Nonuniform transformation behaviour of NiTi in a discrete geometrical gradient design

Shape memory alloys exhibit unique thermomechanical properties, e.g., the shape memory effect and the pseudoelasticity. By proper geometrical gradient design, these alloys can be made to exhibit different and more intricate thermomechanical behaviour to enable innovative applications. This paper reports the design of geometrically gradient NiTi and characterisation of its complex and nonuniform transformation and deformation fields. The study investigated two designs, with one using multiple pseudoelastic NiTi strips of different lengths in parallel arrangement to create a discrete geometrical gradient in the direction perpendicular to the loading axis and the other a tapered plate to give a continuous length gradient in the lateral direction for comparison with the former. The geometrically gradient structures exhibited partial stress gradient during stress-induced transformation. A maximum stress window of 520 MPa was achieved, giving an expanded stress interval for shape memory actuation control. Finite element modelling was applied to characterise the deformation behaviour of such structures under tensile loading and to reveal the complex propagation of the stress-induced transformation in such structures. © 2018