Issue 19

G. Bolzon et alii, Frattura ed Integrità Strutturale, 19 (2012) 20-28; DOI: 10.3221/IGF-ESIS.19.02 23 overlap of two contiguous subsets of the reference image, both in horizontal and in vertical direction. This overlap allows to increase the resolution while maintaining a relatively large subset size. In fact, too small subsets may even decrease the reliability of the results [11]. In this work, the overlapping region was assumed equal to 50% of the subset size in both directions. An example of DIC results is shown in Fig. 4, which represents the displacement field in the heterogeneous specimen during the tensile test (according to the image orientation, the right strip end is fixed, while the displacement is imposed to the left end). Notice that, to improve readability, only one out of 16 sampled results are shown in the figure by arrows superimposed to the original specimen picture. x (pixel) y (pixel) 1000 1500 2000 2500 100 200 300 400 500 600 700 800 Figure 4 : Detail of the displacement field obtained by DIC from the comparison of images 7 (the assumed reference one) and 16 (loaded specimen head on the left, fixed on the right); notice the different scales along the horizontal and the vertical direction. The displacement field can be given the alternative representation of Fig. 5, where plots rely on all available data. The pictures suggest that most deformation concentrates in the area of the aluminium laminate inclusion, while the stiffer paperboard composite transfers the load undergoing an almost rigid body motion. The information provided in Fig. 4 and Fig. 5 also suggests that most part of the aluminium inclusion is almost uniformly stretched in the loading direction and experiences much smaller lateral contraction in the transversal direction, as expected. [mm] [mm] 10 20 30 40 50 60 -10 0 10 20 30 -0.25 -0.2 -0.15 -0.1 -0.05 0 [mm] [mm] 10 20 30 40 50 60 -10 0 10 20 30 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 Figure 5 : The displacement field in the heterogeneous specimen under tensile test (loaded specimen head on the left, fixed on the right), reconstructed by the correlation of the digital image 7 (the reference one) and 16 acquired during the test (see Fig. 3). The situation evolves progressively as suggested for instance by Fig. 6, which visualises the displacement field in the aluminium inclusion reconstructed from the digital image 22, see Fig. 3. Even if not yet visible in the corresponding snapshot, DIC analysis evidences the onset of the degradation phenomena leading to the progressive separation of the