Fracture characteristics of rocks under shear loading

Fracture problems in rock structures quite commonly involve shear loadings including in-plane shearing (mode II) or out-of-plane shearing (mode III). Even though a rock mass may be subjected to simple tension or shear loading, due to the complexity of the geometry and the random orientation of cracks in rocks, the crack growth occurs in mode II or mode III manner. In this study, the fracture toughness and strength of Coconino sandstone under in-plane and out-of-plane shear were determined experimentally. A short beam compression test was employed for in-plane shear loading and a circumferentially notched cylindrical specimen was employed for out-of-plane shear loading. In addition, the mode II fracture toughness and strength were determined at different loading rates to examine the effect of loading rate. As a result, the in-plane (mode II) shear strength was approximately 2 times higher than out-of-plane (mode III) shear strength. The mode II fracture toughness was approximately 1.78 times higher than the mode III fracture toughness. As the loading rate increases, the mode II fracture toughness and the shear strength also increase.