Analysis and optimization of improved hybrid SMA flexures for high rate actuation

The utilization of shape memory alloys (SMAs) as actuators in aerospace applications continues to show promise. These materials, when subjected to controlled changes in temperature, have the capability to provide motion while under loads that exceed thousands of times their own weight and can do so over tens of thousands of cycles. However, the rate of thermally-induced SMA transformation is significantly hindered by low thermal conductivity and latent heat effects observed in this material. The relatively long cooling times observed in SMA geometries such as beams and tubes make it difficult for controlled devices to operate with sufficiently high frequency. Therefore, the application of SMA beams as aerospace control actuators has been limited. Morphing structures such as flight control mechanisms require higher cyclic actuation frequencies than are commonly observed in SMAs, and thus have motivated the effort to increase thermal actuation rates attainable in SMA active components. This work presents an analytical study of a tapered beam actuator and discusses the possibility of using SMAs in conjunction with more conductive materials to enhance actuation performance, especially with regard to actuation cyclic frequency. The analysis involves computing the actuation work output over time of various loaded, thermally cycled active SMA beams using an accurate constitutive model implemented in a finite element framework. This set of analyses considers the solution to a transient thermomechanically coupled problem and includes the effects of latent heat of transformation on the energy balance. The study compares the effectiveness of aluminum, copper, and silver secondary material regions and their geometric configurations in altering the actuation power-to-mass ratio of the beam. An optimization scheme is used to determine the geometric distribution of each secondary material that results in the highest power-to-mass ratio. It is shown that aluminum, when optimally distributed, provides the best overall design solution of the three materials considered.