Research on fabrication and mechanical behaviour of functionally graded graphene platelets/epoxy nanocomposites

Graphene soon attracts extensive attention due to their unique two-dimensional structure and exceptional mechanical performance with Young¿s modulus reaching 1 TPa and tensile strength being 130 GPa. Functionally graded material (FGM) is a kind of essential material in engineering as it can be purposely designed to satisfy various performance requirement and cater for different working environments. Present research aims to incorporate the advantage of graphene and its derivatives into FGMs, producing functionally graded graphene reinforced nanocomposites. Both numerical simulation and experiments are employed. Numerical simulation is first conducted to give an overall idea on the role of GPLs played inside the functionally graded graphene reinforced nanocomposites. Experiments are conducted later to realize the optimized graded structure, obtain reliable testing results, and verify the superior of the graded structures.

To fabricate functionally graded polymer-based nanocomposites, it is not only to realize the graded structure, but also need to ensure the homogeneous dispersion of nanofillers and prevent their natural aggregation. Due to lack of reliable and widely suitable processing techniques, limited experimental attempts have been reported on functionally graded nanocomposites with pristine liquid phase matrix, like epoxy. In the current thesis, a new constructive technique is proposed and developed to fabricate functionally graded nanocomposites with pristine liquid phase matrixes.

This main body of this thesis is composed of three parts.

In the first part (Chapter 1 and 2), the research background, the preliminary motivation of this research and a concise literature review on isotropic and functionally graded nanomaterials are included to identify the research gap. In the second part (Chapter 3 and 4), a numerical study on the effect of graphene nanoplatelets (GPLs) on the static and dynamic behaviours of functionally graded nanocomposites are systematically conducted. A comprehensive parameter study is conducted to give an overall idea on how the GPL concentration, geometry, distribution, plate geometry and boundary conditions affect the mechanical performance of functionally graded graphene reinforced trapezoidal plates. In the third part (Chapter 5), an experimental study is conducted to figure out a technique realizing the layer-wised functionally graded GPLs/Epoxy nanocomposites and then obtain reliable test results, hence verifying the priority of the functionally graded distribution of GPLs.

In summary, this thesis presents a systematically investigation on the influence of GPLs on the static and dynamic performance of functionally graded graphene reinforced materials. Experiments are followed to verify the advantage of the optimized GPLs graded structures. This thesis contributes to the following outcomes: 

a) Layer-wise structure can be accurate enough to simulate the continuously graded nanomaterials.

b) Nanocomposite plates with higher GPL loading, smaller base angle possesses better mechanical performance. Dispersing larger sized GPLs with fewer graphene layers near the top and bottom surfaces of the plate is the most effective way to improve the structural stiffness. However, the reinforcing effect becomes limited when GPL length-to-thickness ratio is bigger than 1000.

c) Two-step route exhibits a clear advantage on the mechanical performance of isotropic nanocomposites than single-step route and solution mixing route. A novel functional graded nanocomposite fabrication route is developed. The fabricated functionally graded GPL/Epoxy nanocomposites demonstrate significantly improved mechanical properties compared to their counterparts with GPLs randomly and uniformly dispersed.