## 4 Results and Discussion
### Influence of the Propagation Medium on AE Signals  
In this section, we focus on the influence of the medium around the fiber on the AE signal due to fiber break. As pointed out by [PERSON] et al. [57], AE parameters such as amplitude or frequency are strongly influenced by the elastic properties of the matrix. For instance, [PERSON] and [PERSON] [58] highlighted that fiber break signals could only be detected provided that the sensor is close enough to the damage location, thus evidencing the major influence of the AE propagation medium. The results of fiber break simulations with different matrices are given in Figure 8. The increase in Young's modulus of the surrounding medium led to a decrease in the recorded amplitude or energy. The signals attributed to polymer media (DGEBD-3 DCM, HexPly and PMMA) exhibited higher amplitudes than those obtained for composite or ceramic media. This was due to the high Young's modulus of the ceramics and the composite, which limited the deformation levels. Even if a perfect point-contact sensor with a flat frequency response was used, the recorded signal reflected the characteristics of the medium rather than those of the source. The mismatch of elastic properties between fiber and matrix strongly affected the AE signal due to fiber break. Figure 5: Comparison between simulated and experimental data obtained with nano 30 sensor (**a**) in time and (**b**) frequency domains; sensor C was 12 mm away from the fiber break. (**c**) Amplitude and roll-off frequency vs. source/sensor distance for experimental signals and numerical signals. For the polymer media, its behavior was characterized by a significant decrease in energy and frequency centroid as the wave propagated through the medium. Higher damping values resulted in more attenuation of high-frequency content, as highlighted in Figure 8b. As a matter of comparison, the frequency recorded by a sensor close to the source was equivalent for all media and hence more accurately represented the source frequency. Figure 8b indicates that the dominant frequency of the source was around 1000 kHz. Due to the viscoelastic nature of the polymer matrix, the AE signals had severe attenuation in high-frequency components. For the ceramic media, the amplitude and the frequency did not change with propagation at this scale. The 2D-FFT obtained with different matrix properties are shown in Figure 9. Plotting the data in the wavenumber/frequency plane allowed the identification of the excited waves obtained for the different media. A strong influence of the elastic properties on the excited waves were observed. Moreover, these representations allowed the identification of parasitic modes due to reflections. Figure 6: Signal without AE sensor effect in (**a**) time and (**b**) frequency domains. For DGEBD-3 DCM and HexPly propagation media, the 2D-FFT showed that the same eigenmodes were excited (Figure 9). For DGEBD-3 DCM, the eigenmodes were distributed over a frequency range of [0-800] kHz (Figure 7a), while for HexPly, they were detected in the [0-1200] kHz frequency range. This was due to the material properties that caused the eigenmodes to be excited at higher frequencies. The discontinuity shown in the 2D-FFT of CFRP, carbon and SiC was due to reflections at the edges of the specimen, which disturbed the propagation of the signals. This phenomenon was not observed in the DGEBD-3 DCM and HexPly polymers, as the reflections were damped and did not affect the incident signals. The dispersion modes of polymer media were emitted at lower frequencies than  
Figure 7: Simulated dispersion curves (wavenumber \(k\) versus frequency \(f\)) via 2D-FFT of signals recorded along a straight line for a fiber break at position P on the top surface \(v_{z}\) of a computational specimen (\(e=2.8\) mm and \(z=0.9\) mm) (**a**) without sensor effect with the perfect local sensor and (**b**) with a nano 30 sensor effect. those observed for ceramic media. This led to many observed dispersion modes over the studied frequency range in the polymer specimens.