The effect of inkjet printed polymer on the mechanical properties of carbon fibre reinforced plastic

A new toughening method is presented in this thesis that uses inkjet printing to deposit toughening materials between carbon fibre reinforced plastic (CFRP) laminate plies prior to the curing cycle. Inkjet printing has the ability of precisely depositing material onto targeted positions, thus a controlled amount of toughening materials can be used to print dimension controllable patterns. Poly(methyl methacrylate) (PMMA) and polyethylene glycol (PEG) were dissolved in suitable solvents respectively to form printable solutions for inkjet printing. Different patterns and substrates were employed to investigate the repeatability of using inkjet printing to deposit polymer solutions. Microscopy showed that the designed patterns can be repeatedly printed onto substrates with controllable dimensions. A range of 0.025 – 0.2 vol.% of toughening material was used to prepare CFRP laminates for mechanical tests.
Mechanical properties of the inkjet printed CFRP laminates were tested by means of double cantilever beam and short beam shear tests to determine mode I interlaminar facture toughness (GIc) and apparent interlaminar shear strength (ILSS) respectively. The GIc of polymer printed laminates increased as the overall amount of polymer deposits increased before reaching to an optimum. A maximum 40% increase in GIc was observed in a system with printed 10 wt.% PMMA deposits that were hexagonally patterned. Different patterns and pattern densities were investigated, among which the hexagon with a higher pattern density performed the best in terms of material usage efficiency. Although laminates with a printed PMMA film possessed the highest GIc compared to the other printed groups that used discrete dots, the crack propagation was unstable. Additionally, the improvement in GIc did not result in a reduction of the ILSS for the polymer printed laminates, except for the film printed group. The damage tolerance of PMMA printed laminates was also enhanced based on the images obtained from X-ray tomography and scanning electron microscopy (SEM).
The proposed toughening mechanism is as follows: the polymer deposits remain strategically dispersed along predicted crack pathways with controllable size. Once microcracks occur between composite plies, the arrayed toughening materials can address crack propagation by plastic deformation and/or deflection of crack pathways due to debonding between toughening material and surrounded epoxy resin. The plasticization of localised epoxy region is also believed to be a contribution to the observed mechanical improvement.