Characterisation of ductile mixed-mode fracture with the crack-tip displacement vector

Abstract

Mixed-mode I/II fracture experiments were performed on a ferritic steel. The crack growth mechanism was studied and the fracture toughness was measured in terms of crack-tip displacement vector δ v and J-integral. A change in the fracture mechanism at high mode II loading accompanied a marked decrease of the J-measured fracture toughness while the δ v-measure was little affected. The stress/strain field in the vicinity of crack tip at the onset of growth was then evaluated via finite element analysis (FEA). A highly strained, narrow band spreading from the crack-tip developed close to mode II in the experimental crack propagation direction. The total energy dissipated prior to crack growth associated with this kind of deformation was lower than in mode I and, therefore, the associated J-integral fracture toughness J c was lower than the mode I fracture toughness J Ic. The decrease of J c with increasing mode II components was finally rationalised in a relationship between δ v and J. Under the condition of a mixed-mode independent δ v – measured fracture toughness, the decrease if J c for increasing mode II of the present experiments and of literature studies was predicted.