The size effect of unmodified and graphene nanoparticle modified matrix fibres is experimentally investigated. Neat matrix fibres show a clear size effect of increasing tensile strength with decreasing volume due to a statistical defect distribution. The nanoparticle modified matrix shows no significant size effect. Nanoparticles act as crack initiators and consume fracture energy. The size of the particles is independent of specimen volume, so that the failure initiating as well as energy absorbing mechanisms are available, independently of the volume. Fractography analysis of SEM images shows different energy dissipation mechanisms such as micro-damage at the graphene particles. Graphene pull-out, layer separation, layer shearing, formation of micro voids as well as crack separation and crack bifurcation are observed that depend on the orientation of the graphite layers to the fracture plane. These mechanisms dissipate energy and so that a graphene nanoparticle modification result in an increased fracture toughness and thus increased strength of an epoxy matrix system if the volume is large enough. The maximum stress in specimen of small volume depends on graphene layer orientation, so that ideally, the covalent bonds of the nanoparticles should be orientated in loading direction.
