Thermomechanical modelling of microstructurally graded shape memory alloys

Abstract

For better controllability in actuation applications, it is desirable to create functionally graded shape memory alloys in the actuation direction. This is achieved by applying designed heat treatment gradient along the length of a shape memory alloy wire, creating transformation stress and strain gradients. This study presents analytical solutions to predict the deformation behaviour of such functionally graded shape memory alloy wires. General polynomials are used to describe the transformation stress and strain variations with respect to the length variable. Closed-form solutions are derived for nominal stress–strain variations that are closely validated by experimental data for shape memory effect and pseudoelastic behaviour of NiTi wires. These materials exhibit distinctive inclined stress plateaus with positive slopes, corresponding to the property gradient within the sample. The average slope of the stress plateau is found to increase with increasing temperature range of the gradient heat treatment.