# Experimental Study on Anisotropic Mechanical Characteristics of Shale under Triaxial Loading
## 3 Experimental Results
### Triaxial Compression Results  
The stress-strain curves of the triaxial compression test for shale samples with different bedding angles are shown in Figure 5, and the stress-strain curves of the uniaxial compression test are also added in Figure 5 for comparative analysis. From this figure, it can be found that:  
(1) For shale samples with different bedding angles \(\theta\), the characteristics of stress-strain curves before peak strength are the same as those of homogeneous rocks, all showing obvious compaction stage (which gradually disappears with the increase in confining pressure \(\sigma_{3}\)), elastic deformation stage, and yield stage (or unstable fracture development stage). For the post-peak stress-strain curve, the shale samples with bedding angles of \(0^{\circ}\), \(30^{\circ}\), \(45^{\circ}\), and \(60^{\circ}\) show obvious brittle failure characteristics, and their bearing capacity decreases rapidly after the peak strength, with no obvious strain softening behavior. On the other hand, the samples with a \(90^{\circ}\) bedding angle show completely different ductile failure characteristics, and the stress-strain curves which do not decrease significantly after reaching the peak strength but fluctuate up and down while the strain increases, maintaining relative stability. The characteristic that the peak strength of the \(90^{\circ}\) bedding angle samples is approximately equal to the residual strength is more pronounced at higher confining pressure \(\sigma_{3}\), and their stress-strain curve are similar to the ideal elastic-plastic model. The reason for the unique stress-strain curve characteristics of the \(90^{\circ}\) bedding angle samples is that under the Poisson effect, radial tensile stresses are generated inside the samples, while for the \(90^{\circ}\) bedding angle samples, the tensile stresses act vertically on the bedding planes with extremely low tensile strength, resulting in tensile cracks along the bedding planes of the samples, so the axial stresses will decrease temporarily. However, the rock matrix is not destroyed yet, so the axial stresses will increase again. From the beginning of the phenomenon of stresses drop, the load-bearing mode of the samples is changed from bearing by a complete cylinder to multiple independent cylinders. At the same time, with the increase in axial stresses, these shale matrix columns will also be stretched and broken into more new columns under the Poisson effect, and their rupture (or instability) is also independent under axial loading. After the bearing failure of the columns with weak strength, the columns with higher strength continue to bear the load, and finally the unique up-and-down fluctuation stress-strain curve characteristics of the \(90^{\circ}\) bedding angle samples are formed. (2) As the confining pressure \(\sigma_{3}\) increases, the compressive strength \(\sigma_{\text{cs}}\) of shale samples with different bedding angles increases, and the axial and radial peak strains of the samples also increase. At the same time, it can be found that the strength of shale sampleswith different bedding angles under different confining pressures \(\sigma_{3}\) is also different, showing obvious anisotropic characteristics, which will be analyzed in detail in Section 4. (3) For the shale samples with bedding angles \(0^{\circ}\) and \(30^{\circ}\) under high confining pressure \(\sigma_{3}\) conditions, the stress-strain curves show a significant yield stage after the liner elastic stage. During this stage, the tangent modulus of the stress-strain curves gradually decreases. However, the stress-strain curves of shale samples with \(60^{\circ}\) and \(90^{\circ}\) bedding angles do not show a significant yield stage before failure, and the axial strains and radial strains exhibit an approximate linear relationship with stress before peak strength, while the plastic deformation is small. The main reason for this phenomenon is the difference in yield characteristics between the rock matrix and bedding planes.