Shape memory alloy (SMA) actuators are becoming increasingly popular in recent years due to their properties such as large recovery strain, silent actuation and low weight. Actuation in SMA wires depends strongly on temperature which is difficult to measure directly. Therefore, a reliable model is required to predict wire temperature, in order to control the transformation, and hence the actuation, and to avoid potential degradation due to overheating. The purpose of this investigation is to develop resistance and natural convection heat transfer models to predict temperature of current-carrying SMA wires using indirect temperature measurement methods.
Experiments are performed on electrically heated 0.5 mm diameter NiTi SMA wire during phase transformation. Convection heat transfer experiments are performed in an environment of air that allows for control of the ambient pressure and in turn the thermofluid properties, such as density and viscosity. By measuring convective heat loss at a range of pressures, an empirical natural convection heat transfer correlation is determined for inclination angles from horizontal to vertical, in the Rayleigh number range of 2.6 × 10−8 ≤ RaD ≤ 6.0 × 10−1. Later, effect of temperature changes on electrical resistance and other control parameters such as applied external stress, wire inclination angle, wire length and ambient pressure is investigated. Based on experimental results a resistance model is developed for SMA wires that combined with the heat transfer correlation previously derived can be used to predict temperature and natural convection heat transfer coefficient of NiTi SMA wires during phase transformation for different wire lengths and inclination angles under various applied external stresses.