A simulation model for cleavage crack propagation in bcc polycrystalline solids

A model for cleavage crack propagation in a body-centered cubic polycrystalline solid is proposed. The model is based on a criterion that one of the three {1 0 0} cleavage planes in a grain located in front of a crack tip is selected so that normal stress acting on the plane is maximum and the grain is cleaved if the maximum normal stress exceeds the cleavage fracture stress. The normal stress is calculated from mixed mode local stress intensity factors, which are calculated by first-order approximations, where crack surface irregularity, crack front non-straightness and crack closing force acting at ridges between cleavage facets are considered. The calculated fracture surface morphology was compared closely with that of a Charpy impact-tested specimen of low-alloy steel. Ridges formed between the cleavage facets play an important role in the fracture propagation process and fracture surface morphology. The influence of stress triaxiality in the process zone on the extent of fracture surface irregularity is also discussed.