A comparison of the DSC measurements of shape memory alloys and the material's thermal characteristics in a large scale actuator

Abstract

An accurate measure of a Shape Memory Alloy's (SMA) transition temperatures is necessary for the development of successful SMA actuator designs. Differential Scanning Calorimetry (DSC) is used to obtain SMA transition temperatures associated with changes in alloy formulations, fabrication processes, and forming methods, and to predict an SMA's thermal characteristics when designed into an actuator. However there is little data directly correlating a material's DSC results with its performance in an actuator configuration, particularly for large-scale actuators producing high force and large displacements. In this paper the authors compare the DSC results of several NiTinol samples with the thermal performance of the same material in a rotary actuator. Data are presented for NiTinol torque tubes 14cm (5.5 in) long by 1 cm (0.4 in) in diameter. The tubes were tested over a range of loads exceeding 17 N*m (150 in-lbs) of torque, with angular displacements of more than 60 degrees, and for durations exceeding 3,500 thermal cycles. Data from various NiTinol suppliers, levels of cold work, and a range of aging temperatures is presented. The DSC data is directly compared to the strain vs. temperature hysteresis curves of the same material under various loads; both before and after extended cycling. The value of the DSC measurements as a predictor of a material's thermal characteristics in an actuator configuration is assessed.