Growth of an initially sharp crack by grain boundary cavitation

A new computational model is presented to analyze intergranular creep crack growth in a polycrystalline aggregate in a discrete manner and based direc tly on the underlying physical micromechanisms. A crack tip process zone is introduced in which grains and their grain boundaries are represented disc retely, while the surrounding undamaged material is described as a continuu m. Special-purpose finite elements are used to represent individual grains and grain boundary facets. The constitutive description of the grain bounda ry elements accounts for the relevant physical fracture mechanisms, i.e. vi scous grain boundary sliding, the nucleation of grain boundary cavities, th eir growth by grain boundary diffusion and local creep, until coalescence o f cavities leads to microcracks. Discrete propagation of the main crack occ urs by linking up of neighbouring facet microcracks. Assuming small-scale d amage conditions, the model is used to simulate the initial stages of growt h of an initially sharp crack under C* controlled, mode I loading condition s. Material parameters are varied so as to lead to either ductile or brittl e fracture, thus elucidating creep constrained cavitation near cracks. The role of the stress state dependence of cavity nucleation on the crack growt h direction is emphasized. (C) 1998 Elsevier Science Ltd. All rights reserv ed.