## 3 Test Results
### Failure Mode  
The failure mode is affected by the temperature, bedding angle, and confining pressure. With increasing temperature, it is obvious that the failure mode of rock samples with the same bedding angle changes from brittle failure to ductile failure [8; 12; 22; 23]. Although the five groups of bedding angle samples basically exhibited shear failure, the failure mode still changed with increasing bedding angle. From Figure 13, it can be seen that under uniaxial compression, the rock sample at \(\beta\) = 0\({}^{\circ}\) was basically split through the bedding plane vertically, the middle part of the sample had a tendency to lateral bulging at higher temperatures, and cracks developed along the bedding plane. The rock sample at \(\beta\) = 30\({}^{\circ}\) exhibited shear failure that passed through the matrix and bedding plane, and the lateral bulge was not obvious at lower temperatures. At higher temperatures, there was a failure trend of a lateral bulge in the middle of the sample, which made the rock sample break along the bedding plane. The rock sample at \(\beta\) = 45\({}^{\circ}\) exhibited shear failure that passed through the matrix and bedding plane. The end of the sample was relatively broken due to stress concentration at low  
Figure 12: (**a**) Variation trend of ultrasonic velocity anisotropy with temperature; (**b**) variation trend of mechanical index anisotropy with temperature. temperature, while the lateral bulge in the middle of the sample was not obvious. The end of the sample was not obviously broken and the lateral bulge was not obvious at high temperature. The \(\beta=60^{\circ}\) rock sample included a shear failure that ran through the whole sample along the bedding plane. The end of the sample was obviously broken due to the stress concentration, and the middle part of the sample showed a failure trend of lateral bulging at higher temperatures. This failure caused cracks to develop along the bedding plane of the rock sample. The rock samples at \(\beta=90^{\circ}\) were fractured along the bedding plane, and the lateral bulge and end crushing were not obvious. From Figure 13, it can be seen that under low confining pressure, the failure mode of the \(\beta=0^{\circ}\) rock sample at various temperatures was matrix-controlled splitting tensile  
Figure 13: Failure diagram of sandstone samples with different bedding angles after uniaxial and conventional triaxial compression. failure through the matrix and bedding plane. The failure mode of the \(\beta\) = 30\({}^{\circ}\) rock samples at 25-800 \({}^{\circ}\)C was matrix-controlled composite tensile shear failure through the matrix and bedding plane. At 1000 \({}^{\circ}\)C, the failure mode was composite tensile shear failure of the local bedding plane and local crossbedding plane jointly controlled by the matrix and bedding plane. At 25-800 \({}^{\circ}\)C, the failure mode of the rock samples at \(\beta\) = 45\({}^{\circ}\) was composite tensile shear failure of the local bedding plane and local cross-bedding plane jointly controlled by the matrix and bedding plane. At 1000 \({}^{\circ}\)C, the failure mode was shear failure of a single shear section through the matrix and bedding plane, jointly controlled by the matrix and bedding plane. The failure mode of the \(\beta\) = 60\({}^{\circ}\) rock samples at various temperatures was shear slip failure along the bedding plane controlled by the bedding plane. The failure mode of the \(\beta\) = 90\({}^{\circ}\) rock samples at various temperatures was tensile splitting failure controlled by the bedding plane along the bedding plane. This is because micropores and microcracks are densely distributed near the bedding plane, and under low confining pressure, the degree of closure of micropores and fractures on the bedding plane is relatively low. The degree of anisotropy in the mechanical properties of sandstone was not significantly weakened.