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Tailoring of Flaw Length-Scale for Mode I and Mixed-Mode Toughness Enhancement in Composite Airframes



Motivated by previous research performed by the authors investigating Mode I toughness enhancement using nanographene in EPON 862 epoxy, this article aims to investigate to cases: (a) the changes in mixed mode fracture properties of a thermoset polymer (EPON 862) reinforced with hydrogen passivated nanographene platelets (HP-NGPs) and, (b) Mode I fracture properties of EPON 862/IM7 unidirectional laminate with and without dispersed HP-NGPs. For case (a), mixed mode fracture experimentation was performed using an asymmetric four-point bending specimen on baseline (0 wt%), 0.1 wt% and 0.5 wt% HP-NGP reinforced EPON 862 polymer. Three different mode mix (KII/KI) ratios (0.78, 1.53, 117) were used to obtain an experimental fracture envelop encompassing pure Mode I to Mode II. Remarkable increase in the fracture envelop both in Mode I (~3 times) and Mode II (~2.5 times) was observed with only 0.5 wt% of HP-NGP. For case (b), Double Cantilever Beam (DCB) experiments were used to obtain the fracture toughness of the unidirectional IM7/EPON 862 laminates with the HP-NGP reinforced matrices as in case (a), in accordance with ASTM D5528. Significant increase (~100%) in resistance to crack propagation in DCB specimens was observed for 0.5 wt% HP-NGP reinforced system in comparison to baseline samples. A brittle to ductile transition at the crack tip due to a nanoscale size effect is postulated as the reason for the toughness increase.


A. Polymer matrix composites, B. Fracture toughness, B. Delamination, Graphene


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