Geometry and loading effects on pure mode II fracture toughness of rock materials

Mode II fracture due to shear loading is a significant mode of failure in rock masses and therefore it is important to study pure mode II crack growth behavior of rock materials using appropriate methods. In this paper, a set of fracture tests was conducted on a soft rock material (Guiting limestone) using two geometries of specimen: centrally cracked Brazilian disc (CCBD) and semi-circular bend (SCB), to obtain the pure mode II fracture toughness (K _IIc). The experimental results show that the mode II fracture toughness is significantly geometry and loading dependent. Specifically, the value for K_IIc obtained using the CCBD specimen is about 2.4 times higher than that obtained using the SCB specimen. These results cannot be interpreted using conventional fracture criteria such as the maximum tangential stress (MTS) criterion. However, it is shown that a generalized form of the MTS criterion, the GMTS criterion which accounts for the effect of the nonsingular stress term (T-stress), can provide very good predictions for the pure mode II fracture toughness of both samples. Accordingly, the mode II fracture toughness of the CCBD specimen is enhanced due to the large negative T-stress that exists in this specimen. Conversely, the mode II fracture toughness of the SCB specimen is reduced by the large positive T-stress.