Low Cycle Fatigue Evaluation of NiTi SESMA Thin Wires

Super elastic shape memory alloys (SESMA) have extensive damping capabilities because of the inherent hysteresis associated with their phase transformations. In most applications of SESMA, wires are often subject to large strain and cyclic loading at various frequencies. Cycling invariably leads to amnesia and accumulation of associated plastic strain. Therefore, degradation and fatigue life of large strain cycling (low cycle fatigue) have been important concerns. It is well known that the transformations depend on the ambient temperatures, rate of loading, amplitude of loading, which in turn determine the life of the material. This paper presents the low cycle fatigue (LCF) experimental studies of SESMA thin wires. The LCF life study of SMA wires is quite involved as these wires go through both solid-solid phase transitions involving latent heat as well as plastic deformations simultaneously during cycles of loading. The difficulty in introducing   a crack and monitoring the growth in thin SMA wires compounds the problem. An experimental setup with precise instruments is required to carry out the experiments and measure accurately the mechanical and thermal characteristics of SESMA during low cycle fatigue. Here in this study the stress-strain and the thermal signatures (using tensile testing machine and Infrared thermographic camera respectively) have been captured simultaneously during the testing. These mechanical and thermal signature responses are correlated to deduce vital phenomena such as the transformation front movement, transformation initiation zones and entropy changes during the exothermic and endothermic processes. One of the main objectives is to look for meaningful inferences that could be drawn from the subtle changes in the mechanical and thermal signatures during the evolution of cycling that can be used for modelling the fatigue behaviour. The details of the experimental results are presented.