Characterization of shape memory alloy micro-springs for application in morphing wings

The incessant quest for more efficient aircraft has driven research into the development of aircraft with morphing wings. This concept aims to increase the flight efficiency of the aircraft by adapting the wing to the present air flow condition. This technology still has some limitations, among them the control mechanism of the shape adaptation mechanism. Some researchers have proposed the application of intelligent materials as alternative to conventional drive systems. Particularly shape memory alloys (SMAs) have been investigated due to their ability to recover large deformations if subjected to a suitable temperature (simple drive mechanism by Joule heating). This work presents a thermomechanical characterization of NiTi SMA micro-springs for application in morphing wings. The actuators work in a regime where the shape memory effect is created by heating of the stress-induced martensite phase, a phenomenon little explored in the literature. An adaptive wing prototype was constructed; the variation in camber deflection is the response of actuator heating by the Joule effect. A 3D wing prototype was tested in a wind tunnel under different air flows, showing the suitability of the actuators for this application. Next, the behavior of these spring actuators when subjected to force and strain under realistic conditions is discussed based on the phase transformation temperatures and the actuator response to heating. These results are fundamental for choosing the actuator size for a particular application and for the parameterization of controllers.