Fracture statistics of brittle materials with surface cracks

Several different statistical fracture theories are developed for materials with cracks confined to the surface. All assume that crack planes are normal to the surface, but are otherwise randomly oriented. The simplest theory assumes that only the component of stress normal to the crack plane contributes to fracture. This theory is in fair agreement with biaxial fracture data on Pyrex glass obtained by Oh. When the contribution of shear is included in the analysis, the crack shape has to be considered. Several shapes are examined and the corresponding fracture statistics are derived. Two failure criteria are employed. In one the fracture occurs when the maximum tensile stress on some part of the crack surface reaches the intrinsic strength of the material. The other is based on a critical strain energy release rate. The assumption of shear-sensitive cracks leads to improved agreement with experiment, but really good agreement appears to require the assumption that the cracks have a preferred orientation.