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.