Multiscale characterization and modeling of progressive failure in nano-graphene reinforced carbon/epoxy composites

This dissertation studies the dispersion of nano graphene platelets in thermoset epoxy polymers for improving the mechanical and hygrothermal properties of nano composites and carbon fiber laminates. Barrier properties of graphene were investigated experimentally by adding 0.1-3 weight percentage of nano graphene to EPON 862 polymer and an analytical model for moisture diffusion in presence of nano graphene was derived assuming time-dependent diffusivity and moisture boundary conditions. Experimental studies were conducted for characterizing the fracture properties of 0.1 and 0.5 weight percentage nano graphene reinforced EPON 862 polymer in comparison to the unreinforced polymer. Pure Mode I, mixed mode and pure Mode II fracture experiments were performed. Remarkable improvement in fracture toughness across all modes was observed. Hydrogen passivation of graphene was employed to improve dispersion of nano graphene in epoxy. Graphene alignment was studied under an alternating current electric field. Mode I delamination experiments were conducted on unidirectional carbon fiber laminates with the polymer phase reinforced with small weight percentage of nano graphene. Significant improvements in initiation fracture energy and resistance to crack propagation was observed in nano graphene reinforced laminates. A theory that accounts for ductile to brittle transition in failure mode was developed to explain the nano scale toughness improvements observed in experiments. An analytical model for determining nano graphene size and orientation for maximum toughness enhancement depending on structural loading was derived and implemented in MATLAB. A hierarchical multiscale modeling technique was used to synergistically couple three different length regimes, nano scale (Molecular Dynamics), micro scale (Generalized Method of Cells) and macro scale (Finite Element Analysis), to capture the physics and length scale effects in a general structural problem (e.g. Open Hole Tension specimen). This work lays the foundation for the use of nano graphene composites for structural light weighting in future aerospace and automobile applications. (Published By University of Alabama Libraries