## 3 Results and Discussion
### Damage Identification Using AE  
Moving further with the damage identification, the total AE activity of the two laminates recorded during the quasi-static tests is plotted in Figure 16. It is important to mention at this point that in order  
Figure 14: DIC \(\sigma\) pattern for a [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{\text{2s}}\) laminate (**a**) before and (**b**) after delamination. Figure 15: Evolution of the Poisson’s ratio of the angle-ply laminates. to guarantee that microdamage related signals would be recorded by the AE software, a quite low amplitude threshold equal to 35 dB was applied. In addition, linear localization was used to eliminate sources out of a certain gauge length (considered as non-relevant or noise). Furthermore, in order to exclude the possibility of electromagnetic noise and triboelectric effects, all of the AE waveforms characterized by zero energy in the software were removed from the analysis, which is a common practice in similar AE studies in the literature [31; 32]. Looking at Figure 16, the first remark is that AE activity was recorded for the \([0^{\circ}/30^{\circ}]_{\text{2s}}\) laminates quite a lot earlier than for the \([0^{\circ}/60^{\circ}]_{\text{2s}}\) laminates. Additionally, more AE events were in total recorded for the \([0^{\circ}/30^{\circ}]_{\text{2s}}\) laminates, in comparison to the \([0^{\circ}/60^{\circ}]_{\text{2s}}\) ones. The deviation between the two laminates is more clearly shown after 70% of \(\sigma_{\text{ult}}\), after which extensive damage started occurring in the \([0^{\circ}/30^{\circ}]_{\text{2s}}\) laminates. In order to achieve a more direct correlation of the AE activity with the stress increase in both laminates, in Figure 17, the AE events rate is plotted versus the normalized stress. The AE rate is calculated with respect to the applied stress and is therefore expressed as \"number of AE events per MPa\". In detail, the AE activity started occurring in the \([0^{\circ}/30^{\circ}]_{\text{2s}}\) laminates quite early, at an applied stress less than 20% of \(\sigma_{\text{ult}}\). At the same stress level, no AE was recorded for the \([0^{\circ}/60^{\circ}]_{\text{2s}}\) laminates. In the absence of observable damage mechanisms until a stress equal to 70% of \(\sigma_{\text{ult}}\) in the \([0^{\circ}/30^{\circ}]_{\text{2s}}\)  
Figure 16: Total AE events recorded for the two laminates during the static tests. Figure 17: AE events rate recorded for the two laminates during the static tests. laminates, and taking into account the fact that the only differences between the two laminates are the higher shear stresses and strains of the \([0^{\circ}/30^{\circ}]_{28}\) laminates, most of this early stage AE activity of the \([0^{\circ}/30^{\circ}]_{28}\) laminates is reasonably attributed to internal shear damage, appearing in the form of intralaminar shear debondings. These shear debondings increased in number as the test progressed, and led to the development of interlaminar delaminations, confirmed by the high AE activity after a stress equal to 70% of \(\sigma_{\text{ult}}\), being in very good correspondence with the microscope observations. For the \([0^{\circ}/60^{\circ}]_{28}\) laminates, the AE activity was initiated at a quite higher stress level, equal to approximately 30% of \(\sigma_{\text{ult}}\). At this stress level, the AE events recorded were related to the initial matrix microcracking, which was not visible with the microscope, and until 60% of \(\sigma_{\text{ult}}\), a rapid increase in the number of the AE events was evident, coinciding with the stage at which matrix cracks were monitored with the microscope. This response is more pronounced in the curve of the AE events rate, shown in Figure 17. This rate of the AE activity continued until a stress equal to 90% of \(\sigma_{\text{ult}}\), following the increase of the matrix crack density, and after this, an almost instant increase in the number of AE events occurred, coinciding with the appearance of delaminations after this stress value.