An analysis of the brittle-ductile transition in dynamic crack growth

Abstract

The brittle-ductile transition in dynamic crack growth is investigated through the numerical analysis of a plane strain edge cracked specimen, subject to impulsive loading at one end. The material is characterized as an elastic-viscoplastic solid, with a temperature dependent flow strength. Thermal softening due to adiabatic heating and a model for ductile failure by void nucleation, growth and coalescence are incorporated into the constitutive relation. The ductile void growth mechanism involves two populations of void nucleating particles; discretely modelled low strength inclusions that give large voids near the crack tip at an early stage and homogeneously distributed small second phase particles that require large strains for void nucleation. Cleavage is modelled in terms of a spatially non-uniform, but temperature and strain rate independent, critical value of the maximum principal normal stress. The numerical results show a clear transition from cleavage dominated crack growth at low temperatures to purely ductile crack growth at higher temperatures. There is an accompanying large increase in the material's resistance to dynamic crack initiation and growth. The computed crack growth behaviour is a direct outcome of the material description; no ad hoc dynamic fracture criterion is employed. Effects of variations in the material model on the brittle-ductile transition are explored.