Use of the stretch zone for the characterization of ductile fracture

Abstract

There is an increasing effort to use the stretch zone ahead of a fatigue crack as a measure of the fracture toughness of ductile materials, particularly in cases where alternative means of measuring the toughness are not possible, e.g. at high strain rates. The stretch zone (SZ) is hence related to parameters such as the J-integral. It was, however, found that the relationship between (SZ) and J IC is affected by other factors including fatigue pre-straining of the material. In this study, the effect of initial pre-straining on the evolution of the stretch zone in a number of ductile steels was investigated. Actual measurements of the deformation of the stretch zone as it is loaded were undertaken using electron microscopy. It is shown that both prior pre-straining and fatigue pre-cracking have a significant work-hardening effect, which controls the width of the stretch zone and hence the measured fracture toughness.