Abstract: Existence of rock microcracks has great influence on rock shear fracture under compressive loadings. However, the effects of microcrack geometries and confining pressures on shear fracture angle are rarely studied in rocks. In this study, an improved expression of stress intensity factor containing crack angle effect is derived, which is based on the proposed expression at crack tips in Ashby’s microcrack model. Using this improved stress intensity factor, the constitutive relationship between crack growth, strain and stress is obtained, and it predicts the peak strength in rocks. Coupling this constitutive relationship at state of peak strength and Mohr-Coulomb failure criterion, the correlations of internal friction angle, cohesion, shear strength and damage are obtained. Effects of confining pressure, initial crack size, crack angle and friction coefficient on shear fracture angle are discussed. The rationality of theoretical model is verified with experimental data. The results show that internal friction angle, cohesion and shear strength decrease with increment of damage; shear fracture angle increases with the increment of confining pressure and friction coefficient, or the decrement of initial crack size; shear fracture angle decreases firstly, and then increases with increment of crack angle.

Key words: rock mechanics, shear fracture angle, crack angle, confining pressure, damage