Abstract
Mode II fracture toughness is an important material parameter of rocks, but accurate measurement of this parameter is still a challenge in rock fracture mechanics. This study aims to modify the mode II fracture toughness of red sandstone measured through shear box testing by emphasizing the critical role of crack initiation angle. Experimental tests combining fracture trajectory scanning and digital image correlation reveal distinct fracture mechanisms of red sandstone under varying loading angles: tensile spalling dominates low angles, and shear fractures emerge at medium angles, while tensile fracture initiates from the rock bridge center at high angles. Although shear fracture initiates from the notch tip, its initiation angle deviates from the initial crack plane, invalidating traditional mode II fracture toughness determination methods. A modified Mohr-Coulomb criterion incorporating fracture angle and Mode I stress intensity factor is proposed to correct the significant errors of traditional methods, and this study establishes a refined framework for mode II fracture toughness determination under compression-shear conditions.