## 4 Analysis of Test Results  
### Effect of High Temperature on Longitudinal Wave Velocity of Sample  
The Longitudinal wave velocity of two sets of specimens at room temperature (25 \({}^{\circ}\)C) and after high temperature are shown in Figure 5. The longitudinal wave velocities are different due to differences in the mineral composition of the two sets of samples. The longitudinal wave velocity of 30 samples in group A is between 3057 and 3630 m/s at room temperature of 25 \({}^{\circ}\)C, the average wave velocity is 3277 m/s, and the dispersion coefficient is 3.95%; The longitudinal wave velocity of 25 samples in group B is between 2276 and 2672 m/s at room temperature of 25 \({}^{\circ}\)C, the average wave velocity is 2486 m/s, and the dispersion coefficient is 4.41%. It can be seen that although the sample is taken from the same layer, there is still a certain difference in the wave velocity. It may be the existence of pores or fissure in rock that results in the variation of wave velocity of rock samples [28; 29]. The ratio  
Figure 4: Electro-hydraulic servo rock test system of RMT-150B. of the maximum difference to the average value of the sample in group A is about 10.8%, and the ratio of the maximum difference to the average value of the sample in group B is about 7.5%. It indicates that the macroscopically homogeneous interior of the sample still exhibits significant heterogeneity. It can be seen from Figure 5 that the longitudinal wave velocities of the samples in group A and B have been reduced to different degrees after different high temperatures. The temperature of 100 \({}^{\circ}\)C has little effect on the wave velocity of group A. After 100 \({}^{\circ}\)C, the longitudinal wave velocity of the sample decreases linearly with temperature. For group B, the longitudinal wave velocity of group B is nonlinear with temperature increase after high temperature. The high temperature of 400 \({}^{\circ}\)C has little effect on the longitudinal wave velocity of the sample in group B, but the effect of high temperature over 400 \({}^{\circ}\)C on the longitudinal wave velocity of the sample increases obviously. After the high temperature action, the structural state and material properties of the sample undergo corresponding physical and mechanical changes, including complex cracks and defects. The ultrasonic waves will undergo refraction and diffraction when they encounter cracks and defects during the propagation process, as a result, the time of penetration of the sample is increased, the wave speed is reduced, and the energy is attenuated. There is a close relationship between the mechanical properties of rock and the longitudinal wave velocity, the longitudinal wave velocity can also indirectly reflect the change of the mechanical properties of the sample after high temperature [30; 31; 32]. The damage factor of temperature D can be defined by formula (1), as shown in Figure 6.  
\[D=1-\left(\frac{V_{\mathrm{pT}}}{V_{\mathrm{p}}}\right)^{2} \tag{1}\]  
where, \(V_{\mathrm{p}}\) is the longitudinal wave velocity value of each sample at room temperature 25 \({}^{\circ}\)C, m/s; \(V_{\mathrm{pT}}\) is the longitudinal wave velocity value of each sample after different high temperature, m/s. Figure 5: Longitudinal wave velocity of two sets of specimens at room temperature 25 \({}^{\circ}\)C and after high temperature. It can be seen from Figure 6 that the change factors of damage factor and temperature of the two groups are basically the same after high temperature action, and the damage factor becomes larger with the increase of temperature. The damage to the sample is also different due to differences in mineral contents. The damage of the A group is higher than that of the B group. The damage factor of the two groups of samples at 400 \({}^{\circ}\)C is 0.42 and 0.13, respectively. When the temperature is at 600-800 \({}^{\circ}\)C, the damage factor of the samples is approximately the same.