# Deep Autoencoder Framework for Classifying Damage Mechanisms in Repaired CFRP
## 3 Results and Discussions
### Damage Characterization Using AE Feature
This variation can be attributed to the fact that the adhesive used for these specimens was applied manually. [PERSON] et al. identified acoustic emission amplitudes for matrix cracking as 50-60 dB, delamination, 60-80 dB, interface failure, 50-70 dB, and fiber breakage, 80-90 dB in fiber-reinforced composites [76]. [PERSON] and [PERSON]'s study on polyethylene composites reported similar amplitudes: matrix cracking below 60 dB, delamination 60-85 dB, interface failure 30-45 dB, and fiber breakage 80-97 dB [77]. [PERSON] and [PERSON] found in glass/polypropylene composites amplitudes of 40-55 dB for matrix cracking, 65-85 dB for delamination, 60-65 dB for interface failure, and 85-95 dB for fiber breakage [78]. Based on these observations, a hypothesis can be formulated. In Table 4, the clustered AE feature entropy versus amplitude may indicate different damage processes. Figure 14: Silhouette values of AE features of (**a**) JSL1; (**b**) JLS 2; and (**c**) JLS3. Cluster 1 represents AE signals generated by matrix cracking; Cluster 2 represents signals from delamination events; and Cluster 3 represents signals from fiber breakage. In this study, time-frequency analysis based on HSA is used to support this hypothesis. Previous research has identified CFRP specimen damage modes based on frequency range and the instantaneous energy of AE waveforms [79]. In the literature, many authors have investigated structural damage using frequency and time-frequency analyses [80]. In this subsection, the analysis relies on Hilbert spectral analysis for further exploration. At random, AE signals are extracted for each cluster and specimen for detailed analysis. The observed spectrum naturally exhibits distinct frequency distributions, complementing previously identified AE features by providing additional insights from the complete frequency-time distribution. This approach offers a more comprehensive understanding of the AE signals and their correlation with various damage modes. As mentioned earlier, HSA is used as a signal-processing technique to extract relevant characteristics from AE waveforms [81]. In Figure 16, instantaneous energy is concentrated between 250 and 375 kHz at 2 ms (Figure 16a). This concentration indicates that the initial stages of damage in this specimen begin in the frequency range of 250 to 375 kHz, which is typical for interfacial debonding and crack growth, as discussed earlier \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{3}{c}{**Cluster Entropy Ranges**} & \multicolumn{3}{c}{**Cluster Amplitude Ranges (dB)**} \\ \cline{2-7}  & **Cluster 1** & **Cluster 2** & **Cluster 3** & **Cluster 1** & **Cluster 2** & **Cluster 3** \\ \hline JLS 1 & 0.01–1.54 & 0.02–1.01 & 0.03–0.8 & 40–48 & 49–65 & 67–100 \\ JLS 2 & 0.03–2.10 & 0.02–1.35 & 0.07–1.03 & 40–58 & 59–78 & 79–100 \\ JLS 3 & 0.01–2.59 & 0.02–1.88 & 0.04–1.52 & 40–53 & 54–75 & 76–100 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Entropy and amplitude cluster ranges across all specimens. Figure 15: Cluster assignments between amplitude and entropy for specimens: JLS1; JLS2; and JLS3. in the section. In the second HSA, the energy is focused between 300 and 350 kHz at 2 ms (Figure 16b). This narrower concentration compared to the first HSA suggests a more localized damage evolution within a tighter frequency band, which may be associated with matrix cracking or delamination. The third has, the spectrum shows higher instantaneous energy, with concentration from 250 to 390 kHz at 2 ms (Figure 16c). The fact that this range exceeds the 375 kHz range from the first and second HSAs indicates an intensification of the energy and more severe degradation. This could suggest that damage has progressed further, affecting the adhesive and fibers more significantly. A progressive increase in instantaneous energy from the first to the third HSA is observed, confirming that the damage intensifies across similar frequency ranges, but with variations in energy levels depending on the progression of damage.