Environmentally-controlled non-equilibrium crack propagation in ceramics

Abstract

A general framework is developed for environmentally-controlled non-equilibrium crack propagation and applied to ceramic materials that exhibit microstructurally-controlled fracture resistance variations. Increasing fracture resistance with crack length, arising from frictional interlocking of predominantly intergranular fracture surfaces, is modelled by the influence of a localized line force behind the crack tip. An indentation fracture mechanics analysis incorporates the fracture resistance variation to describe the inert strength of ceramic materials as a function of dominant flaw size. Non-equilibrium fracture is modelled as the competition between thermally-activated bond-rupture and bond-healing processes, in which the activation barriers are modified by the net mechanical energy release rate acting on a crack. The resulting dependence of crack velocity on mechanical energy release rate is used to describe the strength of ceramic materials as a function of applied stressing rate in a reactive environment. The deconvoluted crack velocity behavior allows both the macroscopic reactive environment fracture resistance and the atomistic lattice traps for fracture to be determined. An implication is that fracture resistance variations are more important in determining observed fracture behavior in reactive environments than in inert environments.