## 3 Results and Discussion
### DIC and Poisson Ratio as Damage Indicators  
Apart from monitoring the damage and creating databases for the full characterization of a material, different techniques also have to be used in order to create damage indicators during testing. Supplementary to the aforementioned damage observations, the DIC can also be used for damage indications at least in a qualitative way. This can be done by using the \(\sigma\) value of the DIC software. This parameter represents the confidence interval for the match at any data point (defined as the middle point of a subset) of the correlated area (with respect to the initial reference image corresponding to the unloaded situation) and is measured in pixels. Any mismatch between the reference image and a deformed image during loading is received as an error in the DIC software, and is expressed as an increase in the \(\sigma\) value. In Figure 13, the \(\sigma\) pattern over a 50 mm by 25 mm area of a \([0^{\circ}/30^{\circ}]_{28}\) laminate (with 50 mm being the vertical dimension of the correlated area, representing the length of the specimen, and 25 mm being the horizontal dimension, representing the width of the specimen) for two subsequent DIC pictures is shown. The loading direction corresponds to the vertical dimension. The images were taken at a stress equal to 80% of \(\sigma_{\text{ult}}\), at which the second delamination in the laminate made its appearance. The significant difference of the \(\sigma\) pattern on the surface of the specimen between the two successive DIC images (corresponding to Figure 13a,b, respectively) shows that the \(\sigma\) pattern could successfully indicate the appearance of delamination. This difference in the \(\sigma\) pattern can be attributed to the out-of-plane displacement of the laminate, due to the occurring delamination between the underlying layers. The same applies for the \([0^{\circ}/60^{\circ}]_{28}\) laminate, for which in Figure 14, the \(\sigma\) pattern at two subsequent pictures at around 90% of \(\sigma_{\text{ult}}\) is shown. The higher \(\sigma\) values in Figure 14b in comparison with Figure 14a indicate out-of-plane movements, confirming the appearance of delamination at this stress level also in this case. At the same time, higher \(\sigma\) values were obtained for the \([0^{\circ}/30^{\circ}]_{28}\) laminates, coming to good agreement with the fact that at a stress value equal to 80% of \(\sigma_{\text{ult}}\), delaminations of a greater extent were observed, leading to higher out-of-plane movements. However, these observations are only qualitative, indicating that a change in the pattern of the \(\sigma\) value can be a sign of the appearance of delaminations. Apart from the NDTs that are commonly applied for indicating damage in composite materials, it is known that the way that simple mechanical properties of a material evolve during testing can also be used as a damage indicator, and can provide additional verifications regarding the damage process. In this direction, this study considers the evolution of the Poisson's ratio, defined as the negative ratio of the transverse strain to the longitudinal strain, calculated using the DIC surface strain measurements. This proves at the same time, that accurate strain measurements by the DIC software assist in the calculation of other mechanical properties, like the Poisson's ratio, as well. In Figure 15, the evolution of the Poisson ratio versus the applied stress is plotted for both of the testing configurations. In the beginning of the test, the Poisson's ratio values corresponded to the theoretically calculated values using classical laminate theory. However, the evolution of the Poisson's ratio was different for the two angle-ply laminates, as damage accumulated during the quasi-static test. As shown in Figure 15, in the [0\({}^{\circ}\)/30\({}^{\circ}\)]\({}_{\text{2s}}\) laminates which are characterized by higher shear strains, the Poisson's ratio was higher and was continuously increasing until the stress level at which delamination occurred.