Composites with Controllable Stiffness

Abstract

High performance carbon fibre reinforced composites with controllable stiffness could revolutionise the use of composite materials in structural applications. Described here are structural materials, the stiffness of which can be controlled on demand. Such materials could have applications in morphing wings or deployable structures. A significant challenge in the field of morphing aerostructures is the development of a stiff skin material that can withstand aerodynamic loads but that can be readily deformed with acceptable actuation forces. To overcome this, two designs of carbon fibre reinforced epoxy composites with controllable stiffness have been developed that contain either thermo-responsive interphases or interleaf layers. At elevated temperatures they undergo large reductions in flexural stiffness and fully recover when cooled, with no discernible damage or loss in properties. These composites, when optimised, could therefore be used as skin materials in morphing wings. In the first design, composites containing thermo-responsive interphases were manufactured by continuously coating polystyrene onto carbon fibres followed by resin infusion with an epoxy resin. Dynamic mechanical thermal analysis and flexural tests showed that reductions in stiffness of up to 30 % could be achieved when the composites were heated above the Tg of the interphase. Composites containing polyacrylamide interphases were also analysed using DMTA. These composites underwent reductions in storage modulus of up to 79 % when the polyacrylamide interphase was partially hydrated and heated above its Tg. In an alternative approach, interleaved laminates containing plies of polystyrene between carbon fibre reinforced laminae were also manufactured. When heated to 120 °C, above the Tg of polystyrene, these composites exhibited up to 98 % loss in flexural stiffness and could undergo large deformations. The process was reversible as the composites could be returned to their original configuration and their flexural stiffness restored to their full values with no damage observed.