Load and Crack History Effects on Fracture

Assessments of the integrity of structures containing defects or cracks require estimates to be made of the elastic-plastic crack driving force (CDF) parameter J. This is the characterising parameter that controls the intensity of the fields of stress and strain close to the tip of a crack. Such estimates of J are inherently made in assessment procedures such as R6, Revision 4 [1]. Engineering components are typically subjected to load cycles, often with significant variations in magnitude. Normal operation cycles or overload (by a proof pressure test for example) may cause a re-distribution of weld residual stresses. A defect can be present at fabrication or develop during operation due to a sub-critical process such as fatigue or stress corrosion cracking. In these two cases, it is reasonable to suppose that the actual crack driving forces are different; since the development of a defect in a region of weld residual stress, in conjunction with additional primary loading, can cause significant non-proportional loading of the crack tip. The objective of the work described in this paper is to provide more accurate estimates of the crack driving force parameter for defects subjected to combined primary and secondary stresses, taking into account the effects of loading hisotory. The eventual aim is to reduce uncertainty in assessments of plant integrity, and to clarify advantage that can be taken from a reduction in crack driving forces due to weld residual stress resulting from overload, operational cycles and the progressive introduction of sub-critical defects. Finite element analyses and R6 calculations are undertaken and compared to examine the effects of inserting a crack at different times during the life of an engineering structure.