ON CRACK TIP SHIELDING DUE TO PLASTICITY-INDUCED CLOSURE DURING AN OVERLOAD

The mechanisms underlying the plasticity-induced shielding of a crack tip during fatigue are investigated at both constant amplitude loading and in a case with a substantial singlecycle overload. A recently developed mathematical model of the stress field around the crack was fitted to maps of isochromatic fringe data obtained from phase-stepped photoelastic images captured during crack opening and closing in a polycarbonate compact tension specimen. The model is derived using Muskhelishvili’s approach and contains additional terms to describe the interfacial shear at the elastic–plastic boundary around the crack and the normal stresses retarding crack growth. These stress fields are characterized by stress intensity factors, KS and KR, respectively, which during and after the overload exhibited substantial changes in behaviour that can be explained in terms of the development of an enlarged plastic zone.