Influence of structural architecture on linear shape memory alloy actuator performance and morphing system layout optimisation

In this work, a design strategy is presented, addressed to adaptive structural systems, driven by shape memory alloy actuators. The peculiar behaviour of shape memory alloy materials, non-linear and fully dependent on three parameters (stress, strain and temperature), and the load path itself complicate the numerical simulation process. This is even more evident if those active elements are integrated within complex structures. Actuators generate forces. Integrated shape memory alloy–based structural actuator capability is strongly influenced by the hosting structure stiffness. It does in turn depend on the geometrical configuration. The structural architecture may be then said to modulate the performance of the aforementioned devices. The layout modifies in fact the structural resistance that opposes the action of a generic shape memory alloy actuator, even if the same topological point is referred to. This opposition affects the change of phase process (martensite ↔ austenite) regulating shape memory alloy peculiar phenomena and then impacts its performance. For simple, linear shape memory alloy actuators, the structure may be represented as an oriented spring where all the information of the parent structure is concentrated in its scalar stiffness property. This value is a function of the specific layout. A way to compute this equivalent structural stiffness comes directly by its definition: the ratio between the generated actuation force and the derived homologue structural displacement, that is, displacement occurring along the same force direction. Because such stiffness is what the shape memory alloy actuator feels, in this article it is referred as ‘perceived stiffness’. In this article, the structural layout effect on linear shape memory alloy actuator performance is initially evaluated for a simple spring. The approach is then extended to a complex active structural system, element of an aircraft wing morphing architecture. The referred device is capable of deforming wing regions while resisting the aerodynamic and the structural loads and recovering the original shape, once the actuation stops. Structural actuator geometry is optimised as a function of the attained structural displacement (figure of merit). The work is concluded with a discussion on the achieved results, namely, rotation, vertical displacement, internal stress (strain) levels and activation temperature.