THE GRIFFITH CRITERION FOR MODE-II FRACTURE

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.