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Toughness Enhancement Mechanisms in Polymer Nanocomposites Due to Length Scale Effects at the Nanoscale



This paper presents fracture studies conducted on nanographene platelet (NGP) reinforced EPON 862 polymer composite tested in pure Mode I and mixed mode. Compact Tension (CT) tests were performed to determine Mode I fracture toughness for baseline (0 wt%), 0.1 wt% and 0.5 wt% NGP reinforced EPON 862 neat resin. Remarkable improvement in fracture toughness, KIC (~197%), and critical strain energy release rate, GIC (~572%), was observed for only 0.5 wt% NGP addition to the polymer system. SEM and AFM studies revealed significantly higher roughness in the fracture surface morphology for NGP reinforced specimens. Asymmetric four-point bend tests were also performed for three different mode mix (tan-1(KII/KI)) ratios (38o, 57o, and 89.5o). Significant enhancement in toughness for each mode mix case was observed. In order to ascertain if the toughness enhancements translated from the neat resin to the carbon composite, IM7/EPON 862 unidirectional laminates were manufactured using hand-layup with 0.1 and 0.5 wt% of NGP reinforcement. Double Cantilever Beam (DCB) experiments were performed to characterize delamination toughness in Mode I (initiation toughness as well as fracture resistance). Significant increases of 48% and 100% in initiation toughness and resistance to crack propagation was observed respectively for 0.5 wt% NGP reinforced EPON 862. The reinforcement potential of NGPs is explained using a novel nanoscale toughness enhancement mechanism, which can be employed to predict the length and orientation of the NGPs for maximum fracture toughness improvement.

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