Measurement of Inhomogeneous Deformation Behavior Arising in Shape Memory Alloy

Shape memory alloy (SMA) has been paid attention as smart material revealing shape recovery by heating. Many researchers have investigated its basic properties and applications. However, many problems have existed in usage of SMA such as change of properties (transformation temperature, Young's modulus) by process of fabrication, shape memory treatment, condition of phase and so on. Therefore, it has been pointing in recent years that expressing these phenomena using conventional constitutive relation of SMA has been difficult. In the present study, phenomena needed to construct novel constitutive relation of SMA, such as nucleation and propagation of phase transformation and so on, were measured using digital image correlation method. Here, tensile loading and unloading tests were performed for NiTi alloy plate in two temperature regions where the SMA shows the shape memory effect (26°C) and pseudoelasticity (60°C). The obtained results were as follows; (1) The propagation of the phase transformation during the stress-induced martensitic transformation and austenite transformation progressed from the both ends to the center of the specimen. (2) Staying phenomena of deformation was confirmed at nucleation point of phase transformation. Deformation behavior of SMA can be classified broadly into three categories, such as “Nucleation of phase transformation”, “Process between nucleation and propagation” and “Propagation of phase transformation”. (3) The transformation had a fixed angle along the longitudinal direction of the specimen, propagating from the both ends to the center of the specimen.