Next Generation Sandwich Composite Structures with Enhanced Impact Properties

The primary aim of this project was to successfully produce a ground-breaking PA6/Graphene nanoplatelet reinforced 3D printing filament material, with the intention to fabricate a novel next generation sandwich composite structure with enhanced impact properties. The novel next generation sandwich composite structure contained an auxetic re-entrant core and Amine-functionalised graphene nanoplatelets. This was to enhance the impact resistance, compressive modulus, compressive strength and energy absorption capabilities, and the residual compressive properties of PA6. Additionally, the auxetic re- entrant core and hybrid (a combination of the re-entrant and honeycomb cells) core topology were compared to the conventional honeycomb core. This was to determine if the auxetic core topology was superior in impact resistance and compressive properties, as well as in residual compressive properties, and to explore the possibility of replacing it. The first set of experimental methods involved analysing the in-plane compressive properties and deformation mechanisms of sandwich structures with a honeycomb, hybrid, or auxetic re-entrant core alongside continuous fibre-reinforcement in the face sheets. The properties were evaluated by performing in-plane uniaxial compression and a drop-weight low-velocity impact test to analyse the residual compressive properties. Kevlar fibres and carbon fibres were used as fibre-reinforcements in various fibre orientations, and a continuous fibre reinforcement 3D printer was utilised for the fabrication of the fibre-reinforced sandwich structures. An additional objective was to investigate if fibre-reinforcement enhanced the compressive properties of the core, and if it enhanced the residual compressive properties after low-velocity impact. A comparison in compressive properties and impact resistance between neat PA6 structures and fibre- reinforced sandwich structures showed that fibre-reinforced structures were inferior to neat PA6 sandwich structures. Secondly, the same set of experiments and analysis was replicated for the novel PA6/Amine-functionalised graphene nanoplatelet reinforced sandwich structures. These sandwich structures were printed using fused filament fabrication. Amine-functionalised graphene nanoplatelets were dispersed into a PA6 matrix at 1%,2%,3% and 4% concentrations using a co-rotating twin-screw extruder. A filament extruder was used for the manufacturing of the novel 3D printing filament material. A scanning electron microscope was used to investigate graphene nanoplatelet dispersion and their bonding behaviour within the matrix. A high depth of field analysis and microscopy approach was taken to investigate fibre lay-ups, 3D printing defects, core failure, face sheet failure, and indentation failure for both types of sandwich structures. The reinforcement of Amine-functionalised graphene nanoplatelets, in particular 2% filler concentration exhibited improved resistance against impact and enhanced compressive properties when compared to neat PA6. The substitution of the auxetic corereported superior compressive properties and residual compressive properties.