# Experimental Study on Anisotropic Mechanical Characteristics of Shale under Triaxial Loading
## 3 Experimental Results  
### Uniaxial Compression Results and Acoustic Emission Characteristics  
The stress-strain curves of shale samples with different bedding angles in uniaxial compression tests, as well as the variation of AE hits, AE cumulative hits, and AE ringing counts with time, are shown in Figure 4. It can be observed that the uniaxial compressive strength of shale samples varies with the bedding angle, and the average uniaxial compressive strength of shale samples with bedding angles \(\theta\) of \(0^{\circ}\), \(30^{\circ}\), \(45^{\circ}\), \(60^{\circ}\), and \(90^{\circ}\) is 171.4, 113.0, 98.6, 61.0, and 117.7 MPa, respectively. This indicates that the uniaxial compressive strength of shale samples first decreases and then increases with the increasing of the bedding angle, and the AE characteristic parameters of shale samples with different bedding angles under uniaxial compression are not identical, as explained below. (1) \(\theta\) = 0\({}^{\circ}\): As shown in Figure 4, at first, the sample enters the compaction stage, and the stress increases slowly. Because the loading direction is perpendicular to the bedding planes which have weak stiffness relative to the shale matrix, the bedding planes are compacted before the shale matrix. In this process, no new microcracks are generated, so only a small number of AE ringing counts and AE hits can be detected. Secondly, as the sample enters into the elastic stage, the stress-strain curve of the sample shows a linear growth trend, and the microcracks in the sample slowly increase, so the AE ringing counts increase slightly. Moreover, most of the collected AE signals do not exceed the threshold, so the AE hits remain relatively low. Subsequently, in the plastic stage, the volume strain of the sample gradually increases, new microcracks are constantly generated, and the original closed microcracks continue to expand, resulting in a rapid increase in AE ringing counts and AE hits. Finally, in the failure stage, the microcracks in the sample are penetrated and the main cracks run through the sample, resulting in the specimen reaching the peak strength and failure. Meanwhile, the AE ringing counts, AE hits, and AE cumulative hits increase sharply and reach the peak. (2) \(\theta\) = 30\({}^{\circ}\): As shown in Figure 4, the original microcracks of the shale matrix and bedding planes in the sample are simultaneously compacted during the compaction stage, so the AE ringing counts and AE hits begin to increase at this stage. As the sample enters the elastic stage, the microcracks inside the sample increase uniformly, and the AE ringing counts also maintain a stable growth. During this period, only a small amount of the AE signal exceeds the threshold value, so the AE hits are collected intermittently, and the AE cumulative hits continue to increase. After entering the plastic stage, the number of microcracks in the sample continue to increase, and the AE ringing counts further increase. However, the AE signal in this stage hardly exceeds the threshold value, so there are no AE hits generated. Finally, the AE ringing counts, AE hits, and AE cumulative hits of the shale sample increase sharply and reach the peak in the failure stage. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \begin{tabular}{c} **Bedding** \\ **Angle \(\theta\) (\({}^{\circ}\))** \\ \end{tabular}  & \multicolumn{4}{c}{**IDs of Shale Samples**} \\ \hline
0 & A\({}_{1}\), A\({}_{2}\), A\({}_{3}\) & A\({}_{4}\), A\({}_{5}\), A\({}_{6}\) & A\({}_{7}\), A\({}_{8}\), A\({}_{9}\) & A\({}_{10}\), A\({}_{11}\), A\({}_{12}\)  
(3) \(\theta=45^{\circ}\) and \(60^{\circ}\): As shown in Figure 4, the uniaxial AE test data of the samples with bedding angles of \(45^{\circ}\) and \(60^{\circ}\) are relatively similar. During the compaction stage, the microcracks inside the sample begin to be compacted, and the stress-strain curve slowly grows. In this process, the microcracks are compacted, the AE ringing counts are collected relatively evenly, and the AE hits are also relatively uniform.