The Griffith energy criterion for brittle fracture is extended! to mod
e II fracture by postulating that crack extension occurs when the maxi
mum energy release rate in non-coplanar crack extension is; equal to t
he fracture energy rate required for mode II crack extension. This fra
cture energy I:ate is estimated from the magnitude of the area of the
frontal process zone at the crack tip. It has been found that the esti
mated K-IIC/K-IC ratio is 1.21 for quite brittle materials, which is v
ery close to the experimental data far several kinds of inorganic mate
rials presented previously.