# Deep Autoencoder Framework for Classifying Damage Mechanisms in Repaired CFRP
## 3 Results and Discussions
### Results of Deep Autoencoder Training
The highest peak is slightly above 4 \(\times\) 10\({}^{-4}\) dB. The frequency spectrum of Cluster 2 has a peak at approximately 250 kHz, with other minor peaks extending up to 300 kHz. The peak amplitude is slightly above 3 \(\times\) 10\({}^{-4}\) dB. Cluster 3 shows multiple high peaks, especially at approximately 300 kHz and extending beyond, up to 500 kHz. The amplitude in this spectrum is significantly higher, reaching approximately 0.5 dB. The frequency analysis of the AE signals from the JLS 2 is shown in Figure 12. The spectrum of the cluster 1 shows a dominant peak at approximately 170 kHz, with other smaller peaks spreading up to approximately 300 kHz. The peak amplitude reaches approximately 3.5 \(\times\) 10\({}^{-4}\) dB. The spectrum of Cluster 2 shows a peak around 250 kHz, with additional marks distributed between 335 and 350 kHz. The peak amplitude is approximately 1.2 \(\times\) 10\({}^{-3}\) dB, which is significantly higher than Cluster 1. Cluster 3 shows significant activity in the frequency range of 100 to 500 kHz, with several high peaks, particularly approximately 300-450 kHz. The amplitude reaches approximately 0.2 dB, indicating a strong and energetic waveform. Figure 10: Latent features clustered by the k-means algorithm for (**a**) JLS 1, (**b**) 2, and (**c**) 3. In the case of specimen JLS3, as shown in Figure 13, the frequency spectrum for Cluster 1 reveals multiple peaks, with the most prominent ones occurring below 135 kHz and several smaller peaks extending up to 315 kHz. The maximum amplitude observed is approximately 3 \(\times\) 10\({}^{-4}\) dB, indicative of relatively low-intensity acoustic emissions. Cluster 2 exhibits a distinct peak at approximately 325 kHz, with a dominant frequency band spanning from 300 to 375 kHz and an amplitude reaching approximately 1 \(\times\) 10\({}^{-3}\) dB. This increase in frequency and amplitude suggests more significant damage, such as interfacial debonding, which tends to produce acoustic emissions within this frequency range due to the dynamic separation at the interface, releasing more energy compared to matrix cracking. Figure 11: Typical AE frequency domain waveforms from JLS 1: Cluster 1; Cluster 2; and Cluster 3 based on latent feature. Figure 12: Typical AE frequency domain waveforms from JLS 2: Cluster 1; Cluster 2; and Cluster 3 based on latent feature. The frequency spectrum for Cluster 3 is characterized by a prominent peak at approximately 330 kHz, with other significant frequencies up to 500 kHz. The amplitude reaches approximately 0.2 dB, reflecting more intense acoustic emissions. This behavior is typical of fiber breakage, involving a higher release of strain energy due to the brittle nature of carbon fibers and the high energy release during a rupture. These observations indicate that higher frequency and amplitude signals are characteristic of severe damage modes, such as fiber breakage. The AE frequency analysis of specimens JLS 1, JLS 2, and JLS 3 provides insights into the progression of damage mode mechanisms in CFRP bonded by adhesive under tensile loading conditions. Each specimen demonstrates a distinct pattern of AE signal frequencies and amplitudes across three damage clusters, revealing the different stages of structural damage. Three primary frequency ranges correspond to matrix cracking, fiber-matrix debonding, and fiber breakage. Cluster 1 in each specimen, with lower frequencies, is associated with matrix cracking and initial adhesive peeling, marking the onset of structural degradation. This is consistent with observations by [PERSON] et al., who identified low-frequency AE signals as indicative of matrix cracking due to the brittle nature of matrix microcracks [66]. Other studies, including our previous work, also confirm that matrix cracking typically produces low-frequency emissions within this range [67].