|The research covered in this thesis aimed to investigate the use of nanofibre and microfibre veils in carbon fibre reinforced composites and assessed the potential of the veils to improve damage resistance during impact and fatigue loading. It was hypothesised that the interleavings would increase the amount of energy required for crack propagation because of toughening due to fibre reinforcement mechanisms such as crack deflection, fibre pull out and fibre breakage. The work was undertaken as a combined project between the University of Waikato (Hamilton, New Zealand) and Revolution Fibres Ltd (Auckland, New Zealand).
During this investigation, six thermoplastic polymers were chosen (acrylonitrile butadiene styrene (ABS), acrylonitrile styrene acrylate (ASA), polystyrene (PS), chlorinated polyvinyl chloride (CPVC), polymethyl methacrylate (PMMA) and polycarbonate (PC)) that could potentially be used for the electrospinning of polymer nanofibre veils. Nanofibre veils were successfully produced from PMMA, and a polymer blend of polyamide 6,6 (PA6,6) and PMMA, (referred to as 'nanoNyplex'). These veils, along with three other nanofibre veils (nanoPA6,6, poly vinyl butyral (nanoPVB), and poly ether sulfone (nanoPES)), three microfibre veils (polyphenylene sulfide (microPPS), polyetherimide (microPEI), and woven polyamide 6 (microtricot)) procured from other manufacturers, and three veils combining one of the nanofibre veils with each of the microfibre veils (microPPSnanoPA6,6, microPEInanoPA6,6, and microtricotnanoPA6,6) were then used as interleaves in the manufacture of carbon fibre reinforced epoxy composite panels. Interleaves were placed between every ply of prepreg. After curing the panels, test specimens were created to assess fatigue, vibration damping and compression after impact performance.
From the vibration damping study, it was found that the nanoNyplex interleaving improved damping the most. It was thought that energy dissipation was due friction brought about by the movement of the interleaving fibres in the matrix, resulting in friction due to weak adhesion between the nanoNyplex fibres and the epoxy matrix.
From the compression after impact (CAI) section of this study, it was found that specimens interleaved with nanoPA6,6, microPPS and microPPSnanoPA6,6 had the highest CAI strengths. From optical inspection, it appeared (in general) that as the CAI strength of the specimen increased, the length of the damage region also increased. However, those identified with the highest CAI strengths had shorter damage regions.
From the fatigue section of this study, it was found that the use of most interleavings, (apart from microtricot) increased the number of cycles to failure. Post fatigue test scanning electron microscopy confirmed that crack deflection was present for most interleaved specimens. Some evidence of pull out and breakage of the interleaving fibres was seen on the fracture surfaces of the nanoPA6,6, microPPS, microPEI, microPEInanoPA6,6 and microPPSnanoPA6,6 interleaved specimens.
For both CAI and fatigue, it was found that improvement was generally greater with veils that had a large number of fibres per unit area and high adhesion strength with the matrix. However, for CAI it seems that high fracture toughness was also desirable.