13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Finally, SEM observations on fracture surface have been performed in order to evaluate the main fracture micromechanisms. In order to check the correctness of the LOM observation procedure, a preliminary test has been performed. An unloaded and metallographically prepared specimen has been deformed up to eng=0% and then LOM observed (Fig. 2a). Subsequently the same specimen (loaded) has been metallographically prepared and LOM observed (Fig. 2b). It is evident the possibility to follow the grains modifications in the investigated SMA only performing the first metallographic preparation. 3. Results Engineering stress strain curve of investigated alloy is shown in Fig. 3, where a plateau has not been observed. The investigated deformation conditions correspond to: 1. eng = 0% - starting test conditions; 2. eng = 5% - near yield point; 3. eng = 10% - plastic zone; 4. eng = 14% - specimen failure. 0 100 200 300 400 500 600 700 0 1 2 3 4 5 6 7 8 9 10111213141516 σeng [MPa] εeng [%] Figure 3. Engineering Stress-Strain curve (red points correspond to the investigated conditions). The failure initiation site (Fig. 4), for eng = 0%, is characterized by the presence of three grains (Fig. 4a). For eng = 5% (Fig. 4b) the three grains show an unchanged orientation, no sub-grains nucleation and a grain boundary deformation from linear to curved shape, probably due to phase transition. This is more evident in the grain on the left, where a surface modification is observed (sort of zig-zag lines on the surface). For eng = 10% (Fig. 4c), an intergranular crack initiate from the lateral specimen surface, with a secondary intergranular crack, that is more or less parallel to the applied load. This secondary crack is probably due to the phases transition in grains with different orientations, with a consequent stress increase at the grain boundary [7]. The increase of the macroscopical deformation implies an increase of the localized damage level, with the coalescence
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