Development of a Shape Memory Material Actuator for Adaptive Truss Applications

Abstract

A shape memory material actuator is being developed to provide active vibration and shape control for large, adaptive space structures. Shape memory alloys have the ability to generate high recovery stresses (>700 MPa) over large strains (> 6 % ), providing potential payoffs for their use in lightweight actuator designs and adaptive com posite structures. However, the utility of these alloys has been limited in past programs due to lack of adequate material characterization studies to optimize mechanism or composite performance, lack of material property stability during transformational cycling, and the dependence on heat transfer rates for rapid cycling capability. In this study, Cory and McNichols' theory of nonequilibrium thermostatics (NET) was applied to quantify and correlate shape memory material behavior for a binary NiTi alloy. NET was then em ployed as a design tool to develop an actuator, utilizing 24 0.5 mm dia. wires (280 mm long) acting against a biasing spring (with stiffness of 305 N/mm), with nominal opera tional stroke of ± 3 mm and force of ± 1000 N. NET state equation parameters were mea sured after stabilizing the NiTi wire pack properties using isothermal transformational cy cling. Isotonic testing in an oilbath was then used to correlate actuator performance with the NET predictions over the design range of force, length, and temperature. Finally, elec trical resistance heating was employed using closed-loop feedback control to assess actua tor response time and mechanical performance in both ambient and vacuum environments. The design methodology employed in this program should provide a viable approach for optimizing the performance of a shape memory actuator specific to its application.