ENERGY CRITERION FOR MODE-II FRACTURE IN CERAMICS

The Griffith energy criterion for brittle fracture is extended to mode II fracture by postulating that crack extension occurs when the maxim um energy release rate in non-coplanar crack extension is equal to the fracture energy rate required for mode II crack extension. This fract ure energy rate is considered to be proportional to the critical magni tude of the area of the frontal process zone at the crack tip of inorg anic materials, where the critical magnitude is the value at crack pro pagation. The frontal process zone area is simply assessed here from t he area enclosed in the iso-stress contours of both the maximum princi pal stress and the maximum shear stress in the vicinity of the crack t ip. The estimated K-IIC/K-IC ratio is 1.21 which is very close to expe rimental results previously presented.