USE OF THE STRETCH ZONE FOR THE CHARACTERIZATION OF DUCTILE FRACTURE

There is an increasing effort to use the stretch zone ahead of a fatig ue crack as a measure of the fracture toughness of ductile materials, particularly in cases where alternative means of measuring the toughne ss 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 ot her factors including fatigue pre-straining of the material. In this s tudy, the effect of initial pre-straining on the evolution of the stre tch zone in a number of ductile steels was investigated. Actual measur ements of the deformation of the stretch zone as it is loaded were und ertaken using electron microscopy. It is shown that both prior pre-str aining and fatigue pre-cracking have a significant work-hardening effe ct, which controls the width of the stretch zone and hence the measure d fracture toughness.