## 3 Results and Discussion  
### Mechanical Response and Damage Development under Static Tension  
Initially, tensile quasi-static tests were performed in order to obtain the mechanical properties of the angle-ply laminates, summarized in Table 2. Both laminates have an equal amount of 0\({}^{\circ}\) layers in the loading direction, and taking into account classical laminate theory, it is expected that the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{28}\) laminates display a higher ultimate strength and elastic modulus than the [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{28}\) laminates. However, the increase in strength acquired by the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{28}\) laminates compared to the [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{28}\) laminates is only 4.1% and the increase in stiffness is equal to 5.2%, showing that the two laminates lead to very similar mechanical behavior, when only the mechanical properties are taken into account. In Figure 3, the stress-strain curves of the two laminates are plotted. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Ultimate Strength** & **E-Modulus** & **Strain to Failure** \\  & **(MPa)** & **(GPa)** & **(-)** \\ \hline
[0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{28}\) & 1375 \(\pm\) 23 & 78.2 \(\pm\) 2.7 & 0.0157 \(\pm\) 0.0007 \\
[0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{28}\) & 1318 \(\pm\) 15 & 74.1 \(\pm\) 0.8 & 0.0171 \(\pm\) 0.00 Despite the fact that the two angle-ply laminates present similar mechanical properties, their main difference lies in the damage sequence during testing. Taking into consideration the tensile quasi-static tests of the angle-ply laminates, specific stress levels were chosen and stepwise tests were then performed by stopping the tests in these stress levels. With this methodology, monitoring of the through-thickness damage of the two laminates with the in situ microscope was possible, by keeping the specimen mounted on the test bench under an applied tensile load. Test interruptions were performed for every 80 MPa of applied stress from the beginning of the test up to 50% of the ultimate strength \(\sigma_{\text{ult}}\), then every 40 MPa up to 75% of \(\sigma_{\text{ult}}\), and then finally every 20 MPa until test failure. In this way, iterative measurements of the developing damage could be assessed at different load levels. Figures 4 and 5 demonstrate the damage state of one [\(0^{\circ}/30^{\circ}\)]\({}_{\text{2s}}\) laminate and one [\(0^{\circ}/60^{\circ}\)]\({}_{\text{2s}}\) laminate, respectively, at different percentages of \(\sigma_{\text{ult}}\). Starting with the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{28}\) laminates, until 70% of \(\sigma_{\text{ult}}\), no damage was observed by the microscope. As shown in Figure 4a, at this stress, fiber splitting at one of the external 0\({}^{\circ}\) layers was observed, and when the stress reached a value equal to 75% of \(\sigma_{\text{ult}}\), interlaminar delamination between this outer 0\({}^{\circ}\) layer and the adjacent thin 30\({}^{\circ}\) layer was observed (external delamination), with no prior matrix cracking of the off-axis layers. Around the same stress level, fiber splitting at the second external 0\({}^{\circ}\) layer occurred and some limited matrix cracks in the thick 30\({}^{\circ}\) layer (two middle 30\({}^{\circ}\) layers) were recorded, which increased negligibly until the end of the test. At approximately 80% of \(\sigma_{\text{ult}}\), a second delamination between the external 0\({}^{\circ}\) layer and the adjacent off-axis layer was visible on the opposing side (Figure 4b). The fiber splitting at both external sides came to a saturation point at a stress equal to 85% of \(\sigma_{\text{ult}}\), whereas the length of the delaminations was increased incrementally until the end of the test.