Microscopic studies of fractures generated under mixed mode loading

A set of fracture profile photomicrographs is presented for fractures generated under mixed mode loading of the Pittsburgh coal. The fractal dimension is used to characterize the irregularity of these profiles and a tentative relationship is presented between the fractal dimension and the fracture toughness. Surface roughness appears to depend solely on the stress intensity and no other dependence was observed with fracture loading mode, loading rate or specimen orientation. Observations of crack bifürcations in fracture profiles can indicate crack growth direction whereas direct surface observations generally cannot in materials such as rock and coal.