## 4 Results and Discussion  
### Processing of Raw Data  
In the experiment, the tension force is controlled by the hydraulic jack. The tension forces for the five tests are presented in Table 1. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Number of Tests**} & \multicolumn{8}{c}{**Tension Force (t)**} \\ \cline{2-10}  & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** \\ \hline
1 st & 0 & 1.223 & 2.007 & 2.962 & 4.050 & 5.202 & 6.013 & 7.055 & 8.010 & 9.005 \\
2 nd & 0 & 0.912 & 1.748 & 2.870 & 3.828 & 4.976 & 5.987 & 6.842 & 8.084 & 8.902 \\
3 rd & 0 & 1.341 & 1.931 & 2.961 & 4.076 & 5.026 & 6.070 & 7.037 & 7.956 & 8.986 \\
4 th & 0 & 0.774 & 1.870 & 2.929 & 3.981 & 5.041 & 6.014 & 7.111 & 8.104 & 8.979 \\
5 th & 0 & 1.526 & 2.199 & 3.488 & 4.342 & 5.463 & 6.403 & 7.388 & 8.346 & 9.440 \\ \hline \hline \multicolumn{10}{c}{**Tension Force (t)**} \\ \cline{2-10}  & **11** & **12** & **13** & **14** & **15** & **16** & **17** & **18** & **19** \\ \hline
1 st & 10.110 & 11.085 & 12.033 & 13.210 & 14.175 & 15.275 & 16.070 & 17.140 & 18.010 \\
2 nd & 9.792 & 10.782 & 12.077 & 12.927 & 13.807 & 14.937 & 15.814 & 16.863 & 17.917 \\
3 rd & 10.011 & 11.011 & 12.191 & 12.991 & 14.006 & 15.014 & 16.011 & 17.131 & 17.969 \\
4 th & 9.874 & 11.071 & 11.952 & 13.064 & 14.037 & 14.945 & 16.159 & 16.935 & 18.006 \\
5 th & 10.345 & 11.382 & 12.261 & 13.264 & 14.401 & 15.361 & 16.396 & 17.238 & 18.306 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The tension force of 5 tests. Figure 7: Signal transmission process. Due to the instability of oil pressure, the magnitude of each tension force varies slightly. The results of the fifth test, with tension forces of 4 t, 8 t, 12 t, and 16 t, are shown in Figure 8. As can be seen, the relationship between the received signals and the four tension scenarios of the prestressing force is not immediately apparent. The magnitude of prestress cannot be directly determined solely from the peak value of the received signal, and the peak value of each received signal does not occur at the same time. There are two possible reasons for this phenomenon: (1) different components of the wave have different propagation velocities (typically, lower energy components have higher velocities, and higher energy components have lower velocities); and (2) lower energy components may propagate directly to the sensor, while higher energy components may experience significant reflection and scattering at the interface before reaching the sensor. Therefore, we calculated and compared the signal energies for each tension case, as shown in Figure 9.  
As described in Section 2.3, the signal received at each load level was subjected to wavelet packet decomposition. Appropriate decomposition layers can preserve the main features of the signal and reduce noise. As recommended by [PERSON] et al. [54], the formula L = int[log(N)] is used to select the appropriate decomposition layer, where N represents the series length. The wavelet packet decomposition layer was set to 5. The energy value  
Figure 8: Received signal results of the fifth test: (**a**) 4 t (**b**) 8 t; (**c**) 12 t; (**d**) 16 t.  
Figure 9: (**a**) The relationship between prestress levels and wavelet packet energy; (**b**) the proportion of energy in each frequency subband; and (**c**) the relationship between prestress levels and energy ratio. after decomposition at each load level was obtained by calculating Equation (14). The relationship between prestress levels and wavelet packet energy was then described, as shown in Figure 9a. When the tension force was small, the growth rate was high and gradually slowed down with an increase in force. In general, the signal energy increases as the tension force increases. This result is consistent with previous findings [35; 38]. It can be explained by the fact that the energy of the transmitted wave is proportional to the contact area, and the actual contact area increases when greater tension is applied.