## 5 The Damage Constitutive Model under a Thermo-Mechanical Coupling Condition Is Considered  
The strength value of the rock is further reduced after high-temperature treatment, and the thermal damage value of the rock is related to the thermal crack from a microscopic point of view. In order to further consider the thermal damage value of rocks under high-temperature conditions, the same method as described above was used to obtain the values of the H-B strength criterion parameters at high temperatures. Test data [29] of granite under high-temperature conditions were selected, as shown in Table 3.  
Figure 7: Relationship between distribution parameters and confining pressure. The relationship between the modified H-B strength criterion values and the test values at different confining pressures can be obtained, as shown in Figure 8. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Temperature/\({}^{\circ}\)C** & **Confining Pressure/MPa** & **Peak Strength/MPa** \\ \hline  & 0 & 118.800 \\  & 10 & 197.785 \\
200 & 20 & 276.863 \\  & 30 & 427.773 \\  & 40 & 450.177 \\ \hline  & 0 & 98.548 \\  & 10 & 231.120 \\
400 & 20 & 335.685 \\  & 30 & 376.763 \\  & 40 & 449.585 \\ \hline  & 0 & 55.974 \\  & 10 & 191.372 \\
600 & 20 & 312.832 \\  & 30 & 364.602 \\  & 40 & 420.354 \\ \hline  & 0 & 41.706 \\  & 10 & 139.573 \\
800 & 20 & 272.275 \\  & 30 & 292.180 \\  & 40 & 360.190 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Rock peak strength and confining pressure at different temperatures. Figure 8: Fitting curves for the modified H–B strength criterion at different temperatures. In order to further investigate the applicability of the intrinsic model based on the modified H-B strength criterion under high peripheral pressure, the uniaxial test data of granite at high temperatures were selected for verification, and the data obtained from the uniaxial test of granite are shown in Table 4.  
As shown in Figure 9, under high-temperature conditions, the model established in this paper is still in good agreement with the test curve. The reliability indices obtained based on Equation (31) are all above 0.65. The predicted residual strength value is also more accurate. In order to consider the change law of parameters \(n\) and \(F\) at high temperatures, the relationship curve between temperature and parameters \(n\) and \(F\) is drawn. As shown in Figure 10, parameter \(n\) decreases exponentially with the increase in temperature, and parameter \(F\) increases first and then decreases with the increase in temperature. The decrease in parameter \(n\) reflects that the brittleness change trend of rock under the condition of increasing temperature is the same as that under the condition of increasing confining pressure. The brittleness drop degree of rock gradually decreases and transforms to ductility. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Temperature/\({}^{\circ}\)C**} & **Peak** & **Residual** & & & \\  & **Strength/MPa** & **Strength/MPa** & **\(\mathbf{\delta}\)** & **Thermal Damage Value** & \(\mathbf{\lambda}\) \\ \hline
200 & 118.800 & 7.412 & 0.968 & 0.086 & 0.820 \\
400 & 98.529 & 11.76 & 0.938 & 0.120 & 0.676 \\
600 & 54.070 & 1.744 & 0.984 & 0.656 & 0.751 \\
800 & 41.132 & 7.724 & 0.901 & 0.724 & 0.786 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mechanical parameters of granite after high-temperature treatment. Figure 9: Stress–strain curve of granite at high temperature. However, parameter \(F\) generally shows a decreasing trend, and the macro strength value of the reaction rock gradually decreases. The reason is that, under high-temperature conditions, the thermal motion of rock molecules is intensified, and the thermal expansion coefficients of various mineral particles composed of granite are different, resulting in a large number of thermal cracks.