Mode I-II combined mode fracture criterion for brittle materials

The Griffith energy criterion for brittle fracture is extended to mode II and combined mode I-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 the mode II or the combined mode crack extension. The fracture energy rate is considered to be proportional to the critical magnitude of the area of a frontal process zone at a crack tip for brittle materials. The area of the frontal process zone is simply assessed here from the area enclosed in the iso-stress contours of both the maximum principal stress and the maximum shear stress around a crack tip. The anticipated ratio of mode II and mode I fracture toughness, K-IIC/K-IC, is 1.20. The value of the ratio, K-IIC/K-IC, is also estimated experimentally for float glass using a disk test. The result shows that the value of K-IIC/K-IC is 1.28 which agrees quite well with the anticipated one.