ANALYSIS OF CRACK DEFLECTION FRACTURE-BEHAVIOR OF AL2O3 POLYCRYSTALS

A crack deflection model reported previously to explain fracture behavior of ideal polycrystalline materials, is applied to analyze fracture behavior of Al2O3 polycrystals. For anisotropic effects within a grain, the experimental results on single crystal Al2O3 are used, and crack propagation through a grain is considered. The calculated fracture toughness and propagation angle when a crack propagates in an anisotropic single crystal are combined with the crack deflection model. As a result, the fracture toughness, transgranular fracture ratio and distribution of crack propagation angle are calculated as a function of grain boundary toughness. The maximum fracture toughness obtainable in the equiaxed Al2O3 polycrystals is estimated to be about 4.8 MPasquare-rootm which is about 3 times that of single crystals, when the grain boundary toughness is 1.4 MPasquare-rootm. The primary reason for the increased toughness of polycrystals over that of single crystals is attributed to crack path restriction at triple and quadruple points. The predicted theoretical relationship between fracture toughness and transgranular fracture ratio showed good agreement with the experimental results. Also the predictions for the distribution of crack propagation angle are consistent well with the experiments.