## 3 Results and Discussion
### Accelerated Fatigue Tests  
This test was carried out at high load (3000 N). Due to this elevated load, which is significantly higher than those experienced in real conditions, acoustic signals appeared at a very early stage. Figure 8 illustrates the spectrogram corresponding to the first 120 s of the test. It shows the absence of events during the initial cycles (from 0 s to 45 s) except for the low-frequency background noise. From that point onward, high-amplitude and -frequency events become more apparent, indicating that the onset of damage had already occurred. It is noteworthy that the frequency of the observed events slightly differs from the cracking phenomenon studied in Figure 7. Specifically, the maximum frequency achieved during the sandwich structure is lower (600 kHz compared to the 1000 kHz during the tensile test). This discrepancy may be attributed to the fact that during tensile testing, more abrupt ruptures occur compared to fatigue testing, potentially explaining the differences in amplitude and frequency. Nevertheless, in both cases, the spectrograms exhibit high-frequency content. This is in good agreement with other authors' findings in their studies using optical microphones [21]. [PERSON], in particular, emphasizes the relevance of the ultra-high-frequency range for process monitoring. These events occurred randomly throughout the entire test duration until the final stage of the test, where the density of the events significantly increased just before rupture. This trend is clearly illustrated in Figure 9, where energy, peak frequency, and cumulative number of events are depicted as a function of time. Three distinct phases can be observed. In the first one, the number of events grows progressively until a lower growth zone is  
Figure 8: Spectrogram of the initial cycles during accelerated fatigue test. reached (refer to Figure 9, cumulative number of events graphs from 1000 s to 2000 s), indicating that damage was not progressing at the same rate. Following this, a sudden increase in events occurs before rupture (from 2000 s onwards). It is also important to note that high-peak-frequency events (orange dots in Figure 9) appear during the periods associated with more severe damage, particularly in the first and third phases. Conversely, in the second phase, where events density is lower, the peak frequency values are considerably lower (see Table 2). This supports the assumption that within this region, no significant damage progression occurred, as high peak frequency values are related to more severe damage. Additionally, the cumulative number of events graph (Figure 9) resembles a crack growth rate curve with three different phases: crack initiation, crack propagation, and final rupture. This is particularly relevant in the third phase where an abrupt change is observed. These findings are in good agreement with the literature, as a sharp increase in acoustic parameters is often found to be indicative of crack initiation [22; 23] in some other tests. The correlation of AE parameters with damage accumulation and the fracture mode has also been used to evaluate different crack types by some other authors [24]. The evolution of acoustic events and the definition of three different phases has also been reported before in [25; 26; 27]. And finally, the relationship between the cumulative AE events and the number of cycles to failure has been previously demonstrated in the literature [28] in the same way as the correlation found in this work between AE cumulative events and severity of the damage. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Phase** & \
\begin{tabular}{c} **High Peak Freq.** \\ **Events (Q-45 kHz)** \\ \end{tabular} &
\begin{tabular}{c} **High Peak Freq. Rate** \\ **(Events/s)** \\ \end{tabular} & **Energy Sum (a.u.)** \\ \hline
1 & 118 & 0.295 & 136.696 \\ \hline
2 & 88 & 0.067 & 328.911 \\ \hline
3 & 319 & 2.13 & 658.303 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Quantitative analysis of acoustic events (accelerated test).