Experimental Exploration of Post Constrained Recovery Mechanics of NiTi

Shape memory alloys (SMAs) have tremendous potential use as actuators in mechanical systems due to their high specific energy density. Large recovery stresses can be generated when Nickel Titanium (NiTi), the most widely used SMA, undergoes constrained recovery where it is held in a deformed geometry and heated from a detwinned martensite phase to austenite phase. Recent experimental results have found that residual stresses can also be generated in NiTi after returning to a low temperature geometrically constrained state. This paper presents experimental results performed on NiTi wire samples where wire was: 1) deformed from a low temperature twinned martensite state to produce a strain that would be recoverable in an unloaded state 2) held at that strain state and heated above the austenite finish transition temperature and then cooled back below the martensite transition finish temperature while recording the forces generated. It was found that a residual load was produced in the low temperature state. Results from further testing beyond this point showed repeatability with application of small and large strains. Post constrained recovery stresses have the potential to be used to generate residual stresses in structures in a low energy, un-actuated state with a remaining potential for thermal actuation.