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.