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

Mixed-mode I/II fracture experiments were performed on a ferritic steel. Th e crack growth mechanism was studied and the fracture toughness was measure d in terms of crack-tip displacement vector delta (v) and J-integral. A cha nge in the fracture mechanism at high mode II loading accompanied a marked decrease of the J-measured fracture toughness while the delta (v)-measure w as 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 ener gy dissipated prior to crack growth associated with this kind of deformatio n was lower than in mode I and, therefore, the associated J-integral fractu re toughness J(c) was lower than the mode I fracture toughness J(Hc), The d ecrease of J(c) with increasing mode II components was finally rationalised in a relationship between delta (v) and J. Under the condition of a mixed- mode independent delta (v)-measured fracture toughness, the decrease if J(c ) for increasing mode II of the present experiments and of literature studi es was predicted. (C) 2001 Elsevier Science Ltd. All rights reserved.