Effect of bedding orientation on failure of black shale under Brazilian tests and energy analysis

The anisotropic characteristics of tensile strength and failure mode of black shale from Longmaxi Formation are investigated by using Brazilian tests. The evolution of absorbed energy during the deformation and failure of black shale is analyzed, and the relationship among the ultimated absorbed energy, bedding angles, tensile strength and AE energy is revealed. The results show that: (1) The tensile strength of black shale is obviously influenced by bedding orientations. The maximum tensile strength appears at angle of 90°, while the minimum tensile strength occurs at near 30°. In the ranges 0°≤ θ≤30°and 60°≤θ≤ 90°, variation of the tensile strength with bedding angles is small, however, it grows rapidly with the increase of bedding angles in the range 30°≤θ≤60°. (2) The failure modes of specimens can be divided into three typical fractures. The specimens with low bedding angles only show single variety fracture and more than one variety fractures are found in samples with high bedding angles. (3) The absorbed energy grows nonlinearly with the increase of load. The growth rate of absorbed energy reflects the violent extent of specimen failure, and the violent extent of failure decreases in turn for all types of failures as follows: tensile-slip failure, slip-tensile failure, tensile failure, sliding failure and pressure tension destruction. (4) The ultimated absorbed energy is small at low bedding angles and is large at high bedding angles. In the range 30°≤θ≤60°, variation of the ultimated absorbed energy is very large. The corresponding quadratic nonlinear relationship presents itself between the ultimated absorbed energy and the tensile strength, AE energy, respectively, which may provide references for the exploitation and development of shale gas.