Effects of Crack Introduction History on Fracture Initiation in Residually Stressed Components

If a crack is introduced progressively into an elastic-plastic material containing a residual stress field, the incremental relaxation of the residual stress field causes the formation of a plastic wake along the crack boundaries. This leads to the reduction in the J parameter for a crack of a given size, compared to a crack with the same dimensions which has been introduced instantaneously, having the crack faces released simultaneously along the whole length, a 40% reduction is observed in the current analysis. This reduction in J is due to the dissipation of strain energy which is otherwise available for further crack extension, as in the instantaneously introduced crack. This is important for the current J-based fracture assessment common in the nuclear and petrochemical industries such as EDF Energy’s R6 and BS7910:2013 as they currently assume instantaneous insertion of cracks as this is inherently more conservative. Although many studies demonstrating this effect in FE are available, there is little experimental evidence for this phenomenon. Especially those including rigorous comparisons with specimens that have been ‘instantaneously’ cracked. This may be due to the difficulty inherent in manufacturing such a specimen as manufacturing processes rely on the incremental removal of material. The aim of this paper is to detail analysis of a novel method of crack introduction that aims to replicate the deformation behavior of an instantaneously introduced crack tip in a model that has had the nodes released in a progressive manner. This will allow specimens to be machined in a way that replicates ‘instantaneous’ cracking allowing for experimental techniques to be developed to display the difference between instantaneous and progressively introduced cracks.