The model of crack propagation in polycrystalline copper at various propagating rates

In this work, the fracture surface of the crack paths in specimens of pure copper have been studied by secondary electron imaging and the dislocation structures in front of crack tip were observed by backscattered electron imaging. It was observed that the propagation of the cracks is mainly transgranular, but the behavior models change in accordance with the variation of dislocation structures located at the front of the crack tip. With a 10−6 mm/cycle rate of cracking, the dislocation structures are in full development with a vast range of area. Therefore, a model for the propagation of cracks is transgranular along the sides of grain boundaries (GB). However, with a propagation rate of 10−7 mm/cycle, the dislocation structures evolve incompletely, and the range of cell or ladder-like walls of persistent slip band is limited to a small area. The sequence of crack propagation is initiation, propagation in strain localization in front of the crack tip, and then coalescence with the crack tip. The crack propagation is transgranular, but the side of the GB is still the preferred path. A model is proposed to explain these behaviors. The major factors of the model are the evolution of the dislocation structures at the crack tips, and the interactions between the grain boundaries and persistent slip bands.