Issue 49

S. Pereira et alii, Frattura ed Integrità Strutturale, 49 (2019) 450-462; DOI: 10.3221/IGF-ESIS.49.43 457 0 m energy d volume    =  (4) where m  is the maximum strain for the differential volume of material, and  is the stress. This behaviour is in line with the results found on the literature, as the NiTi alloys tend to increase the life time with a decreasing diameter. [22] Fracture Surface The fracture surfaces of the rotary bending tests were subjected to a closer look on a SEM (Scanning Electron Microscope). In Fig. 8, one can see that the surface shows some porosities that act like locations of stress concentrations where crack initiation can occur. This feature is not uncommon in other NiTi alloys but is not present in all of them. Also, after the rupture, the material presents a well-seen granular surface through which the fatigue fracture grew. Figure 8 : Detail of the porosities found in the alloy (2000x magnification). The observation of the crack propagation zones showed, for the 0.58mm diameter wire, a similar size of crack propagation zone before the final rupture, for specimens obtained in the LCF zone, Fig. 9(a)-9(d). In Fig. 9(b), it can be seen a detail of a specimen tested under 5% of strain showing a small crack propagation zone near the surface. For lower strain (greater fatigue life time), the crack propagation zone size gets bigger as the strain levels decrease, resulting that the fracture propagates slower, Fig. 9(e). In Fig. 9(f), it can be seen a detail of a specimen tested under 1% of strain showing a greater crack propagation zone. For the 0.25mm diameter wire, the analysis in the SEM showed that the crack propagation zone increases as the strain levels decrease. In Figs. 9(h) and 9(i) can be seen a detail of the crack initiation zone for a specimen tested under 2% of strain and 1% of strain, respectively, where this increase can be observed. FEA Results - Normal stress The results obtained for the normal stress on the x direction, between pin 1 and pin 2, for the different strain levels, are presented in Fig. 10, for both wires. The maximum compression stress (identified as Min, coloured in blue) occurs near the first actuator pin and is slightly greater than the maximum tension stress (identified as Max, coloured in red). The reason for this phenomenon is presumed to be the frictional contact friction between the pins and the wire, in order to mimic the real experiment. The maximum stress randomly floats through the section 2, appearing near the first pin, but without a fixed location. Being a “constant strain region” according to the theoretical deformation model, the floating of the maximum tension stress location can only be attributed numerical error. Therefore, it is expected that the wires will fracture near the pin 1 due to friction