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Abstract
A molecular dynamics (MD) approach to investigating the structural performance of thermosetting epoxy resin in fibre-reinforced composites is developed in this thesis. There has been a substantial amount of interest in modelling resins using MD, but the work has focussed on the mechanical properties of neat resins or resins with additives. Because the fibres are the main load carrying component in fibre-reinforced composites, stiffening the resin does not contribute significantly to the stiffness of a laminate. On the other hand, shear to transfer load between the fibres and resistance to cracking become the dominant properties.
A dendrimeric modelling approach is employed and further improved for crosslinks of epoxy resins with accurate molecular weight. Various algorithms are developed to characterise the resin in the simulation domain. The focus is on modelling the cross-linking process during cure before applying mechanical loads to determine the physical properties, and modelling the constraint provided by the fibres in the final laminate. Prediction of failure of the resin is based on evaluation of the critical dilatational and distortional strain invariants. The approach is then applied to investigate the use of carbon nanotubes (CNTs) to reinforce the resin. MD simulations are used to understand the role of CNTs and their aspect ratio in reinforcing the highly cross-linked amorphous resin.
A series of experimental works are accomplished ranging from the manufacturing to material characterisation process. The aim is firstly focused to validate the modelling and simulation approaches using the identical epoxy formulations modelled in the simulation study. The experimental work is then targeted to disperse CNTs in the viscous epoxy resin and define its property. The focus of the experiment is then directed at defining sources of the resulting properties. Post-analysis of the experimental work is conducted including fractography, thermal stability and dynamic scanning of the nanocomposites. The results from the post-analysis conform to the simulation study that the physical hindrance of bulky CNTs in the three-dimensional epoxy domain decreases the degree of crosslinking. The successful development of a multiscale modelling approach linking nano-scale materials to macroscale physical properties has permitted an innovative study of the effect of aspect ratio of the CNTs on mechanical performance. The multiscale modelling in conjunction with Onset failure theory leads to the conclusion that the nanocomposites with high aspect ratio CNTs may be beneficial to unidirectional composites while structures with off-axis composite can benefit from CNTs with the aspect ratio less than one.