Light-Weight, Fast-Cycling, Shape-Memory Actuation Structures

While bulk shape memory alloys (SMAs) have proven a successful means for creating adaptive aerospace structures in many demonstrations, including live flight tests, the time required to cool such actuators has been identified as a property that could inhibit their commercial implementation in some circumstances. To determine best practices for improving cooling times, several approaches to increase the surface area and reduce the mass of existing bulk actuator technologies have been examined. Specifically, geometries created using traditional milling and EDM techniques were compared with micro-channel geometries made possible by a new electrochemical milling process developed at Northwestern. The latter technique involves imbedding steel space-holders in a matrix of NiTi powders, hot isostatic pressing the preform into a dense composite, and then electro-chemically dissolving the steel. Thus, in a two-step process, it is possible to create an actuation structure with numerous micro-channels with excellent control of geometry, shape, size and placement, to reduce weight and increase surface area (and thus decrease response time) without compromising actuator performance. In this paper, the new, lighter-weight, faster cycling shape-memory alloy actuation structures resulting from each technique are reviewed. Their performances are compared and contrasted through the results of a numerical study conducted with a 3D SMA constitutive law developed specifically to handle the complex, non-proportional loadings that arise in porous structures. It is shown that using micro-channel technology, cooling times are significantly reduced relative to traditional machining techniques for the same amount of mass reduction.