Evolution of hysteresis characteristic of shape memory alloys at incomplete phase transformation cyclic loading

Abstract

The phase transformation ratchetting of shape memory alloys (SMAs) at incomplete phase transformation cyclic loading is experimentally and theoretically investigated. To this end, two different kinds of incomplete phase transformation cyclic loading tests on NiTi wires are performed, i.e. incomplete transformation cyclic loads are respectively applied at the stages of forward martensitic transformation and reverse martensitic transformation. When the cyclic load of incomplete transformation is applied in the forward martensitic transformation stage, a novel phenomenon is discovered: although there is no greater stress to drive the anstenite turn to martensite, the SMAs can still gradually undergo martensitic transformation and accumulation until martensite reaches saturation. The hysteretic behavior finally reaches a shakedown state where the strain–stress curve no longer changes with the number of cycles. When the cyclic load of incomplete transformation is applied in the reverse martensitic transformation stage, a similar phenomenon is obtained. According to the analysis of the temperature evolution during the deformation process of the SMAs, combined with the relationship between the phase transformation yield stress and the temperature of SMAs, the experimental results are reasonably explained. This research is of great significance for a more comprehensive grasp of the mechanical behavior of SMAs.