# Experimental Study on Damage Constitutive Model of Rock under Thermo-Confining Pressure Coupling
## 2 Experimental Program
### Test Procedure  
This experiment involves conventional triaxial tests under various temperature and confining pressure conditions. Based on the studied stratum distribution, the test temperatures are set at five levels: 20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, 80 \({}^{\circ}\)C, and 120 \({}^{\circ}\)C, and the confining pressures are set at six levels: 0 MPa, 5 MPa, 10 MPa, 15 MPa, 20 MPa, and 25 MPa. Relevant studies indicate that thermal cracking in granite with the range of 100 \({}^{\circ}\)C to 500 \({}^{\circ}\)C does not cause structural damage [31], and the effects of temperature and cyclic loading can \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Rock Specim**} & \multirow{2}{*}{**Peak Strength/MPa**} & \multicolumn{2}{c}{**Average Uniaxial**} & \multicolumn{2}{c}{**Standard Value of**} & \multirow{2}{*}{**Upper Limit of**} \\  & & **Compressive Strength/MPa** & & **Compression Strength/MPa** & \\ \hline G-JZ-1 & 119.96 & & & \\ G-JZ-2 & 133.72 & 133.73 & 115.49 & 80.84 \\ G-JZ-3 & 147.50 & & & \\ \hline RS-JZ-1 & 25.40 & & & \\ RS-JZ-2 & 21.98 & 25.04 & 21.36 & 14.95 \\ RS-JZ-3 & 27.75 & & & \\ \hline GS-JZ-1 & 148.26 & & & \\ GS-JZ-2 & 151.24 & 158.17 & 138.47 & 96.93 \\ GS-JZ-3 & 175.03 & & & \\ \hline SH-JZ-1 & 60.72 & & & \\ SH-JZ-2 & 60.74 & 60.29 & 59.28 & 41.49 \\ SH-JZ-3 & 59.42 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Uniaxial Compression Test Results. Figure 2: GCTS high-temperature and high-pressure rock triaxial apparatus. reinforce rock samples under certain conditions [32]. This reinforcement phenomenon is present in most rock types [31; 33]. Some studies suggest that mild temperatures [31] or cyclic loading [33; 34] have a hardening effect on rocks. Triaxial loading and unloading tests on some rocks reveal minimal strain differences during loading and unloading, with a maximum difference of only 6.96% [35]. Moreover, the peak stress (\(\sigma_{f}\)) obtained from the triaxial cyclic loading and unloading tests is similar to the peak stress (\(\sigma_{f}\)) obtained from the triaxial compression tests. Before reaching the peak stress, the number of loading and unloading cycles at high-stress levels is relatively low. As a result, cyclic loading before peak stress has minimal impact on the peak stress of the rock [36]. Based on these research results, the following basic steps are used in this experiment:  
1. The standard granite sample was securely enclosed in a specialized fluoro rubber sleeve to prevent hydraulic oil infiltration, ensuring test accuracy. After clearing debris from the steel block connections, the rock core was placed flat with end blocks at both ends. The axial strain gauge was carefully adjusted to its proper position, and the circumferential strain gauge was installed at the center of the specimen. A slight touch confirmed proper connection by displaying minimal displacement on the computer. Initial values were then set on the computer. 2. Using the GCTS high-temperature and high-pressure rock triaxial apparatus, the rock samples were subjected to an initial confining pressure state (\(\sigma_{2}=\sigma_{3}\) of 0 MPa) and maintained consistently. In this experiment, the loading was controlled by a fixed axial displacement rate. The loading rate was set to 0.03 mm/min in the computer software control system, and the stress limit was set to 70% of the standard uniaxial compressive strength of the rock samples at room temperature [37]. When the axial stress reached the preset stress limit, the system automatically stopped loading. Subsequently, the confining pressure and stress were unloaded to 0 MPa. During the loading process, the system continuously collects and records the stress and strain values of the rock sample in real-time. Peak strength, residual strength, axial strain, and circumferential strain are directly obtained from the recorded data. The real-time stress-strain curve is monitored to ensure that the test data meet the requirements of this study. 3.