Experimental and numerical study on the strength and hybrid fracture of sandstone under tension-shear stress

Hybrid tension-shear fracture of rock usually occurs in underground openings, rock slopes, transtension faults, and en echelon cracks. To investigate the tension-shear strength and the transition between tensile and shear fractures in rock, experimental and numerical tension-shear tests were carried out using an auxiliary device and particle flow code (PFC2D), respectively. The deformation and shear strength of a sandstone under tension-stress were analyzed. The initial crack (the first micro-crack) and the shear failure ratio (the proportion of shear cracks in the total number of cracks) were proposed and used to describe the mechanical properties of hybrid fracture. In addition, the force chains of parallel-bonds in the PFC model were investigated. The results show that the initial crack is a tensile crack if the normal tensile stress is large, whereas it is a shear crack when the normal tensile stress is smaller. The shear failure ratio decreases linearly with increasing normal tensile stress, and the micro-cracks are all tensile cracks in the uniaxial tensile simulation, which suggests that the transition from tensile to hybrid and shear fracture is continuous and linear. Most of the bonds between particles are subjected to tension-shear stress, while some bear compression-shear stress. In the tension-shear tests, shear microcracks may be formed under either tension-shear stress or compressive-shear stress. A new tension- shear failure criterion was proposed, which is more accurate than the existing failure criteria and is associated with the features of microscopic fractures. The proposed criterion can account for the microscopic physical significance of the parameter in the Hoek-Brown criterion.