A model of pre-strain effects on fracture toughness

A simple theoretical model for predicting the effect of tensile pre-strain on fracture toughness has been developed using the local approach. The HRR singularity is assumed to describe the stress-strain field around the crack tip. A stress-modified critical straincontrolled model is assumed to describe ductile fracture (and a critical stress-controlled model for cleavage fracture). The Rice and Tracey void growth model is used to characterize the variation of the critical strain with the stress state. The model further assumes that the fracture process does not change with increasing pre-strain. The effect of prestrain is expressed in terms of an equation for the ratio of the fracture toughness of the pre-strained material to that of the virgin material. The model indicates that the effect of tensile pre-strain on fracture resistance can be characterized in terms of the effect of pre-strain on the stress-strain characteristics of the material, the critical fracture strain for a stress state corresponding to that during pre-strain, and several parameters that relate to the conditions for ductile fracture (or cleavage fracture). Previous experimental studies of the effect of pre-strain on toughness are summarized and compared with the predictions of the model.