Tailoring the response time of shape memory alloy wires through active cooling and pre-stress

Application of shape memory alloy (SMA) actuators is limited to low frequencies due to slow cooling time especially in the embedded conditions where heat transfer rate is the controlling factor. In this study, we investigate various active cooling techniques and effect of pre-stress to improve the response time of two commercially available SMAs: Flexinol from Dynalloy Inc. and Biometal fiber from Toki Corporation. Flexinol and Biometal fiber of equal length and diameter were found to exhibit different actuation behavior under pre-stress. Time domain force response of SMA actuators was found to be dependent upon the applied pre-stress, heating rate, and amplitude of applied electrical stimulus. Compared to Biometal fibers, time domain response of Flexinol was found to decrease significantly with increasing pre-stress indicating the difference in transformation behavior. Fluid flow and heat sinking were found to be suitable methods for improving the response time by reducing the cooling cycle from 1.6 s to 0.30-0.45 s. This is a significant improvement in the actuation capability of SMAs.