## 3 Results and Discussion
### Quantitative Damage Assessment  
For a more quantitative damage description, and in order to better highlight the detrimental influence of shear, Figures 9 and 10 show the evolution of the length of the different delaminations that appeared in the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{\text{2s}}\) and the [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{\text{2s}}\) laminates, respectively, versus the applied stress and also versus the percentage of \(\sigma_{\text{ult}}\), as measured with the microscope. The values on the horizontal axis range between the value at which delamination was first observed and the value at which the final scan was performed, at a stress equal to approximately 98% of \(\sigma_{\text{ult}}\). It is shown that in both cases, delamination accumulated constantly from its onset until the global failure. It is clearly shown that shear has a dramatic influence not only on the damage sequence, but also on the extent of damage. In the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{\text{2s}}\) laminates, the delaminations at the two external layers  
Figure 8: Stress–shear strain curves of the angle-ply laminates. exceeded 100 mm in length, with average values of 125 mm and 115 mm, respectively. The delamination between the internal layers obtained a quite significant length, equal to 60 mm on average. On the other hand, in the [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{\text{2s}}\) laminates, the delaminations were developed to a lower extent, reaching lengths of around 60 mm and 45 mm, close to the final failure of the material. Good repeatability was obtained among all of the different specimens tested for each lay-up. In Figures 11 and 12, the matrix crack density in the monitored length versus the percentage of \(\sigma_{\text{ult}}\) for the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{\text{2s}}\) and the [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{\text{2s}}\) laminates, respectively, is plotted. The crack density is expressed as number of cracks/mm and is measured by using the post-processing tools of the Leica software (as presented in Figure 5). It is shown that the matrix cracks appeared late in the test for the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{\text{2s}}\) laminates, after 1030 MPa stress (75% of \(\sigma_{\text{ult}}\)), and they were stochastically distributed in the thick 30\({}^{\circ}\) layer. The density of these cracks was low, therefore a magnification of its evolution in the same horizontal range is presented in Figure 11. Regarding the thin 30\({}^{\circ}\) layer, matrix cracks only occurred after a stress  
Figure 10: Evolution of delamination length in the [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{\text{2s}}\) laminates. Figure 9: Evolution of delamination length in the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{\text{2s}}\) laminates. equal to 85% of \(\sigma_{\rm ult}\), due to the former developing delaminations between the 30\({}^{\circ}\) and 0\({}^{\circ}\) layers. The crack density in the thin off-axis layer was high, more than two cracks/mm, but it was restricted to a limited length of the layer, equal to 15 mm on average. However, this high crack density proved to be responsible for the occurrence of internal delaminations. On the other hand, in the [0\({}^{\circ}\)/60\({}^{\circ}\)]\({}_{\rm 2s}\) laminates, matrix cracks occurred at around 600 MPa (45% of \(\sigma_{\rm ult}\)) both in the thin and thick off-axis layers. However, it is remarkable that a higher crack density was measured for the thin 60\({}^{\circ}\) layers. The rate of the measured crack density reached an asymptote after a stress level of 900 MPa (68% of \(\sigma_{\rm ult}\)), and the cracks came almost to a saturation after 1100 MPa (83% of \(\sigma_{\rm ult}\)). It is important to mention here that the measurements presented previously correspond to the damage developed on one side of the sample.