ABSTRACT
Shape memory alloys (SMAs) provide actuation via the shape memory effect (SME), which is due to a phase transformation between austenitic and martensitic crystalline structures. The alloy assumes a martensitic crystalline structure at low temperatures, and once heated to a transition temperature, the crystalline structure transforms to austenite. As the austenitic structure is more compact than the martensite, the material contracts (up to several percent strain) during the trans formation, and thus provides actuation. The alloy can be reset to its expanded martensitic form by cooling to a temperature below the transition temperature while also exerting a tensile load. The extent to which the alloy w i l l regain its original form depends upon the amount o f tensile load imposed while cooling. Also, the transformation between crystal states is not instantaneous, but rather occurs across a temperature band. The transformation can thus be controlled pro portionally so the actuator can be utilized in a proportional manner.
