Graphene reinforced aerospace polymer composites with improved toughness and damage tolerance

It is well-known that carbon fibre reinforced polymer composites, despite their widespread use in aircraft structures, are susceptible to delamination cracking due to the low epoxy matrix toughness. Graphene nanoplatelets offer the opportunity to increase the delamination resistance and damage tolerance of carbon fibre-epoxy (CFRP) composite materials and structures. However, the suitability of graphene nanoplatelets for improving the damage tolerant properties of CFRP composites for potential use in aircraft structures is not fully understood. This PhD project aims to experimentally investigate the effect of graphene-based nanoparticles on the material and structural properties of the epoxy matrix and carbon-epoxy laminates. Properties investigated include the mode I interlaminar fracture toughness and interlaminar fatigue resistance, low-velocity impact damage resistance and damage tolerance, and T-joint structural properties. First, the PhD project investigated the preferential ordering by alignment and chaining of GNPs using an externally applied AC electric field as a novel method to improve the mode I fracture toughness and fatigue properties to an epoxy polymer used as the matrix phase in carbon fibre composites. The work investigated the effect of GNP concentration in the epoxy on the fracture toughness and fatigue properties for both random and aligned nanoplatelets. The research revealed that increasing the GNP content increased the mode I toughness of the epoxy polymer, with the maximum improvement occurring at ~1.5 wt.% beyond which the toughness was reduced due to excessive agglomeration of the nanoplatelets. Importantly, the electric-field induced alignment and chaining of GNPs perpendicular to the mode I crack growth direction had a beneficial toughening effect on the epoxy polymer. The epoxy with aligned and chained GNPs also had higher mode I fatigue resistance compared to the polymer with randomised GNPs. Significant improvement to the fatigue crack growth resistance was measured near the threshold region due to unique fatigue strengthening mechanisms induced by the GNPs that promoted a crack-tip shielding effect in the epoxy polymer. The mode I fatigue performance was further improved with electric field-inducedalignment and chaining of the GNPs owing to the more effective fatigue strengthening mechanisms near the crack tip. A physics-based analytical model to predict the mode I fracture toughness of 1D carbon-based and 2D graphene-based polymer nanocomposites was developed. The model is based on the fracture toughening mechanisms induced by 1D and 2D carbon nanofillers when dispersed in a polymer matrix. The intrinsic and extrinsic fracture toughening mechanisms identified for the two types of polymer nanocomposites include (a) debonding of the nanofillers, (b) plastic void growth initiated by the debonded nanofillers, (c) pull-out and crack bridging of the nanofillers, and (d) rupture of the bridging nanofillers. The numerical accuracy of the model was assessed for epoxy nanocomposites reinforced with 1D carbon nanofibres (CNFs) or 2D GNPs. The mode I fatigue properties and strengthening mechanisms of the nanocomposites with 1D CNFs or 2D GNPs are also studied and compared. In addition to the toughening mechanisms, during the cyclic fatigue loading of the epoxy nanocomposites other fatigue strengthening mechanisms occur including (a) unravelling of the inner helical core of the CNFs and (b) crack wedging from the debris formed due to the microcracking induced by the GNPs. This PhD project also examined the mode I interlaminar fracture toughness and fatigue resistance of CFRP laminate with multi-scale reinforcement using nano-scaled GNPs and micro-scaled z-pins made of carbon fibres. Delamination fracture toughness testing of the multi-scale reinforced laminate revealed that the fracture toughness was increased by the GNPs, with additional toughness provided by the z-pins. The GNPs and z-pins had an additive toughening effect on the CFRP laminate, with the pins providing most of the delamination toughening, although the synergistic toughening effect by combining the reinforcements did not occur. Mode I interlaminar fatigue testing revealed that using GNPs + z-pins in combination resulted in higher delamination fatigue resistance when using the GNPs and z-pins separately in the CFRP laminate. The PhD project also experimentally investigated the barely visible impact damage (BVID) resistance and post-impact properties of GNP-doped CFRP laminates at incident impact energy levels up to 50 J. It was found that the GNPs reduced the impact damage area, with cross-sectional profiling revealing high GNP aided crack tortuosity. However, the GNPs reduced the compression strength and compression-after- impact (CAI) strength of the laminates due to high nanoplatelet agglomeration. The efficacy of GNPs to increase the structural properties of T-shaped bonded joints made of CFRP laminate was experimentally investigated. Stiffener pull-out tests were performed of T-joints containing different concentrations of GNFs to determine their effect on the bulk stiffness, damage initiation load, ultimate strength load and absorbed energy capacity of the composite T-joint. DIC was also conducted to gain deeper insights into strain evolution during fracture initiation and progression in the T-joint containing different amounts of GNPs. It was discovered that the GNPs had no significant beneficial effect on the T- joint properties, except for the load-at-first failure. The PhD thesis presents a summary of the major research findings from the project and makes several recommendations for further research in the field of GNP nanopolymers and composite materials.