## 4 Results and Discussion  
### Experimental Results  
During SFFT, we have used a classical 1D linear localization process to determine the positions of AE signal sources. Moreover, the number of localized AE source and the number of fiber breaks observed by means of transmission optical microscope (Figure 6 and Table 3) are in good agreement. The observations demonstrate several fragmentations in the fiber without any significant other processes (Figure 6a). We have established a one-to-one correlation between the numbers of AE located signals and the observed fiber breaks. Therefore, in the tests, every located source originates from a fiber break. Optical observations confirmed the absence of fiber/matrix debonding. In Figure 6, one can observe  
Figure 5: Aperture function for the sensors: evolution of the k factor versus the distance from the centre of the sensor surface. that the two types of sensors used in experimental tests localized exactly the same cumulative number of signals from the same sources. Therefore, they are both suitable for this test. \begin{table}
\begin{tabular}{c c c c c} \hline  & & **Nb. of Localized AE Sources** & **Microscopic Observation** \\ \hline \multirow{4}{*}{**e = 2.8 mm**} & \multicolumn{2}{c}{Nano30} & \multicolumn{2}{c}{PicoHF} & \\  & Test 01 & 56 & 56 & 58 \\  & Test 02 & 45 & 45 & 46 \\
**e = 1.7 mm** & Test 03 & 26 & 26 & 27 \\ \hline \end{tabular}
\end{table} 
Table 3: Comparison between the number of AE-located sources and the number of observed fiber breaks. Figure 6: (**a**) Fiber breaks as observed by optical microscopy. Cumulative localized signal and applied stress versus strain during SFFT detected by (**b**) nano30 and by (**c**) picoHF (e = 2.8 mm and z = 0.9 mm, Test 04). Figures 7 and 8 show the time-frequency maps obtained with Smoothed Pseudo Wigner-Ville Distribution (SPWVD) for signals assigned to fiber breaks for two positions of the source: near the sensor (position P1, at 6 mm from the sensor centre) and far from the sensor position (position P5, at 34 mm from the sensor centre). Figure 7a,b show the dispersion of the wave modes. For the shortest propagation distance, the main energy in the signal is concentrated around 10 \(\upmu\)s and in the range of 200-400 kHz. When the propagation distance increases, the lower frequencies tend to arrive later in the signal. We can observe a significant decrease in the frequency content certainly due to the propagation effect. Their signatures vary due to differences in the propagation distance. For the same signals recorded by the picoHF sensors, the SPWVD (Figure 8) reveals a higher frequency content, especially at the P1 position. In this case, the main energy on the signal is concentrated around 10 \(\upmu\)s and in the range of 400-700 kHz. At the position P5, the main energy is in the range below 300 kHz. The acoustic signature of the fiber break varies due to the choice of the sensor. Figure 8: SPWVD of experimental signals recorded by the sensor picoHF (**a**) near the source (P1 position) and (**b**) far from the source (P5 position). \(\mathrm{e}=2.8\) mm and \(\mathrm{z}=0.9\) mm. Figure 7: Experimental signals recorded by the sensor nano30 with the associated FFT and SPWVD results (**a**) near the source P1 position and (**b**) far from the source P5 position (\(\mathrm{e}=2.8\) mm and \(\mathrm{z}=0.9\) mm). Figure 9 enables us to compare, in the frequency domain, signals detected by nano30 and picoHF sensors, respectively, when they stem from the same source and whether it is close or far from the sensors. Cross-correlation analysis performed between the signals recorded by these two sensors exhibits the maximum values of cross-correlation around 87% far from the source and only 25% near it. Such a low correlation coefficient in the frequency domain clearly indicates that these two kinds of sensors do not record the same information. Near the source, the nano30 is very sensitive around 300 kHz and does not detect high frequency above 500 kHz.