Influence of graphene nanoplatelets on modes I, II and III interlaminar fracture toughness of fiber-reinforced polymer composites

•GNP effects on modes I, II and III ILFT of FRPs are experimentally assessed for the first time.•A new test method is proposed to evaluate mode III ILFT in FRP laminates using torsion test.•The effect of GNP functionalization on the ILFT of FRP composites are established.•Different modes of fracture and associated mechanisms are explored by SEM analysis. The potential capability of graphene nanoplatelets (GNPs), with diverse surface functional groups for the purpose of enhancing the interlaminar fracture toughness of E-glass/epoxy composites was assessed. The GNPs included unmodified GNPs, and two chemically functionalized GNPs. Comparisons were made based on modes I, II and III interlaminar fracture toughness values. The results indicated that the interlaminar fracture toughness was increased significantly under mode I fracture, but not as much for the other modes. Based on the microscopic evaluation of the fracture surfaces, the inclusion of the functionalized GNPs generated the most resilient fiber/resin interface, thus, effectively inhibiting the crack propagation.