Modeling of toughness enhancement mechanisms in graphene nanocomposites

ABSTRACT

The objective of this paper is to develop a mechanism-based theoretical model to explain fracture experiments conducted on nanographene platelet (NGP) reinforced thermoset polymer nanocomposite tested in pure Mode I and in mixed mode by the authors and other researchers. Remarkable improvement in fracture toughness, K_IC (∼200%), and critical strain energy release rate, G_IC (∼570%), was observed with only 0.5 wt% NGP addition to the EPON 862 polymer system. Interestingly, very large increase in ductility of the fracture specimens at peak load was observed with increasing NGP loading. Asymmetric four-point bend tests were also performed for three different mode mix (tan⁻¹(K_II/K_I)) ratios (38°, 57°, and 89.5°). Significant enhancement in toughness as well as ductility for each mode mix case was observed with increasing NGP loading. Further, significant increases of 48% and 100% in delamination initiation toughness and resistance to crack propagation, respectively, were observed for 0.5-wt% NGP-reinforced IM7/EPON 862 unidirectional laminate. In this article, the reinforcement capacity of NGPs is explained through a length-scale based toughness enhancement mechanism, which is then employed to predict the critical length and orientation of nanoparticles for maximum fracture toughness improvement. It is envisioned that the application of this model could lead to significant toughness enhancement in composite laminates through (a) reduced intralaminar matrix cracks, (b) improved fiber/matrix interfacial toughening by controlling the size of nanoparticle growth on fuzzy fibers, and (c) improved delamination toughening as already demonstrated by the authors. For conclusive verification of our proposed model, there is a need for in-situ experiments to fully verify the nanoscale toughening effect by observing the process zone size near the tip of a nanoscale crack in a polymer thin film in-situ while it is being loaded, visualizing the brittle–ductile transition postulated by our theory and MD simulations. Work is currently underway in collaboration with the Air Force Research Laboratory to accomplish this objective.

KEYWORDS:

• Nanocomposites
• fracture toughness
• modeling
• length-scale effect

Funding

The authors would like to acknowledge the support of this work by the NASA Aeronautical Sciences NRA, Contract No. NNX11AI32A.