## 4 Analysis of Test Results
### Analysis of Triaxial Compression Test Results
#### 4.3.2 Strength Properties  
Figure 14 shows the relationship between the three axis strength and the confining pressure and temperature after high temperature. From Figure 14, it can be seen that the peak strength of the triaxial compression test of group B is approximately linear with the confining pressure after high temperature, and this conforms to the Coulomb strength criterion. According to Coulomb strength criterion:  
\[\sigma_{1}=k\sigma_{3}+Q \tag{2}\]  
In the formula, \(Q\) and \(k\) are the material parameter. According to Equation (2), the relationship between the triaxial strength and the confining pressure is obtained, as shown in Table 1. After the high temperature of 800 \({}^{\circ}\)C, the influence coefficient k of the sample is about 5.233-6.833, which indicates that the high temperature of 800 \({}^{\circ}\)C has little effect on the confining pressure influence coefficient \(k\) of the sample. After 1000 \({}^{\circ}\)C, the influence coefficient \(k\) of the sample confining pressure increased significantly, reaching 9.078. From the fitting correlation of the test results of the samples after different high temperature, except for the low correlation coefficient of the sample after 800 \({}^{\circ}\)C high temperature; and the other fitting correlation coefficients are greater than 0.973. The axial strength has a good linear correlation with the confining pressure, which is in accordance with the Coulomb strength criterion. The comprehensive confining pressure influence coefficient is 6.541 and the correlation coefficient is 0.979. Figure 14: Relationship between three axis strength and confining pressure and temperature after high temperature. Figure 14b shows that the peak intensity of the triaxial compression is almost the same under the confining pressure conditions when the temperature is lower than 800 \({}^{\circ}\)C; that is, the peak intensity increases as the temperature increases. When the temperature exceeds 800 \({}^{\circ}\)C, the sample shows two different development trends: under low confining pressure (5 MPa, 10 MPa), the peak strength decreases with increasing temperature; under high confining pressure (20 MPa, 25 MPa), the peak intensity increases with the increase of temperature; while under the medium confining pressure (15 MPa), the peak intensity is weakly changed by temperature, showing a transitional sign. The Strength of the specimen depends on the bonding and friction based on Coulomb strength theory. However, the melting and disappearance of the fusible material inside the specimen at high temperature causes the micro-crack to increase, which will inevitably lead to the change of the cohesive force of the sample. The influence of particle size is very weak, and the roughness of the sample does not change qualitatively. Therefore, for Equation (2), the \(k\) value should be a fixed value, and \(Q\) can be understood as a uniaxial compressive strength at the time of shear failure of the uniaxial compression of the complete sample (the confining pressure is 0 MPa), which can be regarded as the material strength. Figure 15 shows the relationship between the strength of the sample material and the temperature and confining pressure after deducting the confining pressure after high temperature. It can be seen from Figure 15 that although the sample is affected by its own heterogeneity, the test results are somewhat discrete. The measured material strength and temperature change trend are basically the same under different confining pressures, at room temperature of 25 \({}^{\circ}\)C and after 400, 600, 800, and 1000 \({}^{\circ}\)C, the average material strength \(Q\) after deducting the confining pressure is 77.8, 92.1, 94.2, 111.1, and 65.5 MPa, respectively. The high temperature of 800 \({}^{\circ}\)C has a strengthening effect on the sample. After deducting the confining pressure, the sample material strength \(Q\) is positively correlated with the temperature.