The present paper describes improvements in the fracture resistance of epoxy polymers due to the addition of either (a) one-dimensional (1-D) carbon nanofibres (CNFs), or (b) two-dimensional (2-D) graphene nanoplatelets (GNPs), or (c) hybrid combinations of these carbon nanofillers (i.e. using both CNFs and GNPs). The effects of the dimensional shape and concentration (i.e. 0.0, 0.5, 1.0, 1.5 and 2.0 wt%) of the nanoscale carbon fillers are considered. The addition of CNFs, GNPs or hybrid combinations of CNFs and GNPs increases greatly the quasi-static fracture energy, G Ic , of the epoxy due to these nanofillers inducing multiple intrinsic (e.g. interfacial debonding and void growth) and extrinsic (e.g. pull-out and bridging) toughening mechanisms. A mechanistic model is presented to quantify the contributions from the different toughening mechanisms induced by the CNF and the GNP fillers which result in the improvements observed in the fracture energy. The resistance of the epoxy, modified with either the GNPs or the CNFs, to cyclic-fatigue loading is also increased by the presence of the carbon nanofillers.

Graphene nanoplatelets (GNPs) offer great potential for enhancing the multifunctional properties of epoxy polymers, including the cyclic-fatigue crack growth resistance. In the present work we investigate the effectiveness of electric-field alignment of GNPs in increasing the fatigue resistance of such epoxy nanocomposites. The GNPs were aligned in an uncured (liquid) epoxy resin by applying an alternating-current electric field during the curing process, before gelation of the epoxy. The fatigue properties of the cured epoxy containing both random and aligned GNPs were measured using double cantilever beam (DCB) specimens tested in displacement control over a range of cyclic energy release rates from the threshold condition (i.e. below which no fatigue crack growth was observed) to fast fracture. The results show that aligning the GNPs using an electric field yields a greater improvement in the fatigue crack growth resistance than obtained using randomly-orientated GNPs, particularly in the near threshold region. The resistance of the epoxy nanocomposites to fatigue crack growth was increased as the weight fraction of the GNPs was increased up to a certain level, beyond which there was no further improvement in the fatigue resistance. These improvements have been attributed to several toughening mechanisms which retard the fatigue crack growth in the epoxy nanocomposites, including debonding of the GNPs, epoxy void growth, crack deflection and branching of the main fatigue crack caused by microcracking induced by the presence of the GNPs, pull-out and crack bridging by the GNPs, and crack shielding by GNP debris particles behind the advancing fatigue crack tip. These toughening mechanisms become more active when the GNPs are aligned normal to the direction of fatigue crack growth, which results in a higher fatigue resistance for the epoxy nanocomposites containing aligned GNPs than for those containing randomly-orientated GNPs.