Hystoelastic deformation behaviour of geometrically graded NiTi shape memory alloys

Abstract

This paper introduces an idea of geometrically grading NiTi structures to obtain a stable deformation response to loading from NiTi which is originally unstable during stress-induced martensitic transformations. Three types of sample geometry of 2D NiTi structures with linear and quadratic gradients of width are used for tensile experimentation. The graded NiTi samples exhibit widened stress window with distinctive positive stress gradient over forward and reverse martensitic transformations in the alloy. The slope and curvature of the stress gradient can be adjusted by sample geometry design. The average stress–strain slope, the stress window and the strain over stress-induced transformation increase as the width ratio deviates from unity. The deformation behaviour of such components under tension is simulated using elastohysteresis model and finite element method. The constitutive model is based on decomposition of the alloy response into hyperelastic and hysteresis contributions, which describe the main reversible and irreversible components of the deformation, respectively. The numerical results are validated with experimental data.