Thermomechanical Modeling of a Wet Shape Memory Alloy Actuator

Abstract

This paper presents combined thermal and mechanical models of a wet shape memory alloy (SMA) wire actuator. The actuator consists of a SMA wire suspended concentrically in a compliant tube. Actuation occurs as hot and cold water are alternately pumped through the tube to contract and extend the wire, respectively. Although other constitutive models of the behavior of SMA's exist, they generally assume uniform temperature change throughout the SMA actuator. The thermomechanical model presented in this paper accounts for the non-uniform temperature change of the SMA wire due to alternating the temperature of the flow along the wire. The thermal model consists of analysis of the heat transfer between the fluid and the SMA wire. Heat loss to the environment and the temperature change of the fluid through the actuator are taken into account. Based on this analysis the temperature of the wire at segments along its length can be determined as a function of time. The mechanical model approximates the strain-martensite fraction and martensite fraction-temperature relationships. By combining the thermal and mechanical models the strain of the wire can be determined as a function of time. The combined thermomechanical model will be used to model applications in which a wet SMA actuator is desired.