The fracture toughness of nanocomposites may be improved by addition of low content of graphene nanofillers. The improvement is directly related to density of isolated graphene nanofillers dispersed in the nanocomposites. In contrast, for higher content of graphene nanofillers, the fracture toughness improvement may be degraded due to agglomerations. Thus, the main aim of this research is to account for the influence of the agglomeration of graphene nanofillers on the fracture toughness of nanocomposites. In the first step, based on J-integral concept, some relations were presented to estimate the fracture toughness improvement due to toughening mechanisms of nanocomposites reinforced by fully isolated and aligned nanofillers. Subsequently, the influence of agglomerations on the fracture toughness of nanocomposites was considered as a semi-empirical factor. Besides, a modified factor was used to account for the random orientation of the nanofillers. To consider the role of crack pinning toughening mechanism caused by agglomerated nanofillers in the fracture toughness of nanocomposites, another relation was presented. At the end, the experimental data available in literature was applied to validate the proposed comprehensive model. It was observed that the results obtained by the model are in very good agreement with the experimental results.
