A fundamental investigation into large strain recovery of one-way shape memory alloy wires embedded in flexible polyurethanes

Abstract

Shape memory alloys (SMAs) are being embedded in or externally attached to smart structures because of the large amount of actuation deformation and force that these materials are capable of producing when they are heated. Previous investigations have focused primarily on using single or opposing SMA wires exhibiting the two-way shape memory effect (SME) because of the simplicity with which the repeatable actuation behavior of the structure can be predicted. This repeatable actuation behavior is achieved at the expense of reduced levels of recoverable deformation. Alternatively, many potential smart structure applications will employ multiple SMA wires exhibiting a permanent one-way SME to simplify fabrication and increase the recoverable strains in the structure. To employ the one-way wires, it is necessary to investigate how they affect the recovery of large strains when they are embedded in a structure. In this investigation, the large strain recovery of a one-way SMA wire embedded in a flexible polyurethane is characterized using the novel deformation measurement technique known as digital image correlation. These results are compared with a simple actuation model and a three-dimensional finite element analysis of the structure using the Brinson model for describing the thermomechanical behavior of the SMA. Results indicate that the level of actuation strain in the structure is substantially reduced by the inelastic behavior of the one-way SMA wires, and there are significant differences between the deformations of the matrix material adjacent to the SMA wires and in the region surrounding it. The transformation behavior of the SMA wires was also determined to be volume preserving, which had a significant effect on the transverse strain fields.