Over the past decade, mechanical characterization data for nanoparticle-reinforced polymer matrix composites have shown significant improvements in compressive strength and interlaminar shear strength, in comparison with baseline properties. While the synergistic reinforcing influence of nanoparticle reinforcement is obvious, a simple rule-of-mixtures approach fails to quantify the dramatic increase in mechanical properties. Consequently, there is an immediate need to investigate and understand the mechanisms at the nanoscale that are responsible for such unprecedented strength enhancements. A multi-scale and multi-physics simulation approach is considered computationally more viable since relying on a single time or length scale may require huge amounts of computational resources and can potentially lead to inaccurate results. A proof-of-concept case study involving crack initiation in a graphene nanoplatelet is presented, together with a methodology for computing the atomistic J-integral based on Hardy estimates of continuum fields. The same methodology will be used to gain a better understanding of the influence of nanoscale phenomena on continuum-scale mechanical properties of a polymer nanocomposite. It is envisioned that the current research will contribute towards the understanding of advanced nanostructured composite materials within the context of integrated computational materials engineering.