Next Generation Graphene Nanocomposites for Rotorcraft Structural Applications

The loading conditions experienced by components of rotorcraft during flight are predominantly dynamic including both vibratory and periodic flapping and pitching motions. It is therefore very important to enhance the fracture and fatigue properties of composite structures being subjected to these cyclic loading conditions such as flexbeams, rotor blades, fuselage panels, etc. to extend their useful service life and to drive down the costs of replacement/repair. A potential solution to the issues described above may be found in a new class of composite materials consisting of traditional glass or carbon fibers paired with an epoxy matrix infused with a nanoscale graphene network. In this paper we report a novel technique to produce bulk quantities of graphene which is a single-atom-thick, two-dimensional sheet of sp2 hydridized carbon atoms. Next, we describe a method to uniformly disperse these sheets in thermosetting epoxy polymers. We then characterize various mechanical properties (such as tensile strength, Young's modulus, fracture toughness and fatigue crack propagation resistance) of these graphene/epoxy blends. We end the paper with fatigue characterization of glass-fiber/epoxy composites with a graphene network infiltrated into the epoxy resin. Dynamic three-point bending tests were conducted on these samples to generate the classical Stress (S) vs. Number of Cycles to Failure (N) curves at various loading fraction of graphene additives. Our results indicate that addition of only ∼0.2% weight of graphene in the epoxy resin results in up to ∼1000-times enhancement in fatigue life of the composite. In uniaxial tensile mode (i.e. no compressive loading), we also observe enhancement in fatigue life, but the improvements were relatively modest (∼3-5 times increase). This indicates that the dominant mechanism behind the dramatic increase in fatigue life that we report is the ability of the interlacing (interconnecting) graphene network to significantly toughen the glass-fiber/epoxy interface to prevent debonding and buckling of the glass microfibers under compressive load. The nearly three orders of magnitude enhancement in fatigue life of conventional fiber reinforced composites that we report here can translate into significant weight/cost saving as well as enhanced reliability and safety for a variety of rotorcraft structural components.