# Characteristics of Acoustic Emission Caused by Intermittent Fatigue of Rock Salt
## 4 Results and Discussion
### General Features of Acoustic Emission  
Macro-scale rock performance is largely influenced by complex topological geometric defects [22; 23; 24]. The progress of non-equilibrium kinetics directly affects the macroscopic characteristics of the rock. Figure 7 shows the number of AE counts, AE energy, and the strain curve for tests carried out using an interval of 30 s. The trends of the AE counts and energy correlate strongly with the strain curve during fatigue. The number of AE counts produced by the first circle is greater than that produced by the stable fatigue circle. The reason is that the original complete internal crack in the rock expands after the compaction stage arising from the initial damage [25]. The crystals begin to shear, moving along grain boundaries, and transcrystalline cracks emerge as the atoms in the crystals begin to migrate [26]. A representative scanning electron microscope (SEM) image of the first circle is presented in Figure 8, during which the AE phenomenon is very active. The permanent migration of atoms in crystals symbolizes the rock salt's plastic deformation, which predicts the formation of residual strain. During the final phase of fatigue, the AE phenomenon is more active than that during stationary fatigue, and maximum AE counts and energy release are recorded. Initial cracks and a large number of transcrystalline cracks produced by fatigue converge to penetrate into the cracks in this phase [25]. Figure 8 shows that a large number of cracks develop within a short time, which crush the internal structure of the rock salt. This total internal instability leads to ultimate failure of the rock salt, during which large amounts of energy are released and the AE phenomenon is abnormally active. Figure 9 shows the AE features of tests carried out at other time intervals: the behavior is similar to that shown in Figure 7. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Interval Duration (s)**} & \multicolumn{4}{c}{**C/\(y=ae^{bx}+C\)**} \\ \cline{2-5}  & **C\({}_{ceal}\)/\%\({}_{no}\)** & **C\({}_{odd}\)/\%\({}_{no}\)** & **(C\({}_{odd}-C_{ceal}\))/\%\({}_{no}\)** & **Average Fatigue Life** \\ \hline
0 (classic fatigue) & 0.48 & 0.49 & 0.01 & 88 \\
5 & 0.8 & 1.04 & 0.24 & 52 \\
10 & 0.87 & 0.99 & 0.12 & 54 \\
30 & 1.29 & 2.05 & 0.76 & 27 \\
60 & 2.05 & 2.56 & 0.51 & 18 \\
120 & 1.97 & 2.90 & 0.93 & 13 \\
600 & 2.37 & 3.59 & 1.22 & 7 \\
900 & 4.41 & 4.50 & 1.09 & 4 \\
1200 & 2.48 & 3.23 & 0.75 & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: C values of the exponential fit of the residual strain and fatigue life of samples subjected to different fatigue interval durations. The red box area in Figure 7 is defined as a group for the purpose of comparing the AE features of intervals before and after this period. Circle 4 produced the AE counts before the interval and circle 5 produced the AE counts after the interval. Figure 10 shows enlarged detail of the period between 650 and 900 s in Figure 7. The AE counts corresponding to the peak strain in each circle decreased sharply to zero and then gradually increased as the strain decreases (shown by the red dotted lines in Figure 10). The reason for this phenomenon is that there is always a certain delay (about 0.3 s) between the computer instruction and the response of the hydraulic system in the electro-hydraulic servo system: AE counts are not generated during this delay because of the Kaiser effect [12], and a gap manifests during the development of AE counts, although this does not influence the test results. Figure 7: Strain curves and (**a**) AE counts, (**b**) AE energy features for the fatigue tests carried out with a 30 s interval. Figure 8: (**a**) Failure condition of rock salt after experiment. SEM images of micro cracks for (**b**) the first circle and (**c**) the complete circle. Figure 9: Cont.