## 3 Results and Discussion  
### Mechanical Behavior and AE Analysis  
Table 2 summarizes the experimental results, including the total test time, stress and strain at failure, time at which the first AE signals occurred, total number of AE hits, and maximum AE amplitude. The stress and strain at failure of the GFRP specimens decreased continuously with the increasing UV aging time. The tensile strength of specimens irradiated for 300 h was 28.4% lower than that of pristine specimens. The strain at failure decreased in the order 0-100 h \(>\) 200 h \(>\) 300 h. The UV treatment embrittled the material and decreased tensile toughness. All specimens generated large AE amplitudes (99 dB\({}_{\text{AE}}\)) during the last stage of tensile testing. The total number of AE hits exceeded 10,000 for all specimens, but there was no association with UV irradiation time. Figure 4: The concept of \(b\)- and Ib-values [31]. Figure 5 shows the amplitude under tensile loading of specimens for different UV irradiation times. The amplitude (i.e., the highest peak in a detected voltage signal) was converted to decibels (dB\({}_{\text{AE}}\), Equation (1)). The maximum amplitude was the largest amplitude among all of the accumulated AE data. For example, in Figure 5a, an AE hit of 48 dB\({}_{\text{AE}}\) occurred at 16% of the lifetime, and persisted until about 20% of the lifetime, indicating that other hits occurred during the interval but had amplitudes of less than 48 dB\({}_{\text{AE}}\). In the 0 h specimen (Figure 5a), the maximum amplitude increased gradually until 300 h, after which it increased rapidly (Figure 5d). Amplitude corresponds to crack size, so the maximum amplitude relates to crack growth. Crack growth was relatively rapid in the 300 h specimen. Cracking was accelerated due to cracks generated on the surface by UV exposure. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Unit** & **0 h** & **100 h** & **200 h** & **300 h** \\ \hline Total test time & Second & 1741 & 1741 & 1514 & 1404 \\ Failure stress & MPa & 271 & 254 & 113 & 110 \\ Failure strain & \% & 2.9 & 2.91 & 2.53 & 2.34 \\
1 st AE hit time & Second & 281.5 & 214.9 & 159.7 & 110.7 \\ Total AE hits & – & 13,131 & 10,886 & 13,247 & 10,329 \\ Max amplitude & dB\({}_{\text{AE}}\) & 99 & 99 & 99 & 99 \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of fracture test results. Figure 5: Stress–strain response of GFRP specimens and maximum AE amplitudes after UV irradiation for (**a**) 0 h, (**b**) 100 h, (**c**) 200 h, and (**d**) 300 h.  
Figure 6 shows the UV effects for 0 h and 300 h irradiated specimens. The fracture surface in Figure 6b shows a serrated fracture surface with many transverse surface cracks. In a woven textile composite, cracks are first induced in the weft direction (transverse cracks), followed by fiber breakage. The pristine specimens (Figure 6a) showed brittle failure according to the typical failure behavior, whereas the irradiated specimens showed more complex crack behaviors. The phenomenon can be explained by surface hardening and surface cracking, as discussed in previous research [31]. In the 300 h specimen, surface crack leads to larger transverse cracks in the early stage (Figure 5d), and the damaged surfaces and interfaces are weaker than in pristine specimens. The cracks propagated more actively (more AE signals with larger amplitude), creating a serrated fracture surface. Figure 7 shows the mean amplitude of the AEs. To compare crack behavior among different test times, this was expressed as the percentage of lifetime (where final failure = 100%). The mean amplitude displayed a similar increasing trend in all specimens but was higher in UV-irradiated specimens. As noted above, hit amplitude was strongly correlated with crack size, and a higher mean amplitude implied that the cracks were larger, even at an early stage. Crack growth was faster in UV-treated specimens, and not only in terms of maximum amplitude.