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. (C) 2008 Elsevier B.V. All rights reserved

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

Fracture of thermally activated NiTi shape memory alloy wires

This paper presents a study on the fracture of NiTi SMA wire undergoing thermal cycling, activated through resistive heating, under a constant load referred to as thermo-mechanical cycling (TMC). In general, TMC results in initiation of a large number of surface cracks on the SMA wire perpendicular to the wire axis. After initiation, these cracks propagate simultaneously inside the material under cyclic strain. But, the important observation in this study is the generation of cracks in the bulk of the material during TMC, preferentially at the core region of the wire. It has been shown that generation of cracks at the core region is due to strain inhomogeneity during TMC as a consequence of variation in the volume fraction of austenite and martensite phases across the wire cross section. This strain inhomogeneity is attributed to temperature and stress gradients that can exist across the cross section of the wire during thermal cycling. Study shows that nucleation of cracks and their growth at the core region of the wire plays an important role in the fracture of thermally activated SMA wire