Fracture surface analysis of ceramics

Abstract

Flaw size, c, fracture mirror boundaries, r, fracture stress, σ, and critical fracture energy were measured for glasses, glass ceramics, and single and polycrystalline ceramics. The relationship σr1/2 = constant was verified for all these materials. The mirror constants, A, in these materials were shown to be directly proportional to the average critical stress intensity factor for crack propagation, KIC. Based on the A — KIC relationship, the outer mirror to flaw size ratio is shown to scatter about a value of 13∶1. Thus, the mirror constants were used to predict critical flaw sizes in these materials. The observed flaw sizes in most cases correlated well with those calculated. The cases in which poorer correlation was obtained are those in which flaw sizes were smaller than the grain size, flaws were pores or surrounded by porous regions, or where severe microcracking existed. It is shown that the elastic modulus is proportional to the mirror constant and probably to the critical fracture energy, but that the latter is highly dependent on local microstructure. The smaller inner to outer mirror ratios for polycrystalline ceramics over glasses is attributed to the difference in available paths for crack propagation.