## 4 Test Results and Analysis
The time and stress and strain values corresponding to each crack change in the rock destruction process given by the author are shown in Table 4. As shown in the figures, in the initial stage of loading, the sample showed an obvious crack compression stage, and the stress-strain curve of the sample was concaved upwards. During this process, the dominant amplitude curve showed a small downward concave section and then it rose almost at the same slope value of the stress-strain curve. When the axial strain reached 0.83%, the dominant amplitude increased rapidly, reaching the first peak at the strain of 1.83% and then began to fall. A big chunk sloughed on the right side of the observation surface. At this point, the stress-strain curve was still smooth. At this time, the response of the main amplitude of the acoustic wave to rock caving is more sensitive than that of the mechanics. Until the strain was about 1.27%, the stress-strain curve had a small fluctuation for the first time. This is because the cumulative damage in the observation surface caused a sudden drop in bearing capacity of the sample. Then, a tensile crack along the bedding appeared at the observation surface at the axial strain of 2.02% (Figure 13a), and a secondary crack connecting to the failure surface quickly occurred at the end of the crack. This is because the tensile stress in the destruction process of the shale sample was concentrated locally in weak parts such as pores and beddings. As load increased, the two cracks began to extend from the surface to the inside of the rock and increased in aperture, forming macroscopic cracks (Figure 13b). When the strain was 2.27%, another tensile crack along the bedding turned up on the observation surface (Figure 13c), and then the surface collapsed, with a round surface crack formed (Figure 13c,d). When the strain reached a peak of 2.80%, a larger round piece sloughed, and the two round cracks on the surface started to connect (Figure 13e). Then, the sample lost stability and failed (Figure 13f). In this process, there were many small fluctuations in the stress-strain curve, and finally the stress dropped sharply at the peak strain, and the dominant amplitude only had one large sudden change. It was found from the recorded video that sample failure at this stage was completed in a very short period of time; in other words, in this very short period of time, external forces made the internal energy of the test piece increase rapidly. The accumulated internal energy in the sample did not release at once but was released gradually in multiple destructions. However, the gradually released elastic energy was not enough to offset the internal energy produced by external force; thus, the internal energy stored in the rock sample kept increasing and finally released completely at the peak strain, resulting in the ultimate failure of the sample. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Crack No.** & **Time (S)** & \
\begin{tabular}{c} **Strain Corresponding to** \\ **the Crack Image (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Stress Corresponding to** \\ **the Crack Image (MPa)** \\ \end{tabular} \\ \hline a & 1007.32 & 1.64 & 111.79 \\ b & 1007.64 & 1.64 & 110.74 \\ c & 1011.00 & 1.65 & 111.23 \\ d & 1021.64 & 1.66 & 111.79 \\ e & 1022.00 & 1.66 & 111.25 \\ f & 1022.32 & 1.67 & 106.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Stress and strain values corresponding to the crack static images. Figure 12: Stress-strain-dominant amplitude of the square shale sample (z-90-3). Figure 13: Static images of cracks during the shale destruction process. (**a**) Crack initiation stage; (**b–d**) crack propagation stage; (**e–f**) crack penetration stage; (**g**) overall failure stage.