Crack-tip constraint effects on creep fracture

A three-tenn expansion describing the crack tip fields in steady power-law creeping solids is used in a small-scale damage microstructural approach to study the initial stages of creep fracture under plane strain mode I loadi ng conditions. These fields contain three important parameters. The first o ne is C*, which sets the loading level, and the remaining, A(2)* and sigma( infinity) account for the constraint effect imposed by the specific geometr y and loading configuration. The microstructural model incorporates a proce ss window around the tip of the crack, which contains a large number of gra ins that are represented discretely by so-called grain elements. The grain boundaries are described by interface elements that incorporate the princip al damage mechanisms, including cavity nucleation, diffusive-creeping cavit y growth and grain boundary sliding. The process window is surrounded by a standard creeping continuum which is subject to remote boundary conditions corresponding to the crack tip fields in a steadily creeping material. Nume rical results of the model are presented for a range of constraints using v alues of A(2)(*) and sigma(infinity) that correspond to typical test specim ens. The effect of crack-tip constraints is demonstrated and explained for material parameter sets that give rise to either ductile or brittle creep f racture processes. (C) 2000 Elsevier Science Ltd. All rights reserved.