Evaluation of Fracture Strength for Ceramics with Defects by Numerical Simulation (1st Report, Fundamental Theory and Application to Alumina Ceramics)

It is well known that the fracture strength of ceramics with small defects is lower than the strength estimated by linear fracture mechanics, based on the strength of a large defect. This phenomenon can be explained by the increase in the crack resistance with crack propagation. This R-curve behavior is caused mainly by grain bridging in the wake of the crack. Recently the grain-bridging stresses for alumina and silicon nitride have been measured experimentally. In this study, the fracture strength is estimated by numerical simulation, based on grain-bridging stresses. In the simulation, the effective stress intensity factor at the crack tip, K_up, and the effective fracture toughness at the crack tip, K_uP, are used for the conditions of stable and unstable crack propagation. It is found that the fracture toughness for long cracks, K_up, depends on specimen geometries, and the maximum values of K_c approach (K_up)c as the initial crack length approaches the width of the specimen. Furthermore a convenient method to evaluate (K_up)c experimentally is discussed.