# Experimental Study on Anisotropic Mechanical Characteristics of Shale under Triaxial Loading
## 6 Conclusions  
(1) The AE characteristics of shale samples with different bedding angles in the compaction, elastic, plastic, and failure stages are significantly different during uniaxial loading, and the variation of AE hits, AE cumulative hits, and AE ringing counts with time are closely related to the angle between loading direction and bedding planes and the progressive failure mode of those samples.  
(2) The uniaxial and triaxial compressive strength \(\sigma_{\rm cs}\) of shale samples is anisotropic. Under different confining pressures, the compressive strength of shale samples first decreases and then increases with the increase in the bedding angle. The compressive strength of shale samples reaches the minimum value when the bedding angle is 60\({}^{\circ}\), while the maximum values of compressive strength are obtained at the bedding angle of 0\({}^{\circ}\) when the confining pressure is 0\(\sim\)30 MPa and obtained at the bedding angle of 90\({}^{\circ}\) when the confining pressure is 40 MPa. At the same time, the compressive strength and confining pressure of shale samples with the same bedding angle conform to the linear growth relationship.  
(3) The elastic modulus \(E_{\rm\theta}\) of shale samples also has anisotropic characteristics. Under the designed confining pressure, the elastic modulus of shale samples decreases first and then increases with the increase in the bedding angle, and the maximum values of elastic modulus are obtained when the bedding angle is 90\({}^{\circ}\), while the bedding angle where the minimum value is located tends to increase gradually from 30\({}^{\circ}\) with the increase in confining pressure. The elastic modulus of shale samples with the same bedding angle has a linear growth relationship with confining pressure, while the variation of Poisson's ratio with the bedding angle or confining pressure is disordered.  
(4) According to the anisotropic grade of compressive strength \(\sigma_{\rm cs}\), the shale samples have moderate anisotropy. The variation of anisotropic index of compressive strength and elastic modulus of shale with confining pressure is different. In the confining pressure range of 0-35 MPa, the anisotropic index of compressive strength decreases with the increase in confining pressure, while the anisotropy index of elastic modulus increases gradually.  
(5) The failure mode of shale samples is also anisotropic. In the uniaxial and triaxial experiment, the shale samples with 0\({}^{\circ}\) and 30\({}^{\circ}\) bedding angles mainly undergo through-bedding plane shear failure, and the samples with a 45\({}^{\circ}\) bedding angle mainly experience the composite failure of through-bedding plane shear failure and along-bedding plane shear failure. The failure mode of shale samples with a 60\({}^{\circ}\) bedding angle is mainly along-bedding plane shear failure, while the samples with a 90\({}^{\circ}\) bedding angle are dominated by tensile failure and will be accompanied by some tensile and shear cracks through the bedding planes under high confining pressures.  
Conceptualization, Q.D.; methodology, Q.D., J.K. and J.L.; software, Q.D. and J.L.; validation, J.S. and Z.Z.; formal analysis, Q.D. and J.K.; data curation, Q.D., J.K. and J.L.; writing--original draft preparation, Q.D., J.K. and J.L.; writing--review and editing, Q.D. and J.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (52108368 and 42102329), National Key Research and Development Project of China (2021 YFC3100804), Key Research and Development Project of Hubei Province, China (2021 BAD004), and Hubei Provincial Natural Science Foundation (2023 AFB1075).  
Not applicable.  
The data used to support the findings of this study are available from the corresponding author upon request.  
The authors declare no conflicts of interest.