## 3 Results and Discussion
### Damage Identification Using AE
The above confirms that AE can, even from an early analysis of the cumulative total activity, indicate the appearance of the occurring damage in the material as well as the transition to other damage modes or to extensive damage. It is also proven that AE can indicate the damage presence even prior to any optical observations, therefore allowing its application for the structural health monitoring in real composite structures by predicting macroscale damage based on early AE activity. It should be noted here that the above results regarding the onset and the evolution of the AE activity depend on the choice of the applied threshold. However, as the laboratory conditions allowed the application of a quite low amplitude, equal to 35 dB, and taking into account the small cross section of the specimens, it is not considered likely that other significant damage sources were not recorded. At the same time, the application of the zero-energy filter allowed for the limitation of the noise signals. Of course, the AE difference between the two laminates is not only quantitative, but also qualitative. This means that the signals related to specific damage sources have certain patterns, and a features analysis is necessary in order to distinguish the damage modes and to detect the damage mode transition. It is proven that the two angle-ply laminates under consideration have different features related to their damage, which is shear dominated for the \([0^{\circ}/30^{\circ}]_{28}\) laminates, whereas it is matrix cracking dominated for the \([0^{\circ}/60^{\circ}]_{28}\) laminates. In the current study, the rise time proved to be the most characteristic feature to prove that different multiaxiality and damage modes lead to different shapes of the AE waveforms. Undoubtedly, different AE features exist, like, for instance, the frequency and the energy, which may contribute to a more complete damage characterization. However, the rise time and its evolution was proven to be the most indicative AE feature for proving the variation of the AE waveforms when variable stresses and damage mechanisms are dominant in the CFRE laminates. The rise time is defined as the time interval between the first threshold crossing and the signal peak, as shown in Figure 18, for a typical AE waveform. In Figure 19, the average rise time values for the two angle-ply laminates measured at specific stress levels during the quasi-static tests are plotted. It should be mentioned that, as fracture in composites is a stochastic phenomenon, large scatter in AE is inherent. However, what is more important is whether consistent trends of the average values of the AE features are obtained, which is the case for the rise time recorded throughout the quasi-static tests for both laminates under consideration. As shown in Figure 19, there is a significant difference in the average rise time values between the two laminates. More specifically, the rise time was constantly higher throughout the static test for the \([0^{\circ}/30^{\circ}]_{28}\) laminates. Even from a low stress, high average rise time values were recorded, related to potential intralaminar shear debondings due to the high shear stresses. The rise time continued to increase constantly, until the end of the test, following the increase in the shear stresses, the development of shear debondings, and the appearance of delaminations after 1000 MPa. It is clearly shown that these shear related mechanisms, or in other words, mode II damage incidents, in which a shear stress is acting parallel to the plane of the crack and perpendicular to the crack front, lead to AE signals with high rise time values. A characteristic example is presented in Figure 20, in which the evolution of the average rise time values versus the average delamination length in the \([0^{\circ}/30^{\circ}]_{\text{2s}}\) laminates is plotted. The rise time constantly increases following the increase of the delamination length and the release of mode II AE signals.