Crack propagation mechanism of single- and double-flawed rock specimens under tension–shear stress condition

The rock mass of underground engineering is often faced with tensile–shear failure disaster due to excavation. However, the failure mechanism of flawed rocks under tensile–shear stress state is not well understood at present and there is no relevant experimental investigation due to the difficulty of test technology. To reveal the crack propagation mechanism of flawed rocks under tensile–shear stress, experimental investigation using a developed tension–shear auxiliary device and numerical simulation were conducted in this study. The results obtained from direct shear tests of single-flawed sandstone specimens under constant normal tensile stress indicate that the flaw inclination angle and normal tensile stress significantly affect the shear strength and crack pattern. The cracks are mainly subjected to tensioning for different flaw inclination angles. It is easy to develop secondary cracks to produce the nucleation failure with the primary cracks when the flaw inclination angle is an acute angle. Digital image correlation (DIC) analysis suggests that the strain concentrations are greater gradually near the two tips of the flaw and the cracks grow along the strain concentration areas. Furthermore, the crack propagating process and stress field evolution in single-flawed and double-flawed specimens were examined by DEM simulations. The primary cracks are initiated at the tensile stress concentration areas of flaws, and then the concentration areas transfer to the crack tips and impel the crack propagating. The nucleation failure formed by two wing cracks is a main rock bridge failure pattern in double-flawed specimens. The nucleation failure area becomes small and maybe disappeared due to the increase in normal tensile stress. The conclusions can provide the theoretical reference for the stability evaluation of underground rock mass excavation.