Interaction between structural instability and functional fatigue in SMAs: phenomenological modeling of the return-point memory during subloop deformation

Abstract

Localization of the stress-induced martensitic phase transformation plays an important role in the fatigue behavior of shape memory alloys (SMAs). The phenomenon of return-point memory that is observed during the subloop deformation of a partially-transformed SMA is a clear manifestation of the interaction between localized transformation and degradation of the functional properties. The goal of the present study is to demonstrate this structure--material interaction through modeling the phenomenon of return-point memory. A gradient-enhanced model of functional fatigue is developed for this purpose. The model is first employed to reproduce the hierarchical return-point memory in a pseudoelastic NiTi wire under isothermal uniaxial tension with nested subloops. Additionally, a more detailed analysis is carried out for a NiTi strip with more complex transformation pattern. Our study highlights on the subtle morphological changes of the phase transformation under different loading scenarios and resulting implications on the cyclic degradation and return-point memory.