Probabilistic analysis for the functional and structural fatigue of NiTi wires

The present work aims to investigate the functional and structural fatigue of Ni_49·8Ti_50.2 (at.%) shape memory alloy (SMA) wires subjected to thermal cycling under a constant stress. More than 70 specimens were tested under 7 stress levels ranging from 140 MPa to 200 MPa. Probabilistic analysis of the fatigue life were performed, including analysis of final plastic strain and recovery strain, the evolution of the plastic strain and recovery strain through the whole life span. Corresponding probabilistic curves under the reliability of 0.5, 0.9, 0.99, and 0.999 were given. Moreover, the influence of the stress level on fatigue life, plastic and recovery strains were discussed. Finally, fracture surface analysis was also performed to determine the effect of stress level on behavior of the crack nucleation and propagation.