Effects of bedding orientation on the failure pattern and acoustic emission activity of shale under uniaxial compression

The bedding orientation influences the failure pattern and brittleness of shale. Clarifying the relation between acoustic emission and the effects of bedding orientation on the failure behaviors of shale is important to understand the mechanism of the formation of fracture networks. In this study, we analyzed the failure patterns and crack complexity of the shale specimens with various bedding orientations under uniaxial compression using X-ray computed tomography. We proposed an improved criterion based on the acoustic emission waveform parameters to identify the mechanism of the acoustic emission events and showed that the failure mechanism changed from tension to shear during the failure process of each specimen because the shear cracks link independent tensile cracks to form macrofractures in the later loading stage. The proportion of shear-mode events in the specimens declined when the bedding orientation angle increased from 0 degrees to 90 degrees, whereas that of the tensile-mode events exhibited an opposite trend; these changes were consistent with the observation results of X-ray images, indicating the transition of the dominant failure mechanisms with the bedding orientation. The large b value of the acoustic emission events corresponded to a complex crack distribution; thus, this b value may be useful to evaluate the brittleness of shale. These results denote the relation between the mechanisms of the failure patterns in shale and the acoustic emission activity and improve our recognition to the fracture network characteristics.