SIMULATION OF MICROFRACTURE PROCESS AND FRACTURE STRENGTH IN 2-DIMENSIONAL POLYCRYSTALLINE MATERIALS

Microfracture processes of microcracking and crack propagation are sim ulated along with fracture strengths for 2-dimensional alumina polycry stals which have thermal anisotropy within a grain. Microcracks are ge nerated by thermally induced residual stresses at a grain boundary. Th e stress concentration near the microcrack is calculated numerically b y the body force method, and superposed on the pre-existing residual s tress. Stress intensity factors at the microcrack tip are also obtaine d by the method, and the location at which the next microfracture occu rs is determined by the competition between microcracking and crack pr opagation in the new stress state. The microfracture stress increases with the progress of the fracture and decreases after maximum indicati ng a fracture strength. In many cases, the propagation of microcracks induces an unstable fracture. With decreasing grain size and increasin g grain boundary toughness, the number of microfractures prior to the unstable state decreases, while the fracture strengths increase. For a lumina of grain size 17.5 mu m, when the fracture toughness of the gra in boundary is 0.6 times that of the grain or greater, unstable fractu re occurs prior to stable microcracking.