## 4 Discussions  
Rock deformation is accompanied by energy release, which can be recorded by AE sensors. The released energy is equal to the sum of the absolute value of the area between the envelope of the signal wave and the threshold line. Figure 8 presents the energy release process of rock salt under different confining pressures during the triaxial loading test. According to the energy fluctuation, the damage process of rock salt can be divided into four stages, as shown in Figure 8. The characteristics of each stage are described below. (1) Stage I: At the beginning of the test, the AE activity of rock salt was weak, displaying very little energy release. At this stage, the rock sample was under a confining pressure and axial loading stress on the top, which can be regarded as external energy input. The sample was experiencing elastic and compression deformation, and the energy absorption from the surrounding system was at its highest level. Figure 7: AE event maps of rock salt under different confining pressure during the triaxial loading test: (**a**) confining pressure of 5 MPa; (**b**) confining pressure of 10 MPa; (**c**) confining pressure of 15 MPa; (**d**) confining pressure of 20 MPa; and, (**e**) geometric relationship between micro-crack direction and stress direction. (2) Stage II: With increasing axial loading stress, the energy input from the external system continued to increase. However, energy in the sample reached its limit, and the rock salt had no further ability to absorb energy. Therefore, the excess energy had to be released in another way: it was consumed in the sliding and embedding actions among the salt crystals, followed by the generation and expansion of micro-cracks. This stage corresponds to the brittle-ductile transition of the rock salt in which irreversible rock damage occurred. The AE activity continued to increase throughout this stage. (3) Stage III: At this stage, the steady rise in AE energy release changed to an irregular fluctuating trend and the energy release rate remained at a high level. Due to the rapid expansion of internal cracks in the rock, the rock salt produced large deformation. The AE activity was extremely intense, and the released energy reached its peak at the end of this stage. Some researchers thought [38; 39] that there was a period of stagnation of AE before it reaches the energy peak. After the peak point, the energy release intensity dropped significantly. Due to the large deformation characteristics and strain hardening properties of rock salt, the triaxial compression strength cannot be accurately determined. In this study, the stress when the AE energy is at its peak is defined as the \"damage ultimate stress\", which has the same unit and physical meaning as the triaxial compressive strength. The triaxial compression strength was replaced with the \"damage ultimate stress\", from which the friction angle and cohesion of rock salt can be determined. (4) Stage IV: When the axial stress exceeded the damage stress limit, significant structural damage occurred with a large number of connected cracks in the sample. However, the rock salt still had a stress carrying ability. The intensity of AE activity decreased significantly, but it was still active. In the above study, four critical stresses were involved: crack initiation threshold stress, dilatancy boundary stress, short-term strength, and damage ultimate stress. These critical stresses of rock salt  
Figure 8: Histogram of AE energy of rock salt under triaxial compression test: (**a**) confining pressure of 5 MPa; (**b**) confining pressure of 10 MPa; (**c**) confining pressure of 15 MPa; and, (**d**) confining pressure of 20 MPa. under different confining pressures during the triaxial loading test were presented, as shown in Table 1. The crack initiation threshold is the end of the linear elastic deformation stage, after which the salt samples enter a process with elastic and plastic deformation coexisting when the axial stress exceeds the crack initiation threshold.