In this paper, graphene nanoplatelets GNPs have been used as continuum phases within polymer composite materials. For such a purpose, a multi-scale strategy embedding the constitutive law of each material phase is accounted for to obtain the effective properties. The elasto-plastic response is established in the framework of the J2 plasticity. An expression of the algorithmic tangent operator for each phase is obtained and used as uniform modulus for homogenisation purpose. Using the Mori-Tanaka scheme, the effective non-linear behaviour is predicted for micro-parameters such as the aspect ratio and volume fractions. The results are compared to available experimental data as well as modelling from the literature. They show an enhancement of the equivalent macro stress-strain response with respect to low aspect ratio corresponding to platelet-like inclusions. Also, the volume fraction is seen to improve the composite response.