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 directly on the underlying physical micromechanisms. A crack tip process zone is introduced in which grains and their grain boundaries are represented discretely, while the surrounding undamaged material is described as a continuum. Special-purpose finite elements are used to represent individual grains and grain boundary facets. The constitutive description of the grain boundary elements accounts for the relevant physical fracture mechanisms, i.e. viscous grain boundary sliding, the nucleation of grain boundary cavities, their growth by grain boundary diffusion and local creep, until coalescence of cavities leads to microcracks. Discrete propagation of the main crack occurs by linking up of neighbouring facet microcracks. Assuming small-scale damage conditions, the model is used to simulate the initial stages of growth of an initially sharp crack under C* controlled, mode I loading conditions. Material parameters are varied so as to lead to either ductile or brittle fracture, thus elucidating creep constrained cavitation near cracks. The role of the stress state dependence of cavity nucleation on the crack growth direction is emphasized.