## 4 Analysis and Discussion
### Time-Domain Analysis for Multi-Feature Extraction and Assessment  
The health condition of Segment 2 was deteriorated, and it had a debonding of about 2 mm in thickness in the current monitoring stage after being subjected to train loads and environmental loads during long-term service. The discriminations between the time-domain features of the two track segments are shown in Table 3. Based on the mechanism of instantaneous baseline, if the features of one segment are significantly different from those of the other segment in the current monitoring stage, distinct damage may occur. From Tables 2 and 3, it can be found: (i) compared to those in the early stage of monitoring, the relative differences of all the six time-domain parameters in the current stage of monitoring were larger, reflecting a debonding-induced change in responses; and (ii) the difference in peak acceleration in current monitoring stage was significantly higher than that in the early stage, indicating it is a dominant debonding-sensitive feature  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Features** & **Segment 1** & **Segment 2** & **Difference** \\  & _(a)_ & _(b)_ & _(1b\(-\)a\(\,\)1/a)_ \\ \hline Peak acceleration & 0.0268 & 0.0336 & \(2.537\times 10^{-1}\) \\ Shape factor & 3.560 & 3.548 & \(3.371\times 10^{-3}\) \\ Crest factor & 17.509 & 17.503 & \(3.427\times 10^{-4}\) \\ Impulse factor & 68.832 & 68.797 & \(5.085\times 10^{-4}\) \\ Clearance factor & 517.106 & 517.055 & \(9.863\times 10^{-5}\) \\ Kurtosis factor & 17,616.660 & 17,616.722 & \(3.519\times 10^{-6}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Six features of the two segments and their relative differences in the early stage of monitoring.  
Figure 8: (a) Scatter plot of peak acceleration of Segment 1 in early monitoring stage. (b) Scatter plot of peak acceleration of Segment 2 in early monitoring stage.  
among the six features. Therefore, the peak acceleration feature was used in the subsequent analysis of Section 4.5. According to Table 1, these features were related to characteristics of response (i.e., the maximum, the mean value, and the RMS, etc.), so the reason for the enlarged differences between the early and current monitoring stages can be attributed to the debonding-induced stiffness reduction and the resultant change in track response.  
Typical responses of Segment 1 and Segment 2 in the current stage of monitoring are illustrated in Figures 9 and 10. The debonding-sensitive feature, peak acceleration, is illustrated in Figures 11 and 12 to observe their discrimination. In Figures 11 and 12, the horizontal coordinate displays the sequence number of passing-by vehicles in a typical day, and the vertical coordinate exhibits the vertical acceleration of three sensors. The black rectangle and blue triangle represent the vertical acceleration of sensor No. 1 and sensor No. 3 in the cable trough of cross-sections 1-1 and 3-3 of the track segments, respectively, and the red circle stands for the vertical acceleration of sensor No. 2 in the cable trough of the cross-section 2-2 of the track segments. It can be found by comparing Figures 11 and 12 that the vibration level of Segment 2 was about three times higher than that of Segment 1, indicating the high sensitivity of this feature to debonding. The tolerable limit of peak acceleration of bridge deck on a viaduct of the ballastless track is 0.5 g [58], so both Segment 1 and Segment 2 were still in a safe state, although Segment 2 had a debonding of 2 mm in thickness. The CRTS-II-slab ballastless track system is famous for its low vibration and noise, but the vibration level has deteriorated due to the increased severity of debonding. Therefore, it is necessary to pay more attention to the track segment with debonding.