## 3 Results and Discussion
### Mechanical Response and Damage Development under Static Tension
At around 85% of \(\sigma_{\text{ult}}\), multiple matrix cracks were recorded in one of the thin 30\({}^{\circ}\) layers along a limited length of the layer in the area of the previously formed delamination (Figure 4c), and this led to the appearance of a third internal delamination between this layer and the inner 0\({}^{\circ}\) layer shortly afterwards, at around 87% of \(\sigma_{\text{ult}}\) (Figure 4d). The growth of all of these delaminations resulted in the final failure of the material, after catastrophic fiber breakage. For the [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{28}\) laminates, the damage sequence was remarkably different (Figure 5). Damage was detected by the microscope much earlier than in the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{28}\) laminates, starting from a stress equal to 45% of \(\sigma_{\rm ult}\). However, the first appearing damage mode was matrix cracks and not delaminations. The matrix cracks occurred in all of the off-axis layers randomly, and they progressively increased in number. After 60% of \(\sigma_{\rm ult}\), multiple matrix cracks were monitored in all of the off-axis layers, with a higher crack density for the thin off-axis layers (Figure 5b,c). The matrix cracks became saturated only at a level of 90% of \(\sigma_{\rm ult}\) and, only after this point, was an initial delamination between one of the external \(0^{\circ}\) layers and its adjacent thin \(60^{\circ}\) layer observed (Figure 5d). The corresponding delamination on the other side of the specimen occurred at around 95% of \(\sigma_{\rm ult}\) and, moreover, only during the application of the last 5% of \(\sigma_{\rm ult}\), was fiber splitting evident at the external \(0^{\circ}\) layers. Taking all of the above into consideration, in Figures 6 and 7, the total damage progression during the quasi-static tests for the \([0^{\circ}/30^{\circ}]_{\rm 2s}\) and the \([0^{\circ}/60^{\circ}]_{\rm 2s}\) laminates, respectively, is shown as a percentage of the applied stress. The loading periods during which the different damage modes appeared in the test are plotted, defining which modes appeared first and which were overlapping in a specific percentage of the stress. The values of the stress are averaged, with the deviation being very small, in the range of 1-1.5%. In both of the laminates, the final catastrophic failure occurred because of brittle fiber breakage in the loading direction. Looking at the different damage progression of the two angle-ply laminates, one can recognize the influence of the different multiaxiality in the sequence of the appearing damage modes. Taking into account the \([0^{\circ}/30^{\circ}]_{\rm 2s}\) laminates, in which the shear stresses are dominant in the off-axis layers, delaminations are prone to happen even before matrix cracking, which is in disagreement with  
Figure 6: Damage process during the quasi-static tests for the \([0^{\circ}/30^{\circ}]_{\rm 2s}\) laminates. Figure 7: Damage process during the quasi-static tests for the \([0^{\circ}/60^{\circ}]_{\rm 2s}\) laminates. the damage sequence that is generally accepted for the composite materials, presuming that matrix cracking is always the onset damage mode. Indeed, in the case of the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{\text{2s}}\) laminates, interlaminar delaminations start at a total stress of around 1030 MPa (around 75% of \(\sigma_{\text{ult}}\)), with no prior matrix cracking. Therefore, delaminations are reasonably attributed to the developing shear friction and intralaminar shear debondings due to the high shear stresses that are developed in the laminate layers. The delamination events together with the continuously increasing shear stresses lead to the appearance of matrix cracks, which then lead to the development of internal delaminations at around 1200 MPa stress (87% of \(\sigma_{\text{ult}}\)).