## 4 Test Results and Analysis  
In the process of shale destruction, the expansion and convergence of microcracks and cracks are the root causes of shale damage and destruction. As the propagation process of cracks will be accompanied with changes in acoustic parameters, we came to the idea of establishing the relationship between acoustic parameters and rock stability. This section focuses on the development and destruction mechanism of shale cracks, division of the process of shale destruction and the relationships between acoustic parameters and crack propagation at different stages of shale destruction. The rock mechanical parameters of the shale used in this experiment are shown in Table 1.  
Figures 8 and 9 show monoaxial stress-strain curves of the square shale sample with a bedding dip angle a of 90\({}^{\circ}\) and static images of crack changes during the destruction of the shale sample. Moreover, the authors also provide the time, stress and strain values corresponding to each crack change in the rock destruction process, as shown in Table 2. As shown in the figures, at the initial stage of loading, the sample was in the crack compression stage. The sample had larger deformations, but no cracks were observed on the surface, and the curve of this stage was convex. With the increase in external loads, the curves of stress-strain and dominant amplitude-strain rose smoothly. When the strain was around 1.27%, the stress-strain curve shows a small fluctuation for the first time, and the dominant amplitude curve has an obvious sudden change. This is because the work performed by the external loads on the rock was mainly stored inside the rock in the form of elastic potential energy, which resulted in the development of the rock to the critical high internal energy, when the rock would change to another new steady state abruptly, and this change corresponded to rock destruction. After elastic energy was released, with internal energy dropped, the rock was in a new stable state. Therefore, the dominant amplitude increased first and then decreased to a new stable value. The reason no failure cracks were observed on the observation surface is the sample failure occurring inside the sample or other observation surfaces. Then, the dominant amplitude and the stress-strain curve still increased with an increase in load. When the strain reached 2.04%, a local crack appeared for the first time in the upper right corner of the observation surface (Figure 9a). At this time, the dominant amplitude also had a sudden change, indicating another release of the stored elastic energy in the sample and corresponding to another rock damage. At this time, the stress-strain curve still rose smoothly, failing to reflect the time of rock damage accurately. After rock damage, the dominant amplitude dropped to a new stable value. Then, the dominant amplitude suddenly increased sharply in a short time. By examining the recorded video, it was found that, in this very short period of time, a wing crack at 45\({}^{\circ}\) relative to the axial direction occurred in the observation surface (Figure 9b), and the stress-strain curve still did not show this damage. When the strain reached 2.27%, the stress-strain curve was still smooth, while the dominant amplitude changed suddenly again. Looking at the static map of the observation surface, a crack parallel to the first crack was found at 45\({}^{\circ}\) relative to the axial direction, which came up in the upper right corner of the observation surface (Figure 9c). Until the strain reached 2.79%, the stress-strain