Thermomechanical hysteresis and cyclic testing of resistance controlled heated NiTi SMA wires for low force and high cycle actuator applications

Every single day, billions of electronic actuators provide mechanical work in embedded systems for a wide range of technical applications all over the world. Beside the commonly used servo motors, solenoids, piezo elements or other conventional technologies shape memory alloys (SMA) are increasingly considered to be a serious alternative for future actuator applications. SMA can support engineers in the development of more lightweight, more energy- and cost-efficient solutions and therefore they are highly appreciated if they are able to fit application requirement. Unfortunately, for the majority of applications structural fatigue or too low martensite finish temperatures constrain SMA for different reasons to enter the markets. The current work presents thermomechanical hysteresis data and experimental results of 75µm thin NiTi SMA wires under cyclic loading conditions. The preconditioned and electromechanically crimped wires were activated by custom made electronic resistance-heating controllers while bias springs were applied to mechanically restore the former shape of the wires by detwinning of martensite during cooling. For the first time hysteresis data is presented that show martensite finish temperatures of approximately 85°C under loaded conditions for binary and pretrained SMA wires. In cyclic testing experiments more than 10^6 cycles were achieved while the actuator stroke was set to 4%. An experimental characterization regime for SMA wire actuator evaluation is presented and underlines the enhanced properties of these improved SMA materials in comparison to the state-of-the-art. New opportunities arise to establish NiTi SMA even for applications that didn’t work yet due to the aforementioned constraints.