Improvement of the Attainable Working Frequency of SMA Wires by Means of Active Cooling and Working Strain-Ratios

Abstract

Shape Memory Alloys (SMAs) are one of the most widely used smart materials. Their attractive characteristics make them interesting for the development of novel devices. Most of these devices make use of their shape memory effect or superelasticity; these effects are based on diffusionless phase transitions. SMAs can be used in different forms. In particular, wires are often used for actuation purposes because they exert linear forces and large strokes, up to the 8% of their original (memorized) length. However, they work at very low frequencies. SMAs are thermally activated. The limitation on their actuation bandwidth is a consequence of their capability to increase and decrease their temperature. The most common way of heating SMA wires is by Joule heating. Afterwards, they normally cool down by releasing thermal energy to their surroundings by conduction or convection. The heating and the cooling come from different physic’s principles and the cooling is a slower process than the heating one. Therefore, the cooling of SMAs is the main concern regarding to the SMA wire’s maximum attainable working frequency. In this paper, the effect of different applied heating and cooling rates on the resulting SMA wire’s working frequency is studied. Different heating rates have been applied to an SMA wire by applying different levels of electrical power. In a similar manner, different cooling rates have been applied to the wire by applying different forced airflows around the SMA wire. The use of a forced airflow has shown to increase the convective heat transfer coefficient between the wire and the surrounding air up to eight-fold. SMA wires’ working frequency depends on the amplitude of strains at which it performs. The higher the amplitude, the lower the maximum attainable frequency. Moreover, the relationship between the temperature and the strain is highly non-linear. For that reason, the range of strains in which the wire works has also an effect on the resulting attainable frequency for a given working amplitude. This is of great interest for SMA actuators since a similar working amplitude can result in very different attainable frequencies, depending on the range of temperatures within which the wire performs. Experimental results show that the SMA wire’s working frequency can be increased up to threefold by making it work within the appropriate temperature range. Afterwards, this improvement is reflected on the overall performance of the actuator in which the SMA wire is embedded.