compact-tension specimens provide a useful means for studying the behaviour of cracks within residual stress fields. The magnitude of calculated crack driving forces due to residual stresses is influenced by the approach used to introduce cracks into the stress field, with progressive cracks providing lower levels of crack driving force than instantaneously introduced cracks. The J R-curve associated with cracks under primary or combined primary + secondary loading can apparently be rationalized when the total crack driving force is calculated using methods that take proper account of the influence of prior plasticity on the J-integral. However, it is noted that due to differences in the form of the crack-tip stress and strain fields for static and growing cracks, such values of J may be path dependent and influenced by the magnitude of the growth increment.