13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- observed. Figure 6 Shear stress-strain curves at various strain rates 3. Measurement of strain fields 3.1 Digital Image Correlation (DIC) setup To acquire images, different devices were used, depending of the performed test. For low frequency acquisition, a reflex camera (Canon EOS) was used and for high frequency acquisition, fast cameras (PHOTRON) were used. The main difference between the two ways to get pictures is that for fast camera, the resolution is lower than for reflex camera. Subsequently, the spatial resolution is poorer for dynamic tests: the resolution of the fast camera is in inverse proportion to frame rate even if, for the last test, a more efficient camera was used in comparison with the previous test. This impact directly the accuracy of the optical measurement. In order to allow DIC computations, a speckle was applied on the observed surface of the specimen with black and white paint. Moreover, in order to avoid heating of the specimen surface, the lighting was done with LED lights during the quasi-static experiments and with short time light exposition of the specimen during the dynamic experiments. The principle of the used DIC computation program (CorreliQ4) is given in [31-32]. A first analysis of DIC results allows to give qualitative information on the phenomena taking place during the tests. Basic observation on figure 7, resulting from a quasi-static tensile test at 0.01 mm/s, shows that only two admissible strains are visible: a 1 % strain zone (supposed to be the austenitic phase) and a 6 % strain zone (supposed to be the stress-induced martensitic phase). An enlargement of the SIM region during the test can be observed.
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