Issue 49

S. Pereira et alii, Frattura ed Integrità Strutturale, 49 (2019) 450-462; DOI: 10.3221/IGF-ESIS.49.43 460 It can be seen in Fig. 12, for the region between pin 1 and pin 2, that some specimens, with low strain levels, show a linear change of stress along the cross section, suggesting that are still fully austenitic. In the other hand, the remaining specimens, under greater strain levels, show an inverted ‘S’ profile, meaning that they are partially austenitic (in the central zone, showing a straight line) and partially in the transformation phase (in the outer part, which is approximately flat) [6, 8, 23, 24]. Comparing the information obtained in these charts with the uniaxial tension results, the results converge as the 1% strain specimens present a fully austenitic cross section, as obtained in these tests. For strains above 1%, the wire starts to exhibit the transformation phase from Austenite to Martensite across the cross-section from the outer circle of the wire’s cross- section towards its centre axis. Final Results After comparing the FEA results and the theoretical approach, as the results in terms of strain slightly differ, the combination of these two is imposed in order to improve the accuracy of the fatigue results presented. C ONCLUSIONS AND F UTURE W ORK he work developed and presented in this paper show the results obtained in the rotating bending machine developed by Carvalho et. al [10] and, later, the analysis and validation of both theoretical model and method used for that purpose. The method used, as well as the machine used, allow to address a range of different issues and is able to analyse them based on sound Engineering foundations and Materials science. The versatility of the machine allows one to perform tests for different ranges of strains in a simple and intuitive way. A series of fatigue tests were conducted using Alfa Aesar® Nitinol wires of two different diameters, one of 0.58mm and the other of 0.25mm diameter. The wires, when under strain levels in the elastic isotropic austenitic phase corresponding to strains of 0.6% and 0.8% for the 0.58mm and 0.25mm diameter wires, respectively, showed infinite (above 〖 10 〗 ^6 cycles) life time. When under strain levels in the transformation phase condition, both wires showed a reduced fatigue life that is almost constant during this phase: between 145 and around 1000 cycles for the thicker wire with 0.58mm; and between 1989 and around 15000 cycles for the thinner wire with 0.25mm. This happens due to the stress-strain relation during this phase being a: Ø0.58mm wire b: Ø0.25mm wire Figure 12 : Stress Profile along the beam cross section. Figure 13 - Stress vs Life Time concerning FEA and Laboratorial Results. a – Stress vs Life Time b – Strain vs Life Time