Issue 49

S. Pereira et alii, Frattura ed Integrità Strutturale, 49 (2019) 450-462; DOI: 10.3221/IGF-ESIS.49.43 455 Figure 5 : Uni-axial tension test results. Material Property Alfa Aesar® Isotropic Elasticity Young's Modulus (GPa) 60 Poisson’s Ratio 0.33 Superelasticity Starting stress value for the forward (Austenite to Martensite) phase transformation σ AS s (MPa) 522 Final stress value for the forward (Austenite to Martensite) phase transformation AS f  (MPa) 555 Starting stress value for the reverse (Martensite to Austenite) phase transformation SA s  (MPa) 280 Final stress value for the reverse (Martensite to Austenite) phase transformation SA f  (MPa) 235 Maximum residual strains L (mm/mm) 0.0645 Table 2 : List of Material Properties to Alfa Aesar® Nitinol. From Fig. 5, it can be seen that, while having the same overall behaviour with an elastic plateau (which is typical of NiTi alloys), there is a large variation in the ultimate tensile strength and phase transformation stress/strain necessary to start the phase transition, but not as such with regards to the overall strain and the “length” of the phase transformation region (i.e., the quasi-horizontal plateau). Fatigue Tests The results for the Strain vs Number of Cycles for the fatigue tests under different levels of strain induced in the 0.58mm and 0.25mm NiTi wires are presented in Fig. 7. In this figure it is also possible to see the fatigue results for two others different NiTi alloys tested in the same apparatus [19]. For the wire of 0.58mm diameter, the fatigue tests were conducted on a range of maximum extension from 0.6% to 5%. Where the specimens were subjected to strains under 1%, i.e., where the crystallography of the NiTi alloy still is austenitic, the material showed a large fatigue life, when compared with tests where specimens were subjected to larger levels of strain. More specifically, the 0.6% strain specimens, showed an infinite fatigue life (i.e., above 6 10 cycles). The specimens with strains equal to and above 1% show a fatigue life nearly constant, in the order of magnitude of 2 10 to 3 10 , as expected for superelastic alloys [20, 21]. The specimen with the shortest life time failed after 145 cycles.