Effects of structural geometry on the localized deformation of superelastic NiTi sheets

Abstract

Due to unique reversible martensitic transformation, superelastic NiTi can be strained to several percents and fully recover upon unloading. It is well accepted that superelastic NiTi under primarily tensile deformation is localized rather than homogenous. In this paper, we examine the heterogeneous deformation of superelastic NiTi sheets with various aspect ratios (the ratio of gauge width to gauge length). For the specimens with aspect ratios lower than 2, the deformation at quasi-static loading condition is realized by the nucleation and propagation of a major transformation band. For the specimen with aspect ratio of 4, multiple transformation bands coexist at the loading rate as low as 1 × 10−4/s, and the geometry of the transformation band is no longer well-defined. Finite element analysis indicates that the transition of localized deformation mode is attributable to the enhanced stress biaxiality and stress heterogeneity with the increasing aspect ratio.