A study on the contraction and cooling times of actively cooled shape memory alloy wires

Shape memory alloys (SMAs) are thermally activated and show a strong thermomechanical coupling (i.e. the relationship between strain, stress and temperature). SMA wires are commonly used to develop SMA-based actuators. The most important characteristics of SMA wires is their capability to exert linear forces with strokes up to 8% of their original length. This make SMAs an attractive smart material for actuation purposes. However, the main weakness of SMAs is their limitation on actuation bandwidth. This limitation comes from the rate at which the SMA wires are able to heat up and cool down, releasing heat energy to the surroundings. This paper focuses on the effects that different heating and cooling rates have on the SMA wire's working rate that can be attained. An SMA wire has been experimentally tested under different conditions of heating, cooling and applied load to find the influence of these parameters on the contraction and cooling times of an SMA wire and, therefore, on its attainable frequency. In addition, a model for SMAs has been implemented in a finite element analysis software and the experiments have also been simulated, which has been used to corroborate the conclusions drawn from the experiments.