▎ 摘　　要

The transversely isotropic elastic properties of the multi-scale composites made of unidirectional T650-35 graphite fibers embedded in graphene nanoplatelets (GNPs)-enriched PMR-15 polyimide resin are evaluated. A multi-step micromechanical model consisted of generalized method of cells (GMC) and Halpin-Tsai approaches is hierarchically developed. The main advantage of this micromechanical methodology lies in its ability to consider the size and the agglomerated state of GNPs. The numerical results of the model are compared to the existing experimental data and a quite good agreement is found. The influences of amount, size, and agglomerated degree of GNPs, volume fraction of graphite fibers on the Young's moduli, Poisson's ratios, and shear moduli along the axial and transverse directions are investigated. The results clearly demonstrate an improvement in the elastic properties of the graphite fiber-reinforced multi-scale composites by the uniform dispersion of nano-graphene fillers in the polyimide resin. Furthermore, the change of GNP size can notably affect the multiscale composite properties. Thinner GNPs would be preferable to improve the mechanical characteristics. However, the nano-graphene agglomeration drastically reduces the stiffening effect of the GNPs.