## 4 Test Results and Analysis
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Num**} & **Density** & **Modulus of** & & **Compressive** \\  & **(g/cm\({}^{-3}\))** & **Elasticity** & **Poisson’s Ratio** & **Strength** \\  & & **(10\({}^{4}\) Mpa)** & & **(MPa)** \\ \hline Z0-6 & 2.15 & 1.30 & 0.18 & 156.83 \\ Z45-6 & 2.13 & 1.78 & 0.13 & 111.85 \\ Z90-3 & 2.18 & 1.19 & 0.24 & 144.49 \\ \hline \hline \end{tabular}  
* Crack evolution characteristics during the destabilizing and cracking process of shale samples with different beddings under compressive load.  
\end{table} 
Table 1: Mechanical properties of the shale used in the experiment. curve had a small drop again, and the dominant amplitude also had a sudden change. The shearing cracks in the upper right corner of the observation surface began overlapping and connecting (Figure 9d). The original tensile cracks penetrated more beddings, and a tensile crack penetrating the entire sample appeared in the middle of the observation surface. At this time, the rock sample is in the yield stage, and the stress change is not as obvious as the main amplitude of the acoustic wave. When the strain was 3.49%, in addition to the original cracks, a surface crack appeared in the lower left corner on the observation surface (Figure 9e). When the strain was 3.50%, the surface crack on the left side of the observation surface started to expand inward (Figure 9f). These two almost simultaneous failures made cracks on the observation surface penetrate the entire sample (Figure 9g), and the stress-strain curve at this time had a large drop, indicating the failure of the sample. In the process of shale shear failure, the acoustic properties are in good agreement with mechanical properties. \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 & 634.64 & 2.04 & 80.11 \\ b & 652.32 & 2.10 & 83.46 \\ c & 705.64 & 2.27 & 97.24 \\ d & 861.60 & 2.79 & 125.65 \\ e & 1072.56 & 3.49 & 143.71 \\ f & 1075.64 & 3.50 & 143.37 \\ g & 1091.64 & 3.56 & 127.89 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Stress and strain values corresponding to the static crack images. Figure 8: Stress–strain-dominant amplitude of the square shale sample (z-0-6). observed on the observation surface, indicating that destruction did not occur on the observation surface. When the axial strain was 1.64%, near the peak strain (1.66%), two main cracks turned up from the same point on the right side of the observation surface (Figure 11a,b): one was a shear crack along the bedding direction, and the other was a tensile crack on the right side of the surface. With the increase in external load, secondary cracks appeared at the end of the shear crack, reaching the boundary of the sample (Figure 11d). Almost at the same time, a tensile crack along the axial stress came up at the initial point of the observation surface. Then, a round surface crack appeared between the two tensile cracks (Figure 11d-f), which was caused by the release of stress on both sides of the tensile cracks. At this point, the sample reflected a sharp drop in stress until destructed completely. Figure 11: Static images of cracks during the shale destruction process. (**a**) Crack initiation stage; (**b–d**) crack propagation stage; (**e**) crack penetration stage; (**f**) overall failure stage. Figure 10: Stress–strain-dominant amplitude (z-45-6) of square shale sample. Figures 12 and 13 show uniaxial stress-strain curves of the square shale sample with bedding dip angle a of 0\({}^{\circ}\) and static images of cracks during destruction of the sample.