It has been recognised for some time that Shape-Memory-Alloy (SMA) materials have a significant potential for long-stroke deployment actuators. SMA-based actuators would not suffer from end-of-deployment shocks, a common problem with spring-driven systems, while avoiding the complexity and cost associated with the use of electric motors. However, the number of applications of SMA-based actuators to the present day is still quite small. Potential users are interested in this technology, but need a deeper understanding of the thermo-mechanical behaviour of SMA's and how it might be exploited in the design of working actuators.
The broad aim of this report is to provide a user's guide to the design of SMA-based actuators for deployable structures.
Shape Memory behaviour is associated with a reversible thermoelastic transformation between high and low symmetry phases (austenite and martensite, respectively). Formation of martensite under uniaxial stress produces a preferred orientation (detwinned martensite), and is accompanied by strains of the order of 10%. Depending upon temperature, this strain can be recovered either by unloading or by heating. These transformations are best explained by means of a stress-temperature phase map. Of the many alloys that exhibit shape memory, the particular alloy considered in this report, NiTi, has the largest transformation strains.
Two thermo-mechanical tests are carried out in order to measure the key parameters that govern the behaviour of any SMA. These tests are the standard tension test, carried out at different constant temperature, and the thermal cycling test under different constant loads.
Two additional tests are carried out, the Differential Scanning Calorimeter test, which determines the phase transformation/temperature relation for a small, unstressed specimen of the material, and the restrained recovery test, where a length of the material is kept approximately constant while the temperature is varied.
The design process of a ``powered hinge'' actuator based on SMA elements is illustrated by means of two examples and the design procedure is validated by comparing the predictions to the measured performance of two model actuators.
The effects of temperature variation are also discussed.