Issue 18

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 18 (2011) 45-53; DOI: 10.3221/IGF-ESIS.18.05 49 a) b) Figure 5 : FE analysis of the testing condition: a) FE model and b) equivalent stress fringes at the maximum elongation. In Fig. 6a a comparison between the experimentally measured engineering stress-strain curve of Fig. 3 and the numerically simulated one is illustrated, and a good agreement is observed. Fig. 7 shows a comparison between the numerically simulated stress-strain curve relative to net and gross engineering strain. This latter was calculated from the displacement of the specimen head, according to the experimental conditions. As expected, gross strain is significantly greater than net strain and the difference increases when increasing the applied stress. Furthermore, this effect is also evident in the early stage of loading, i.e. in the range of elastic deformation of austenite, resulting in an apparent smaller value of the Young’s modulus. This effect can be attributed to two different mechanisms: the compliance of the miniaturized testing machine and the deformation on the specimen heads. Note that a linear correction of the gross strain cannot be used here as the material non-linearity causes a marked non-linear relation between gross and net strain. For a better understanding of the results reported in the following section, Fig. 7 illustrates the relation between gross strain and net strain within the range of deformation of the experiments. Figure 6 : Comparison between experimentally measured stress-strain curve and numerically simulated one. Prior to investigate stress strain behavior and in order to investigate microstructural transitions it is necessary to evaluate the initial XRD spectrum. The first diffraction obtained on the calibrated length of the miniature specimen in stress free conditions, shows two picks at 42.39 and 77.54°, corresponding to [011] and [022] crystallographic plains, typical of the