Interleaved composites for controllable stiffness, shape memory and easy repair capabilities

Abstract

Thermoplastic interleaved carbon fibre/epoxy composites with controllable stiffness and shape memory capabilities have been developed in Imperial College London. Simple structures of this material have exhibited excellent re-shaping and shape recovery capabilities on heating. This thesis describes research exploring the properties and potential of these interleaved composites. Firstly, an investigation was presented which examined the polystyrene (PS)-interleaved carbon fibre/epoxy composites in three deployable configurations. These were an integrated hinge, an interleaved linear actuator in a sine wavy profile and an expanded composite mesh. Experimental results showed that specimens of each configuration exhibited good re-shaping and shape recovery performance and confirmed the potential of interleaved composites for deployable structure applications. The viscoelastic behaviour of the thermoplastic interleaf influences the stresses during re-shaping and the time required for shape recovery of these polystyrene interleaved composites. The viscoelastic behaviour of the interleaf material in shear and of interleaved composites in three-point flexure tests at elevated temperature was therefore investigated. Test results showed that the interleaved composites exhibited increasing stiffness when loaded at increasing crosshead speed. For specimens tested at higher crosshead speeds and then held at a fixed displacement, the load relaxed with time due to the stress relaxation of the thermoplastic interleaf. When unloaded, the specimens recovered towards the original shape. The shape recovery rate was highest at the beginning as the stored elastic stresses in the composites plies were released, but became slower as time progressed due to the viscoelastic behaviour of the thermoplastic interleaf. A FE investigation was conducted into how finite element (FE) analysis can be used to model the shape memory capability of interleaved composites. A FE model was developed of a PS-interleaved specimen in the shape of a circular arc which was to be re-shaped to a flat configuration in its low stiffness state at 120°C, cooled down to room temperature to return it to its high stiffness state and retain the flat shape and finally re-heated to 120°C to recover to the original shape. The ‘Restart’ facility in Abaqus was used to transfer data between the various stiffness states. Results showed that upon re-shaping, the bending stresses within composite plies were of right magnitude but were smaller than the predicted value from the simple beam theory. When cooled down, the FE predicted a 0.23 mm springback of the specimen upon removing the applied pressure. Finally, when re-heated, the specimen almost recovered to the original, circular arc configuration and the remaining bending stresses were small. Such a modelling approach could be applicable for the design of complex structures made of interleaved composites. The FE analysis was used to model the viscoelastic behaviour of polystyrene-interleaved composites subjected to three-point flexure tests at 120°C. Results predicted that the interleaved composites exhibited higher loads when loaded at higher crosshead speeds. When held at the 5.0 mm displacement, specimens of the higher crosshead loading speed cases relaxed in load with time towards the value in the low crosshead speed cases. Upon unloading, the specimens recovered rapidly at the beginning due to the release of stored elastic stresses in the composite plies, then gradually, more slowly towards the original shape, as the result of the viscoelastic behaviour of the PS interleaves. These predictions were consistent with the experiment results. Laminated composites are susceptible to interlaminar damage due to impact. Conventional composite repair methods are often expensive, time consuming and require skilled operators. In the final part of this thesis, a concept is proposed for using a suitably interleaved carbon fibre/epoxy composite to enable easy repair of interlaminar damage. In this concept, the laminate is interleaved with a thermally bondable polymer with suitable mechanical properties so that damage due to low energy impacts will occur preferentially within the interleaves, i.e. the interleaf acts as a weak link. The damage could then be healed by applying heat and pressure to return the laminate to its pristine state. To demonstrate the concept, two interleaved carbon fibre/epoxy composites, one interleaved with polylactide (PLA) and one with polycarbonate (PC), were subjected to three-point flexure tests and static indentation tests. It was showed that interlaminar damage occurred at the interleaf locations and this could be repaired by heating the laminate whilst applying external pressure. The shear strength, as measured in the flexure tests, showed a 66% overall recovery in the PLA-interleaved specimens after two damage-repair cycles and a 68% overall recovery in the PC-interleaved specimens after three cycles. In the static indentation tests, the critical load associated with the onset of interlaminar damage was measured and the recovery was 45% for the PLA-interleaved specimens after two damage-repair cycles and 30% for the PC-interleaved specimens after three cycles.