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 maximum energy release rate in non-coplanar crack extension is equal to the fracture energy rate required for mode II crack extension. This fracture energy rate is considered to be proportional to the critical magnitude of the area of the frontal process zone at the crack tip of inorganic materials, where the critical magnitude is the value at crack propagation. The frontal process zone area is simply assessed here from the area enclosed in the iso-stress contours of both the maximum principal stress and the maximum shear stress in the vicinity of the crack tip. The estimated K-IIC/K-IC ratio is 1.21 which is very close to experimental results previously presented.