Issue 19

G. Bolzon et alii, Frattura ed Integrità Strutturale, 19 (2012) 20-28; DOI: 10.3221/IGF-ESIS.19.02 22 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 Force [N] Specimen total elongation [mm] Figure 2: Force versus total specimen elongation in tensile test. The region of interest in the investigated material sample is monitored by a digital camera (Allied GX3300, with a 3296 × 2472 pixel 4/3” CCD sensor) equipped with 50mm Zeiss optics. The photographs are acquired with a grab frequency of 1Hz for the full length of the test. A snapshot sequence is represented in Fig. 3. These images can be processed by Digital Image Correlation (DIC) techniques, as introduced in [2]. The reliability of the method is improved by a speckle pattern produced on the specimen surface [9], visible in Fig. 3. DIC permits to recover the displacement field developing in the region of interest by the comparison of two subsequent images of the marked specimen, acquired in the course of the experiment at different loads. The first picture, assumed as a reference, is split into rectangular areas (subsets) of fixed size. The subset size can change substantially with the experimental conditions [10], commonly in the range between 11×11 and 45×45 pixel. In this application, the subset size is 24×24 pixel. DIC algorithm is applied to each subset of the reference picture in order to find the second image portion which is the ‘most similar’ to the starting one, i.e. the region that corresponds to the minimum value of a proper figure of merit, which assumes small values for similar images and larger values in the opposite cases. A proven effective correlation algorithm [10] minimizes the quadratic difference between the intensity pattern of each subset of the first (reference) image and of the next (deformed) image portions. This approach was chosen for this work. Figure 3 : Sequence of digital images showing crack appearance and evolution during the test. DIC techniques include the possibility to apply a deformation to the subset of the first image in order to better fit a portion of the second image and therefore improve the accuracy of the results. A further improvement results from the Im 16 Im 7 Im 27 Im 30 Im 22