Experimental Study on the Uniaxial Compression Failure and Ultrasonic Transmission of Brittle Shale Considering the Bedding Effect

The rock failure process, especially the crack propagation process in rock, can essentially reveal many mechanical problems in the rock. The bedding structure of shale is one of the main influencing factors of rock failure. This study performed an ultrasonic dynamic transmission test on Longmaxi Formation shale specimens with different bedding angles under uniaxial compression and comprehensively analyzed the mechanical and ultrasonic evolution characteristics. The deformation and failure of the shale samples indicated obvious brittle characteristics, and the final failure mode was dominated by shear cracks along the bedding planes and tensile-compressive cracks along the maximum principal stress direction. The difference index of the bedding calculated by the ultrasonic response speed showed that with increasing deformation and failure, the influence of the bedding structure on the failure decreased. In the process of uniaxial compression, the ultrasonic transmission velocity changed periodically. Relying on the obvious jumping points of the ultrasonic velocity curve, we could initially judge the possible key points of the changes in the pores and cracks during the rock failure process. A new method to calculate the rock brittleness index based on the characteristic response velocity of ultrasonic waves was proposed. Compared with the classical method, this method revealed a good applicability. This study can further clarify the relationship between the acoustic characteristics of ultrasonic transmission and rock deformation and failure, and can lay a certain theoretical foundation for the subsequent study of ultrasonic transmission and rock structure, along with its mechanical, physical and chemical properties.