## 4 Analysis of Test Results
### Analysis of Triaxial Compression Test Results
#### 4.3.3 Failure Patterns  
Figure 16 shows the fracture morphology of group B samples after triaxial compression after high temperature action. Compared with the failure mode of the uniaxial compression test in Figure 11, the damage mode in the triaxial compression test is relatively simple, and most of them are typical shear failures, it is likely to be due to the presence of a confining pressure. The shear surface of the individual samples is conical, and the main shear fracture surface has fine powdery substances with obvious friction marks. Most of the shear plane starts at the edge of the end face of the specimen, and the individual specimens are cracked at the end of the specimen. The projection of the entire shear plane in the horizontal plane covers the cross section of the whole sample, and a few horizontal cracks perpendicular to the loading direction appear on the shear surface of a few specimens. For example, in Figure 16, the B8 sample under the action of confining pressure 15 MPa after 400 \({}^{\circ}\)C high temperature and the B12 sample under the action of confining pressure 10 MPa after 600 \({}^{\circ}\)C high temperature may cause the main rupture surfaces slip each other and a horizontal force causes the slip block to break laterally.  
Figure 16: Rupture morphology of samples under conventional compression condition after high temperature.  
Figure 17 shows the relationship between the fracture angle of the sample, the temperature, and confining pressure after the high temperature action. Although the sample is affected by its own heterogeneity, the test results are somewhat discrete. The triaxial compression fracture angle of the sample under different temperature and confining pressure fluctuates. At high temperatures, the actual fracture angle of the sample is consistent with the theoretical calculation value. The effect on the fracture angle of the sample is small, and the fracture angle of the sample increases slightly as the temperature increases, as shown in Figure 17a. Compared with the actual rupture angle of 64.6\({}^{\circ}\) at room temperature of 25 \({}^{\circ}\)C, the average actual rupture angles of the samples after 400, 600, 800, 1000 \({}^{\circ}\)C is 64.6, 63.0, 66.6, 66.0, and 67.7\({}^{\circ}\), the increase rate is \(-\)2.5%, 3.1%, 2.2%, and 4.6%, respectively; according to Coulomb strength criterion theory, the fracture angle (45\({}^{\circ}\) + \(\varphi\)/2), is 67.5, 66.4, 69.1, 67.4, and 71.7, the average actual rupture angle of the visible sample is less than the theoretical calculated rupture angle of 1.4\({}^{\circ}\)\(-\)4.1\({}^{\circ}\). This can explain the friction effect of the sample on the upper and lower end heads. During the compression of the sample, the axial compression of the sample is laterally expanded, while the stiffness of the upper and lower end heads is much larger than the stiffness of the sample, which restricts the lateral expansion of the sample to produce friction, thereby affecting the crack propagation at the end of the sample, so that the actual cracking and outward rupture angle becomes small.  
Figure 17b shows that the actual fracture angle of the sample is negatively correlated with the confining pressure after the high temperature action, and the actual fracture angle monotonously decreases with the increase of the confining pressure. Compared with the actual rupture angle of 68.4\({}^{\circ}\) of 5 MPa, the average rupture angles of the specimens under the confining pressures of 10, 15, 20, and 25 MPa is 67, 66.6, 64.2, and 61.6\({}^{\circ}\), with a decrease of 2.04%, 2.63%, 6.14%, and 9.94%, respectively. It shows that the influence of confining pressure on the fracture angle of the sample is greater than the influence of temperature.