STATIC AND DYNAMIC ANALYSIS OF A SHAPE-MORPHING, THIN-SHELL CYLINDER UNDER ACTUATIONS OF SHAPE-MEMORY ALLOY

Shape-morphing turbofan bypass nozzles, incorporating shape-memory alloy actuators, have been studied in terms of engine performance and fuel economy in recent years. This study uses such a nozzle to investigate the possibility of reducing vibrations by morphing the shape of the structure, thereby moving structural resonances away from tonal excitation frequencies. Accordingly, the principal objective was to develop a cylindrical morphing nozzle with reliable working performance and effective vibration-reducing capability. The morphing behaviour of the nozzle was simulated using finite element analysis and verified by physical experiments. The initial results revealed that the manufacturing precision of the cylinder and actuators dramatically affected the static deformation. A second morphing nozzle was built under more strictly controlled tolerances, with actuator dimensional accuracy being improved using a specially designed clamp. Improved correlation between simulation and experiment was achieved using this nozzle. The design process revealed the importance of actuator stiffness and development of fatigue within the actuator material. The investigations showed that several heating/cooling cycles tended to stabilise performance and therefore these were incorporated as a pre-test process.
The simulation work carried out on the second nozzle showed that the natural frequencies of the first six modes achieved shifts from 4.1% to 0.3% when the actuators were activated providing an adequate range to allow resonance-avoidance. As morphing altered mode shapes, the use of individual frequency response functions was considered an unreliable measure of overall vibration suppression. Instead, the measure used was the kinetic energy frequency response of the structure. Over the frequency range spanning the first six vibration modes of the structure, it was shown that kinetic energy levels of the cylinder could be reduced by more than 97% if the excitation frequency remains stationary at the at the frequency of an initial resonance. For scenarios where the excitation ramps up or down, the morphing cylinder was able to reduce time “on resonance” by activating or deactivating the actuators at appropriate times. This novel approach offers considerable benefits in the reduction of noise and vibration-induced fatigue.