Issue 47

L. Marsavina et al., Frattura ed Integrità Strutturale, 47 (2019) 266-276; DOI: 10.3221/IGF-ESIS.47.20 271 Figure 5 : Comparison between fracture criteria and experimental results C RACK RELATIVE DISPLACEMENT FACTOR ESTIMATION BY DIGITAL IMAGE CORRELATION n the present paper, the fracture process was evaluated trough two parameters, the Stress Intensity Factor and the Crack Relative Displacement Factor (CRDF), respectively. As shown above, the calculation of SIF is most often based on the analytical solutions. The analysis of the analytical equations allowing calculation the SIF shows that its estimation depends on sample geometry and the loading amplitude. Based on the displacement fields amplitude the CRDF can be related with the kinematic state near the crack tip [19, 20, 25]. In this case, the CRDF can be calculated from the displacement fields measured by optical metrologies [19, 20, 25]. In the present paper, we propose to use the Digital Image Correlation (DIC) in order to measure the displacement fields and to evaluate the CRDF. It should be added that the evaluation of CRDF allows to separate the mixed mode loading configurations and to identify the part of each mode in the fracture process. Principle of Digital Image Correlation As mentioned above, in the present study, the CRDF was investigated by means DIC. Using this optical full field method the evolution of displacement fields was recorded during the fracture test. Now concerning the principle of DIC, it is important to specify that this technique is based on comparison of two images acquired before and after sample deformation [29-31, 42]. As described in Fig. 6, the displacement was calculated in the Zone Of Interest (ZOI) meshed by small groups of pixels, called subsets [19, 20, 29-31, 51]. According with the DIC hypothesis, the light intensity distribution during the test does not change. By supposing that the displacements may be approximated as homogeneous and bilinear inside the subset, the displacement fields were estimated by searching the subset distortions in terms of translations, rotations and rigid body motions. In fact, the displacement field represents the displacement vectors of the center of gravity of all subsets. Concerning the sample preparation, it should be noted that prior to testing, a very thin black and white speckle pattern was sprayed on the specimen surface. Then, as had been indicated above the displacement fields are calculated by tracking the deformation of a random grey speckle pattern applied to sample surface. In the present study, the ZOI was meshed by using the 32  32 pixels² subset sizes. The optical device configuration used in the present study consist of an AVT Marlin F-201B with a Pentax 12.5-75 mm lens and a LED light source. The measurement was realized using an Aramis a non-contact and material-independent measuring system based on digital image correlation. The image analysis was performed using Correla software’s, developed by PEM team of Pprim Institut of Poitiers [38-39]. The estimating uncertainty of displacement is about 0.026 pixels. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 K II /K Ic K I /K Ic MTS SED Gmax CS 16 CS 25 SENB 16 SENB 25 I