Unnotched fatigue behavior of an austenitic Ni-Ti shape memory alloy

Constant stress amplitude fatigue life of an austenitic Ni (55.88 wt.%)-Ti shape memory alloy (SMA) within the stress amplitude range of 180-450 MPa was evaluated. The stress-strain hysteresis loops were monitored throughout the fatigue loading. They reveal that with the increasing number of fatigue cycles, the critical stress required for the stress-induced martensitic transformation, width of the hysteresis loop, recoverable and frictional energies of each cycle, all decrease while accumulated plastic strain increases. Post-mortem characterization of the fatigued specimens by employing differential scanning calorimetry (DSC), X-ray diffraction (XRD), and fractography were carried out, in order to understand the fatigue micromechanisms. Results indicate that the progressive accumulation of stress-induced martensite in the alloy is the source for the fatigue failure. Implications of these observations are discussed within the context of fatigue performance of SMAs and other materials that undergo stress-induced transformations.