Validity for measuring mode II fracture toughness using the short beam in compression method

Mode II fracture toughness is a crucial intrinsic property of rocks, and accurately measuring this parameter is a challenge in rock fracture mechanics. Among the various approaches for measuring the mode II fracture toughness, the short beam in compression (SBC) method is a convenient and effective option. Nevertheless, the validity of measuring the mode II fracture toughness using the SBC method is controversial. This study conducted numerical analyses and experiments on the SBC specimen with different geometries (C/H) to identify the fracture mode and initiation location. In addition, the improved fracture criterion was developed by introducing three components of T-stress to determine the fracture mode at crack tips and the crack initiation angle. The results reveal that the pure shear fracture can occur on notch tips of SBC specimens with C/H=0.1 and 0.2, indicating that the SBC method is valid for measuring the mode II fracture toughness of rocks. The modified fracture criterion can predict the fracture mode and the angle of shear crack initiation observed in the SBC experiment well. Another important finding is that, due to the compressive stress on the notch tip, the measured mode II fracture toughness of rocks using the SBC method should be corrected using the Coulomb friction model. It is also discovered that the length of the fracture process zone and the corrected mode II fracture toughness depend on the value of C/H. A small C/H leads to a significant error between the measured and corrected mode II fracture toughness.