Investigations into sample geometry effects on the superelastic and fatigue behavior of Nitinol: Modeling and experiments

The sample geometry by way of its influence on stress distribution is expected to control the superelastic and fatigue behavior of Nitinol. A variety of sample geometries are reported in literature for studying the fatigue behavior of Nitinol and it is unclear which geometry is the most suitable. To establish this, we conducted finite element simulations on the different geometries employing the ANSYS software package. Guided by the simulation predictions, samples bearing two different geometries, g_1 and g_5, were fabricated from Nitinol sheetswhich were apriori heat treated at 300 degrees C. The heat treated Nitinol was characterized by optical microscopy and differential scanning calorimetry to reveal the grain structure and transformation temperatures. Tensile samples of this material were subjected to cyclic uniaxial tensile tests where the maximum stress was increased gradually from 600 to 800 MPa in steps of 100 MPa every 10 cycles. The residual strain at the end of every 10th cycle was noted and found to be higher for g_1 compared to g_5. X-ray diffraction investigations on the cycled samples indicated a higher residual martensite phase in g_1 compared to g_5. The higher residual martensite is considered to be the cause of the poorer fatigue life of samples bearing geometry g_1 compared to that of g_5. Scanning electron microscope studies validated the fatigue measurements as the g_1 fractographs demonstrated higher void fraction vis-a-vis g_5.