Abstract
Typical shape memory alloy actuators provide a unique combination of large stresses and strains that result in work-per-volume larger by more than two orders of magnitude than all other actuation methods that are based on active materials. High-rate actuation of shape memory alloys can provide improved energy efficiency, and shorter response and total actuation times, along with large travel-per-wire-length, with respect to slow-rate SMA applications. In this article, we review the different aspects of high-rate actuation of shape memory alloy wires in the high-driving-force regime. We briefly survey previous experimental results about the kinetics and thermodynamics of the phase transformation in view of its practical implications. New experimental results, regarding energy efficiency, total actuation time, repeatability, and fatigue, are presented and discussed. The paper provides general design guidelines for obtaining high actuator performance, as well as guidelines for selecting the source of the electric pulse and its parameters. Finally, we construct and solve detailed simulations of actuator response that can serve as accurate design tools.
Highlights
Actuators 2021, 10, 140. https://Shape memory alloys (SMA) are smart materials that undergo a phase transformation between two solid phases, martensite and austenite [1]
In accordance with Equation (2), during the phase transformation the temperature of the SMA wire should be much higher than the stress-free transition temperatures. (Effects of fatigue and durability of the wire under high levels of stress and temperature are discussed in Section 6.) The desired levels of stress and temperature in the SMA wire are determined by a compromise between energy efficiency and actuator durability
The kinetics and thermodynamics of the high-rate phase transformation under an abrupt heat pulse have been studied using the clamp-clamp mechanical configuration in which the wire was clamped at both ends [34,38]
Summary
Shape memory alloys (SMA) are smart materials that undergo a phase transformation between two solid phases, martensite and austenite [1]. Heating/cooling of an SMA element beyond the transformation temperature yields large and reversible strains that can be employed in actuator mechanisms. The martensitic and reverse martensitic phase transformations are diffusionless firstorder phase transformations between a high-temperature austenite phase and a lowtemperature martensite phase. SMA exhibit strong nonlinear thermo-mechanical behaviors associated with abrupt changes in their lattice structure. The crystallographic symmetry of the austenite is higher than that of the martensite, leading to a variety of symmetry-related degenerate states in the martensite phase called variants [2]. Local domains within the martensite phase that are oriented at different variants are termed twins and the interface between them is termed twin boundary
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