## 3 Results and Discussion
### Non Accelerated Fatigue Tests  
With respect to non-accelerated tests, where loads were much lower (600 N), initial acoustic events appear at a later stage of the test (560 s). This suggests that damage is not generated until several cycles have been run (5600 cycles). After that, few events are observed which have, as a principal feature, high peak frequencies (up to 170 kHz), most probably related to cracking phenomena (phase 1). This trend is maintained until 40,000 cycles, above which a considerable growth in the cumulative number of events occurs (phase 2), followed by a sudden increase in acoustic events, especially from 7000 s onwards (70,000 cycles, phase 3). This sudden appearance of high-peak-frequency events indicates severe damage prior to part rupture (see Figure 14) as it occurred with structure condition (phase 3).This fact again proves the assumption that high peak frequency values are a particular feature of cracking. Figure 13: Four different locations strain along the whole tests. Signal discontinuities represent sequences that were not analyzed due to the high number of images. These numbers are summarized in Table 3, which, in comparison with Table 2 showing values from the accelerated test, indicates less damage. This is clearly observed when comparing different features such as energy and high-peak-frequency events in Figure 15. Another sign of less severe damage is the total number of events identified during this test, which was 207, instead of the 910 counts in the accelerated test. It has to be mentioned that the acoustic event's amplitude was lower for non-accelerated tests while no significant differences were observed in peak frequency values. Figure 14: Evolution of different acoustic parameters and the cumulative number of events along time during non-accelerated test. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Phase** & \begin{tabular}{c} **High Peak Freq.** \\ **Events (>45 kHz)** \\ \end{tabular} & \
\begin{tabular}{c} **High Peak Freq. Rate** \\ **(Events/s)** \\ \end{tabular} &
\begin{tabular}{c} **Energy Sum (a.u.)** \\ \end{tabular} \\ \hline
1 & 19 & 0.0047 & 24.850 \\ \hline
2 & 0 & 0 & 232.553 \\ \hline
3 & 25 & 0.09 & 37.801 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Quantitative analysis of acoustic events (non-accelerated test). Figure 15: Comparison between accelerated and non-accelerated tests in terms of energy and number of high-peak-frequency events. When it comes to event classification, a similar behavior compared to the accelerated test is observed. This is clearly illustrated in the polar graph (Figure 16 where, on average, all the features are considerably higher for outliers except for the counts and peak frequency. The machine learning algorithm clearly captures the two main classes (as observed in Figure 17 Principal Component Analysis) where two classes may be distinguished by the naked eye, but again, this is related to variability due to different energy and amplitude events. However, in order to distinguish cracking from other noise sources, frequency domain features should play a more important role as was demonstrated in the higher presence of peak frequency events at the last stage of the test (refer to Figure 14), where the most severe damage in the sandwich structure occurred prior to rupture. This is backed up by some other works [32] in which different frequency signals were clustered and related to different damage mechanisms in carbon fiber composites testing. In particular, the highest frequencies were attributed to fiber breakage. Figure 16: Polar graph representing the average values of the two classes. Figure 17: PCA analysis and the two classes that are distinguished. In the case of the strain evolution from DIC system, the level remained constant until the last stages of the tests, where a clearly noticeable redistribution was observed. In this particular case, Figure 18, strain 4's level was kept at a lower level while strain 1 and strain 2's levels increased dramatically until rupture.