The Fatigue of Carbon Fibre Composites Containing Interlaminar Inkjet Printed Polymer Droplets

The objective of this research was to investigate a novel method of increasing the interlaminar toughening of prepreg composites to improve their fatigue performance, with particular emphasis on the retardation of delamination. Thermoplastic polymers droplets were deposited onto a composite prepreg using inkjet printing. Poly(methyl methacrylate) (PMMA) and polyethylene glycol (PEG) were dissolved in suitable solvents, creating polymer solutions that could be deposited onto prepreg substrates with excellent volume and position control. The prepreg laminae were then laid up to create complete laminates which contained the toughening polymers exclusively in their interlaminar regions, therefore leaving the bulk matrix properties unchanged. Both four point bending and tensile mechanical tests were used to evaluate the performance of printed composites. Results showed that multidirectional laminates printed with a solution of 10% by weight 20,000 molecular weight (Mw) PEG in deionised water exhibited significant retardation of delamination, being shown to reduce the rate of delamination by almost half in comparison to unprinted laminates. These laminates also showed increases of tensile strength and modulus of 4.9% and 12.3% respectively. Whilst laminates printed with PMMA and lower molecular weight PEG solutions were also shown to improve static mechanical properties, they also resulted in greatly increased rates of delamination in cyclic loading. Scanning electron microscopy was also used to analyse the delaminated surfaces of tensile samples. It was found that PMMA did not affect the bulk matrix properties in the interlaminar region. However, PEG was shown to result in increased matrix toughness and fibre/matrix bonding. PEG 20,000Mw was shown to exhibit the greatest increase of fibre/matrix bonding, whilst PEG 1,500Mw was shown to increase the ductility of the interlaminar matrix to an extent which was detrimental to the delamination resistance of laminates. The work presented in this thesis generated new understanding of the damage mechanisms operating at the interlaminar interface of cyclically loaded inkjet printed composites. It also demonstrated that such printed composites could potentially outperform unprinted laminates.