Mechanical characterization of NiTi SMA wires using a dynamic mechanical analyzer

An investigation into the thermo-mechanical response of trained Ni–Ti shape memory alloy (SMA) wires with two-way shape memory effect (TWSME) induced into them was conducted in order to examine the influence of the cold work on quasi-static and fatigue behavior of SMAs. Use of dynamic mechanical analyzer (DMA) for such tests on thin wires was examined and successfully utilized. Quasi-static stress–strain responses of the wires at different temperatures were obtained to determine the critical stresses for forward as well as reverse transformations. Mechanical stress-controlled fatigue tests were conducted on the wires in the austenitic state (130°C). They showed a two-stage deterioration of fatigue life—a rapid decay when the maximum stress of the fatigue cycle is higher than the critical transformation stress, and a more gradual deterioration at stress levels considerably lower than the critical stress. The fatigue life of the wires was found to increase with the load ratio, R, whereas the frequency of cyclic loading had only a marginal effect. Differential scanning calorimetry (DSC) of the fatigued austenite specimens indicated the presence of residual stress-induced martensite in the austenitic state. The high concentration of the austenite–martensite interfaces act as potential sites for stress-concentration and are the main source of the drastic decrease of the fatigue life in the two-phase region.