## 3 Test Results
### Stress-Strain Curve  
Figure 7 shows the axial stress-strain curves of sandstone samples with different bedding directions after heat treatment at different temperatures under uniaxial and triaxial compression. These stress-strain curves show similar changes, including the compaction stage, linear elastic deformation stage, unstable crack growth stage, and failure stage. Because the stress-strain curves of sandstone with the same dip angle and different temperature treatments are basically the same, and the stress-strain curves of sandstone with different dip angles and the same temperature level are basically the same, only part of the curve has been selected for analysis in this paper. Figure 7 shows that the shape and deformation characteristics of the sandstone stress-strain curve are jointly controlled by the temperature and bedding dip angle. Figure 7a shows the change in the stress-strain curve of sandstone with the same bedding dip angle after treatment at different temperature levels. For sandstone with the same bedding angle, when \(T=200\)\({}^{\circ}\)C, the peak position of the stress-strain curve shifts slightly, but the shape does not change significantly, indicating that the temperature of \(200\)\({}^{\circ}\)C slightly affected the strength of the sandstone, but the effect is not significant. When \(T=400\)\({}^{\circ}\)C and \(600\)\({}^{\circ}\)C, the stress-strain curve of the sandstone showed significant changes at each deformation stage compared with \(T=200\)\({}^{\circ}\)C, indicating an increase in peak stress and peak strain, an extension of the pore compaction stage, indicating that temperature increased the strength of the sandstone, but at the same time, the sample exhibited many cracks under the influence of high temperature. When \(T=800\)\({}^{\circ}\)C, the peak strain of the sandstone further increased, the peak strength decreased, the pore compaction stage was further prolonged, and the elastic stage shortened, indicating that the temperature caused the internal cracks of sandstone to develop more, and the peak strength of sandstone decreased. When \(T=1000\)\({}^{\circ}\)C, compared with the normal temperature state, the stress-strain curve of sandstone has a significant reduction in peak stress and strain softening. After the rock sample reached the peak stress, the stress drop rate slowed significantly,  
Figure 6: (**a**) Variation trend of the S-wave velocity variation rate with temperature for rock samples with different bedding dip angles; (**b**) variation trend of S-wave velocity with dip angles for rock samples treated at different temperatures. and the axial strain continued to grow slowly during the drop process, indicating that the temperature makes the internal cracks of sandstone develop and connect with each other, leading to a significant reduction in the strength of the sandstone. Figure 7: \(Cont.\)  
Figure 7b shows that the sandstone stress-strain curves at the same temperature level with different bedding dip angles are also different. When \(T=25\)\({}^{\circ}\)C, compared with other bedding dip angles, the stress of \(\beta=45\)\({}^{\circ}\) sandstone decreased before reaching the  
Figure 7: (**a**) Stress-strain curve of the rock sample at the same dip angle in the uniaxial compression test; (**b**) stress-strain curve of the rock sample at the same temperature in the uniaxial compression test; (**c**,**e**,**g**) stress-strain curves of rock samples at the same dip angle in the triaxial compression tests under confining pressure of 5–15 MPa; (**d**,**f**,**h**) stress-strain curves of rock samples at the same temperature in the triaxial compression tests under confining pressure of 5–15 MPa. peak value, indicating that the rock sample was partially damaged along the bedding plane before complete failure. When \(T\) = 200 \({}^{\circ}\)C, sandstones with all bedding dip angles showed typical plastic elastic stress-strain curves. When the rock sample reached the peak strength, obvious brittle failure occurred, and the stress rapidly decreased.