Precision instrument for characterizing shape memory alloy wires in bias spring actuation

Some metallic alloys such as Nitinol (NiTi) exhibit the shape memory effect, which is suitable for generating force and displacement when the alloy changes phase during a heating and cooling cycle. These shape memory alloys are often formed into one-dimensional wires, tubes, and ribbons that are preloaded by bias springs to create inexpensive actuators for electromechanical devices. This article describes a new instrument for measuring the quasistatic characteristics of the alloy and the transient performance of bias-spring actuators when resistively heated and convectively cooled. The instrument achieves more accurate measurements by eliminating rolling friction and by sensing force and displacement in line with the bias spring and shape memory alloy wire. Data from the instrument enables calculation of stress and strain at constant temperatures and during actuation cycles.