Modelling and experimental investigation of geometrically graded shape memory alloys with parallel design configuration

The shape memory effect and pseudoelastic effect of NiTi shape memory alloys occur within respectively a narrow temperature and stress window associated with its martensitic transformation. This renders difficulty in accurate and reliable control for actuation applications using these properties. One approach to improving controllability of an actuation component is to design a geometrically graded shape memory structure to create a nonuniform transformation field within the structure. This paper presents analytical modelling and experimental evaluation of geometrically graded NiTi structures with parallel design configuration. Closed-form solutions are obtained to describe the stress-strain relationship of such structures under tensile loading conditions, which can be used as an engineering tool for optimizing shape memory performances of such components. The geometrically graded structures exhibited partial stress gradient over stress-induced transformation. A maximum stress window of 420 MPa was achieved over transformation stage, giving enlarged stress interval for shape memory actuation control.