Enhancement of Mode I fracture toughness properties of epoxy reinforced with graphene nanoplatelets and carbon nanotubes

Relative to the conventional metals used in structures, epoxy-based composites have poor fracture toughness properties. This has long been a weak link when using epoxy composites for structural applications and therefore several efforts are being made to improve their fracture toughness. In this experimental study, the enhancement of fracture toughness brought about by the addition of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) was investigated. CNT-Epoxy and GNP-Epoxy Compact Tension (CT) samples were fabricated with 0.1% and 0.5% nanofiller weight concentrations. The potential synergistic effects of dual nanofiller reinforcements were also explored using CNT/GNP-Epoxy CT samples at a 1:1 ratio of CNT:GNP. Displacement controlled CT tests were conducted according to ASTM D5045 test procedure and the critical stress intensity factor, K-IC, and the critical fracture energy, G(IC), were calculated for all the material systems. Significant enhancements relative to neat epoxy were observed in reinforced epoxies. The CNT-Epoxy samples at 0.5% nanofiller concentration experienced the most enhancement with a similar to 118% and similar to 311% increase in fracture toughness and fracture energy, respectively. The GNP-Epoxy and CNT/GNP-Epoxy samples also demonstrated significant enhancements relative to neat epoxy, but the enhancements are in general weaker than those of the CNT-Epoxy samples. Fracture surfaces were analyzed via scanning electron microscopy. Instances of CNT pullouts on the fracture surface were observed, indicating the occurrence of crack bridging. Furthermore, increased surface roughness, an indicator of crack deflection, was observed along with some crack bifurcations in the GNP-Epoxy samples. A further exploration is needed to understand the poor synergy between CNTs and GNPs in the CNT/GNP-Epoxy samples, which may be due to the CNT:GNP ratio and particle sizes used in the current study.