# Experimental Study on Mechanical Properties of Anisotropic Slate under Different Water Contents
## 3 Experimental Results and Analysis  
### Stress-Strain Curve Characteristics  
In rock mechanics, the stress-strain curve of a rock typically describes the material's deformation characteristics, with relevant parameters calculated from it. The standard rock stress-strain full-process curve divides into five distinct stages (as shown in Figure 5) [20; 21; 22]; the compaction stage of pores and fractures (OA section), the elastic deformation stage (AB section), the stable development stage of microplastic fractures (BC section), the unstable cracking stage (CD section), and the post-failure stage (DE section). Figures 5-7 show the stress-strain curves of dried, natural, and saturated samples from these tests, respectively. These curves generally represent the five stages mentioned. In these figures, the positive half-axis of \(x\) indicates axial strain, and the negative half-axis represents hoop strain. Figure 4: MTS 815 triaxial rock testing system. 1, reaction frame; 2, normal loading chuck; 3 and 6, washer; 4, heat-shrink tube; 5, sample; 7, triaxial pressure chamber; 8, exhaust pipe; 9, oil inlet; 10, oil outlet; 11, signal transmission line; 12, test data acquisition system; 13, valve. Figure 5: Typical stress–strain curve of rock. Under uniaxial compression conditions, as depicted in Figures 6a, 7a and 8a, the hoop and axial stress-strain curves of the slate samples initially exhibit a pronounced linear elastic trend. The microelastic fracture stable development stage is brief. Without confining pressure, internal defects in the slate evolve and merge rapidly. At peak strength, fractures are fully developed, and the sample quickly disintegrates, with a sharp stress drop, demonstrating typical brittle destructive deformation. Samples with identical bedding angles show similar curve shapes in triaxial compression conditions. Increasing confining pressure restricts slate deformation and postpones failure. The stress decline at peak values in most samples becomes more gradual and pronounced in the post-peak stage. Brittle failure characteristics diminish, while plastic failure traits intensify. Additionally, as confining pressure rises, the peak strength of the slate increases. If the natural state specimen is considered an example (Figure 6), a 5 MPa increase in confining pressure on average boosts the compressive strength of 0\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) samples by 17.93, 14.17, 11.6, 14.15, and 31.54 MPa, respectively. The 45\({}^{\circ}\) and 60\({}^{\circ}\) samples are more notably influenced by the bedding plane, which has weaker mechanical properties. After compression, the slate is prone to significant plastic deformation, and the energy release rate decreases, making the yield stage more apparent and the post-peak stress reduction rate slower, as seen in Figures 6a and 8b,c. Under varying water content conditions, the peak strength of samples typically follows the pattern: dry \(>\) natural \(>\) saturated. For example, at the same confining pressure, the average compressive strength of dry 0\({}^{\circ}\) samples is 1.26 times that of natural state samples, and the saturated samples' average compressive strength is 0.73 times that of the natural state. Water immersion in slate leads to mineral dissolution along the bedding direction, increasing the number and size of micro-fractures and making shear failure along the bedding plane more likely. The failure of loaded slate is fundamentally an energy absorption and release process. Shear slip along the bedding plane causes energy loss, reducing energy released during failure. The 60\({}^{\circ}\) sample curves in Figures 6d, 7d and 8d reveal that the elastic-brittle characteristic of slate gradually changes to an elastic-plastic (ductile) characteristic with increasing moisture content. Figure 6: Stress-strain curve of the slate sample in a dry state. (**a**) \(\sigma\)3 = 0 MPa; (**b**) \(\sigma\)3 = 5 MPa; (**c**) \(\sigma\)3 = 10 MPa; (**d**) \(\sigma\)3 = 15 MPa.