## 4 Results and Discussion
### Comparison between Fiber Break and Fiber/Matrix Debonding Signals Computed in DGEBD-3 DCM Medium  
The results obtained for the instantaneous debonding (Model A) are shown in Figure 10a. The figure shows the evolution of the amplitude as a function of low-frequency content between 0 and 125 kHz for fiber break and fiber/matrix debonding. The data correspond to the signals calculated for different distances \(d\) between the source epicenter and the sensor (Figure 2a). In all cases, the amplitude of the signals resulting from debonding was smaller than that of the fiber break even when the debonding size was large (the smallest debonding surface was about eleven times larger than the cross section of the fiber). The AE signal amplitude or energy for the debonding was proportional to its area or to the velocity of the crack. Consequently, higher crack surface areas resulted in higher acoustics emission signal energies for the same conditions of debonding. On the other hand, instantaneous debonding and fiber break admitted similar centroid frequencies. The frequency descriptors showed a frequency content independent of the debonding length (Table 2). Nevertheless, the differences were highlighted. The frequency centroid attributed to the fiber break coincided with that of the debonding up to a propagation distance of 18 mm. Beyond this distance, the frequency centroid of fiber break became slightly weaker. The opening frequency or the partial power in the 0 to 125 kHz frequency range showed a low-frequency content more important in the case of fiber break. Indeed, debonding does not excite the same modes as does fiber break, which excites the fundamental anti-symmetric mode of low frequency. The median value for the frequency centroid for the fiber break was around 370 kHz, whereas the value obtained the fiber/matrix debonding was around 400 kHz.  
For a given value of debonding length (L = 100 \(\upmu\)m), the energy carried by the wave decreased with increasing source duration or debonding propagation velocity (Figure 10b). Instantaneous debonding in one step corresponded to the highest velocity, whereas the non-uniform progression corresponded to the smallest velocity. The main effect was observed on the energy or amplitude. The energy or the signal amplitude did not only depend on the length of the debonding, but also on the speed of crack propagation and on the kind of damage mechanism. Moreover, the frequency was inversely proportional to the duration of the source. A gradual decrease in frequency content was observed as the source duration increases from 0.1 to 4 \(\upmu\)s (Table 2). The frequency content mainly depended on the debonding kinetics rather than on its size. A decrease in the high-frequency content of signals was observed when the debonding velocity was slow. As expected, the highest frequencies corresponded to the signals produced by the fastest source.  
Figure 8: Variation of the descriptors (**a**) amplitude, (**b**) frequency centroid as a function of the distance between the source fiber break epicenter and perfect sensor location for the simulated signals. (**c**) Frequency centroid as a function of amplitude.  
Figure 9: Simulated dispersion curves (wavenumber k versus frequency f) via 2D-FFT of fiber break signals recorded along a straight line on the top surface of a numerical specimen (e = 2.8 mm and z = 0.9 mm) without the sensor effect. (**a**) Matrix \(\alpha_{2}\) (Hexply), (**b**) Matrix \(\beta\) (carbon) and (**c**) Matrix \(\gamma\) (composite CFRP).