## 3 Experimental Results
### AE Activity  
Energy release and rock damage are associated with AE activity. Salt rocks are sensitive to AE during uniaxial loading because the crystal structure contributes to the friction effect [31]. AE events were consistent with the loading process, and were 0.04%, 2.66%, 13.8%, 75.27% and 8.23% of the total AE activity during the five deformation stages (Table 2). Based on seismological principles, AE events can be divided into three types: shock, swarm, and isolated. It is obvious that salt rock belongs to  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Number** & \begin{tabular}{c} **Uniaxial Compression** \\ **Strength/MPa** \\ \end{tabular} & \
\begin{tabular}{c} **Damage** \\ **Strength/\%** \\ \end{tabular} &
\begin{tabular}{c} **Dilatancy** \\ **Dtrength/\%** \\ \end{tabular} \\ \hline SA-1 & 19.5 & 42.5 & 62.6 \\ SA-2 & 16.4 & 45.0 & 66.4 \\ SA-3 & 18.2 & 49.7 & 63.1 \\ Mean value & 18.0 & 45.7 & 64.0 \\ Standard deviation & 1.56 & 3.66 & 2.04 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Damage and dilatancy strength for salt rock samples under uniaxial compression.  
Figure 5: P- and S-waves’ velocity attenuations in sample SA-2 at different loading phases.  
the second category because there was no extreme, high-level shock, and the energy was released by repeated activity with similar magnitudes within a longer time period, as shown in Figure 4b.  
During testing, four AE sensors were used to locate the AE position, which clearly showed damage development inside the rock samples. From the compression fissures to accelerated-growth cracks stage, AE gradually became active and reached a peak, the AE located in the area with the most significant damage (Figure 6). In the uniaxial compression test, most of macro-cracks were parallel to the direction of stress, which presented a vertical splitting characteristic as shown in Figure 7. More serious damage and inconsistent distortions occurred in the lower part of the rock, as the loading stress came from the base of the test machine. Comparing the damaged salt rock after testing with AE events location (Figure 7), we clearly recognized that the location of the AE was in good agreement with the macro-cracks of the damaged samples. Where the damage was most serious, AE events were the most concentrated.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample Number**} & \multicolumn{4}{c}{**AE Event Percentage/\%**} & \multicolumn{2}{c}{**Event Sum**} \\ \cline{2-6}  & **Stage 1** & **Stage 2** & **Stage 3** & **Stage 4** & **Stage 5** \\ \hline
1 & 0.06 & 3.04 & 14.4 & 71.9 & 10.6 & 6904 \\
2 & 0.05 & 2.21 & 11.9 & 79.4 & 6.4 & 7751 \\
3 & 0.02 & 2.74 & 15.1 & 74.5 & 7.6 & 8233 \\ Mean value & 0.04 & 2.66 & 13.8 & 75.27 & 8.23 & 7629 \\ Standard deviation & 0.02 & 0.42 & 1.68 & 3.81 & 2.16 & 673 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Acoustic emission events of salt rock in different stages under uniaxial stress.  
Figure 6: AE locations in sample SA-2 during different deformation stages; (**a**) compression fissures stage; (**b**) linear deformation stage; (**c**) stabilized-growth crack stage; (**d**) accelerated-growth cracks stage and (**e**) after peak stress stage. The purple part was the rock sample; the black grid helped to locate AE positions; the red spots represented the location of AE enents.  
Figure 7: Comparison of damaged rock after uniaxial compression and AE event locations for sample SA-2.