Abstract
Natural fibre reinforced composites have been extensively used in non-structural components, mainly in automotive industry. For these composites to be used in structural applications, an understanding of fracture toughness behaviour is important. In this study, the influence of water absorption and hybridisation of flax and flax/basalt hybrid laminates are presented with the aim to investigating the Mode I and Mode II interlaminar fracture toughness characteristics. Four types of composite laminates namely, neat vinyl ester (neat VE), flax fibre reinforced vinyl ester (FVE), flax fibre hybridised basalt unstitched (FBVEu) and flax hybridised basalt stitched (FBVEs), were fabricated by vacuum assisted resin infusion technique. Double cantilever beam (DCB) and Three-point-end-notched flexure (3ENF) tests were performed to evaluate the critical strain energy release rates, GIC and GIIC (initiation and propagation) as well as the crack length (R-curve) in dry and wet conditions by using different data reduction methods. The morphology of delamination and the fracture shear failure of composite laminates were evaluated using scanning electron microscopy (SEM) and X-ray micro computed tomography (µCT).From the experimental results, it was found that the Mode I fracture toughness initiation GIIC init. and propagation GIIC prop. of water immersed FVE composites were decreased by an average of 27% and 10% respectively, compared to the dry specimens, whereas the fracture toughness propagation of water immersed FBVEu and FBVEs composites were increased by approximately 15% and 17% compared to dry specimens. The results of Mode II fracture toughness obtained experimentally exhibited that the fracture energy of FBVEu composites, GIIC init. and GIIC prop. were improved by 58% and 21%, respectively compared to that of FVE dry specimens. Moisture absorption behaviour caused an increase in the ductility of matrix which resultantly improved the resistance to crack initiation. However, there was a reduction in the fibre/matrix interfacial strength of FBVEu wet composites and a deterioration in the delamination resistance to crack propagation. The critical strain energy release rate of neat VE increased about 52% with reinforcement of flax fibre composites. The fracture mechanisms showed energy dissipation through matrix deformation, fibre pull-out, fibre debonding, and fibre breakage. The experimental results confirmed that basalt fibre hybridisation enhanced the durability and water repellence behaviour of flax fibre reinforced composites.
Finally, this thesis provides a unique manufacturing technique to improve the interlaminar fracture toughness of flax fibre and flax/basalt hybrid composite laminates to be used in load bearing applications as an alternative to E-glass fibre reinforced composites. The outcomes of this study will be beneficial to automotive, marine and construction industries. In addition, the findings of this study will be useful for academic and researchers who are involved in the research and development of sustainable composites for light-weight structural applications.
| Date of Award | Jan 2018 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Hom Dhakal (Supervisor), Zhong Yi Zhang (Supervisor) & Stephanie Barnett (Supervisor) |