## 5 Conclusions  
For a better understanding of the relationship between AE descriptors and a damage mechanism, we have performed a series of AE simulations in a model composite. Indeed, finite element simulations provide a powerful tool for understanding the effect of the source, the medium and sensors on AE signals and descriptors.  
The main conclusions without the sensor effect and for the same matrix are the following:  
1. The amplitude of the signals resulting from debonding was much smaller than that from fiber break, even when the size of the debonding was large. On the other hand, instantaneous debonding and fiber break had very similar centroid frequencies.
2. Debonding also generated high-frequency waves but did not excite the same modes as did fiber break in the near field. Fiber break rather excited the fundamental antisymmetric mode at low frequencies. Nevertheless, this difference in the frequency distribution smeared out with propagation distance.  
In near field with the sensor effect, it was possible to easily separate fiber break and debonding signals for a limited propagation distance.
3. The acoustic signature for debonding was mostly affected by the debonding conditions (instantaneous debonding or progressive debonding). The signals obtained with progressive debonding had lower amplitude and frequency. For instantaneous debonding for different lengths, the amplitude was affected more than the frequency content. The effect of debonding conditions suggests that the use of a mechanical model of debonding would be more suitable for future work.  
The effects of the medium on the AE signature of a fiber break are summarized below:  
1. The dispersion modes depend on the mechanical properties of the matrix (Young's modulus, density, etc.). The dispersion modes detected in polymer materials were much more numerous than in a ceramic material over the same frequency range up to 1 MHz. The viscoelastic nature of polymer materials significantly attenuated the frequency compared to ceramic materials. Young's modulus plays an important role in the energy content of the signals. The higher the Young's modulus, the lower the energy content of the signals is. This simulation confirmed the impossibility to obtain a universal signature for the same fiber break in several media. Our study shows that it is not possible to generalize the results of a fiber break signature (amplitude and frequency content) to all composite materials. Therefore, it is necessary to treat each medium independently.  
The combination of the sensor effect and the acquisition threshold makes it possible to define the detectability of the source according to its nature and size. After classification of signals, the labeling of each cluster must rely not only on the microscopic observation of damage mechanisms but also on the simulation results of damage due to sources of different sizes and nature. The notion of detectability of the source is rarely considered for the labeling of clusters.  
Moreover, AE simulations could provide substantial data for machine learning to increase the robustness of supervised classification. These validated results open the way to combine the experimental and the simulated data to generate a library in order to identify real-time damage mechanisms.  
**Author Contributions:** Conceptualization Z.H., N.G., C.F., T.M. and A.D.; data curation Z.H.; formal analysis Z.H.; investigation Z.H.; methodology Z.H., C.F., N.G., T.M. and A.D.; supervision N.G., C.F., T.M. and A.D.; writing N.G., C.F. and A.D. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interests.