## 3 Experimental Results
### Ultrasonic Wave Analysis  
When elastic waves encounter obstacles during propagation, such as inclusions, holes, and cracks, the wave velocity decreases, which can be calculated by the length of the sample and the propagation time [9]. The propagation time is obtained by interpreting the received waveform information. Figure 4 shows a set of the excited and thereceived ultrasonic waveform. Figure 3: Comparison of rock salt failure characteristics: (**a**) picture of rock salt before test; (**b**) picture of rock salt after triaxial loading test; and, (**c**) AE locations of rock salt under triaxial loading test. The change of ultrasonic wave velocities through rock salt under triaxial loading stress is shown in Figure 5a. Both P-waves and S-waves had slightly increased wave velocities at the elastic compression stage. Subsequently, their velocities remained stable during the brittle-ductile transition process. This finding was unexpected and different from typical brittle rock [18], and it suggested that the new cracks that were generated inside the rock were negligible during this period. One possible reason is that the sliding and embedding actions among the salt crystals are controlling rock deformation, even if the axial strain has reached 0.5%. When the crystal slide exceeded their limits at the plastic deformation stage, a large number of micro-cracks were produced and then connected with each other. The P-wave and S-wave velocities began to decline, as shown in Figure 5a. Based on the ultrasonic wave velocities, the crack density of rock can be calculated. Previous research [10; 11] presented the relationship between ultrasonic wave velocity and crack density, as shown as Equation (1):  
\[\xi=\frac{9[(1-\frac{\overline{V_{p}^{2}}}{V_{s}^{2}})(3\frac{\overline{V_{p} ^{2}}}{V_{s}^{2}}-4)(5\frac{V_{p}^{2}}{V_{s}^{2}}-8)-4(\frac{V_{p}^{2}}{V_{s} ^{2}}-\frac{\overline{V_{p}^{2}}}{V_{s}^{2}})][3\frac{\overline{V_{p}^{2}}}{ V_{s}^{2}}-2]}{32\frac{\overline{V_{p}^{2}}}{V_{s}^{2}}(3\frac{\overline{V_{p}^{2}}}{ V_{s}^{2}}-4)(3\frac{V_{p}^{2}}{V_{s}^{2}}-4)} \tag{1}\]  
where \(\xi\) is the crack density in the rock sample, \(V_{p}\) and \(V_{s}\) are the P-wave and S-wave velocities in the damage-free rock sample, and \(\overline{V_{p}}\) and \(\overline{V_{s}}\) are the P-wave and S-wave velocities in the damage-bearing rock sample. Figure 5b presents the crack density curves of rock salt under different confining pressures in the triaxial compression tests. The four curves have a consistent trend: first, decreasing remaining constant for a while, and then the increasing significantly. The lower the confining pressure, the greater the slope of the fracture density curve and the faster the crack development rate. The above results are effective verifications of crack growth inhibition by confining pressure. Additionally, the ultrasonic wave velocity and crack density can indeed illustrate the development of rock damage. However, we do not recommend regarding them as the damage prediction index for rock salt under triaxial compression tests, for the wave velocity change is not significant before the plastic deformation phase, and the interpretation of the waveform information, especially S-waves, was confounded by the subjective influence of the tester. Ultrasonic waves will decay when there are granular structures, defects, and impurities in the medium. In addition, a part of the ultrasonic energy will convert into heat because of the friction between rock grains. All the above will lead to loss of ultrasonic energy, which is presented by the reduction of the ultrasonic wave amplitude. Figure 5c shows the changes of the P-wave and S-wave amplitude during the process of rock salt damage.