Abstract
The Griffith energy criterion for brittle fracture is extended to mode II fracture on the postulation 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 estimated from the magnitude of the area of the frontal process zone at the crack tip. It is seen that the estimated KUCKIC, ratio is 1.21 for quite brittle materials. New experimental measurements on soda-lime float glass have been performed to measure values of mode I and II fracture toughness using the circular disk technique. The results obtained for the KUCKIC ratio for float glass is 1.27 which is approximately equal to the theoretically calculated value.