# Deep Autoencoder Framework for Classifying Damage Mechanisms in Repaired CFRP
## 3 Results and Discussions
### Fractographic Analysis  
The damage mechanisms identified through AE signal analysis matrix cracking, fiber-matrix debonding (or interfacial debonding), and fiber breakage were confirmed through fractographic analysis, providing a comprehensive view of failure progression in the JLS specimens. Fractographic examination post-failure corroborated these mechanisms, revealing specific structural features associated with each damage mode.  
Matrix cracking, initially detected as lower-frequency AE signals, was identified as the first stage of damage. This mechanism typically involves the formation of microcracks within the matrix material, marking localized stress concentrations that set the stage for further damage progression. Fractographic analysis showed that initial adhesive peeling occurred at the knee of the bend in the overlap region (see Figure 19a), suggesting that mechanical stress concentrates in this area, leading to early adhesive debonding under tensile load. This peeling and microcracking weaken the bond between the adhesive layer and CFRP laminates, initiating interfacial crack growth, a critical damage mode highlighted by acoustic emissions in the 250-375 kHz frequency range.  
Figure 18: Typical AE frequency domain waveforms from JLS 3: (**a**) Cluster 1; (**b**) Cluster 2; and (**c**) Cluster 3.  
As matrix cracking and adhesive peeling progress, the bond between the matrix and fibers deteriorates, resulting in fiber-matrix debonding or interfacial debonding (see Figure 19d). Fractographic evidence supported this observation, showing compromised integrity of the adhesive layer and leading to laminate debonding, as illustrated in Figure 18b. This interfacial separation detected by AE signals in the mid-frequency range corresponds to a stage where the laminates are no longer fully bonded, creating a higher risk of delamination. The fractographic analysis confirmed this delamination, revealing that, as peeling and debonding advance, individual fibers become exposed to irregular forces. These forces are not fully supported by the matrix, increasing susceptibility to rupture.  
The Delamination along the boundary of the adhesive layer interface is observed in Figure 19b. The adhesive not only detached from the lamina but also triggered fiber/matrix debonding within the same layer. Figure 19c provides a magnified view of the circled region in Figure 19b.  
The final stage of damage, fiber breakage, is marked by high-frequency AE signals. Fractographic evidence in Figure 19c shows fiber breakage at the end of the lamina, where exposed fibers fail under unbalanced forces as matrix support diminishes. The combination of fiber-matrix separation and initial fiber breakage exacerbates the problem of fiber-matrix adhesion, leading to a complete breakdown in the load-bearing capability of the composite. These findings confirm that fiber breakage is the ultimate mode of failure in the JLS specimens, coinciding with the highest recorded AE frequencies and corresponding to catastrophic structural failure. The AE results, confirmed by fractographic analysis, establish interfacial debonding and interlaminar crack growth as the primary failure mechanisms in the JLS specimens. The majority of AE signals, centered in the 250-375 kHz band, indicate that load transmission primarily occurs through the adhesive layer, with interfacial crack growth responsible for most acoustic activity. The convergence of acoustic and visual data underscores the significant role of adhesive integrity in load transmission, with interfacial cracking emerging as a key indicator of structural damage in adhesively bonded CFRP composites.  
Figure 19: Fractographic analysis of CFRP bonded in JLS (**a**) peeling at the knee of adhesive; (**b**) delamination along the boundary of the adhesive layer interface; (**c**) fiber breakage; (**d**) fiber/matrix debonding.