Environmentally-controlled non-equilibrium crack propagation in ceramics

A general framework is developed for environmentally-controlled non-equilib rium crack propagation and applied to ceramic materials that exhibit micros tructurally-controlled fracture resistance variations. Increasing fracture resistance with crack length, arising from frictional interlocking of predo minantly intergranular fracture surfaces, is modelled by the influence of a localized line force behind the crack tip. An indentation fracture mechani cs analysis incorporates the fracture resistance variation to describe the inert strength of ceramic materials as a function of dominant flaw size. No n-equilibrium fracture is modelled as the competition between thermally-act ivated bond-rupture and bond-healing processes, in which the activation bar riers are modified by the net mechanical energy release rate acting on a cr ack. The resulting dependence of crack velocity on mechanical energy releas e 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 fractur e resistance and the atomistic lattice traps for fracture to be determined. An implication is that fracture resistance variations are more important i n determining observed fracture behavior in reactive environments than in i nert environments. (C) 1999 Elsevier Science S.A. All rights reserved.