High-performance shape memory composites with intrinsic heating capabilities

Abstract

Shape morphing structures have played a significant role within the field of aerospace for more than a century. While the shape morphing aerostructures of the past and present have depended on hinges and motors to achieve morphing, their future is expected to rely on smart materials and structures that have intrinsic shape morphing capabilities. One such smart material, that has previously been developed at Imperial College London, is the carbon fibre reinforced epoxy polymer (CFRP) composite with thermoplastic (TP) interleaves. These interleaved composites exhibit controllable stiffness (CS) and shape memory (SM) capabilities under suitable thermal conditions. While these interleaved composites showed excellent shape morphing capabilities, they had several drawbacks. These composites showed poor flexural modulus and through-thickness shear strength compared to the epoxy-based non-interleaved CFRP. These composites also used an oven to achieve the high temperatures required to exhibit the CS and SM capabilities. This thesis describes studies conducted to mitigate these drawbacks. In the first study described in this thesis, the source of the premature through-thickness shear failure in TP interleaved CFRP composites was discovered to be the low shear strength of the polystyrene (PS) interleaves used in previous works. It was then demonstrated that replacing PS with Poly(styrene-co-acrylonitrile) (SAN) could improve the through-thickness shear strength of the interleaved composites to be almost as high as that of pristine CFRP. Furthermore, the SAN-interleaved CFRP laminates also exhibited excellent CS and SM capabilities. In the next study described in this thesis, it was demonstrated that the flexural modulus of TP interleaved CFRP composites can be substantially improved by two different methods- (i) reducing the thickness of the TP interleaves, and (ii) introducing reinforcements within the TP interleaves. The following study describes how intrinsic heating capability was achieved in TP interleaved CFRP composites, through resistive heating of heater elements such as stainless steel (SS) meshes and woven carbon fabric (WCF) embedded within the layup of the composite. This intrinsic heating strategy was used to supply the temperature necessary for the corresponding composites to exhibit CS and SM capabilities. As a result, these intrinsically heated TP interleaved CFRP composites exhibited successful out-of-oven morphing capabilities. In the final study described in this thesis, composite structures that were initially flat in their as-cured state, but were capable of deployment into planar and curved meshes were designed. Finite element numerical models were used to predict the deployment capabilities of these composite structures. Finally, the deployable composite mesh structures were manufactured and characterised.