Composites with aligned carbon nano-reinforcements: synergistically improving the damage tolerance and detection

摘要

Advanced fibre-reinforced polymer composites offer numerous advantages over metallic alloys, including higher specific strength and stiffness, resistance to corrosion damage and fatigue cracking. However, in the absence of through-thickness reinforcement, laminated composite materials are susceptible to interlaminar and intralaminar delamination damage due to accidental impact from bird strikes, hailstones, and tools dropped during maintenance. In addition, the low through-thickness conductivity of composites and its bonded structure presents challenges in protecting of aircraft against lightning strikes and detection of damage using traditional electrical based non-destructive techniques. In light of these challenges this PhD project investigated the effects of carbon nano-reinforcements alignment on the fracture and electrical properties of epoxy composites. The alignment of the nano-reinforcements was investigated using two different external field based techniques, namely electric-field and magnetic-field. The carbon nano-reinforcements used for the investigations include one-dimensional carbon nanofibres (CNFs) and two-dimensional graphene nanoplatelets (GNPs). The project explores key parameters for reinforcing the epoxy composites with carbon nano-reinforcements, including their weight content, shape (one-dimensional and two-dimensional), orientation (random and aligned), and alignment techniques (electric- and magnetic-field). The PhD also investigates the influence of the alignment of CNFs on the damage detection ability of the CNF-reinforced epoxy adhesive bonded composite joints. The study showed that the addition of just 1.0 wt% of aligned CNFs and GNPs increase the fracture energy of their epoxy nanocomposites by about eleven and seven fold, respectively. A mechanistic model is presented to quantify the contributions from the different toughening mechanisms induced by CNF and GNP nano-reinforcements which lead to the dramatic improvements in fracture toughness of the nanocomposites. The CNF-reinforced epoxy composites also showed a greater resistance to fatigue cracking when subjected to cyclic loading. The improved fatigue resistance of CNF-epoxy nanocomposites was due to a combination of intrinsic and extrinsic toughening mechanisms induced by the CNFs. In addition, the alignment of the nano-reinforcements also increased the electrical conductivity and simultaneously lowered the percolation threshold necessary to form a conductive network in the nanocomposites. Compared to the unmodified epoxy, the improvements in electrical conductivity of the nanocomposites with aligned CNFs and GNPs were increased by about ten and seven orders of magnitude, respectively. The improved electrical conductivity of the CNF-reinforced epoxy enabled real-time in-situ detection of fatigue cracking using a DC potential drop technique. A further study investigated the effects of using through-thickness nano-scale (CNFs) and micro-scale (z-pin) carbon reinforcements on the delamination resistance of carbon fibre-epoxy laminates. The delamination toughness of the composite laminate reinforced concurrently with CNFs and z-pins increased (by about 400%) in comparison to the control laminate, suggesting a synergistic toughening mechanism. The new class of fibre composites with multi-scale through-thickness reinforcements offers a unique opportunity to greatly enhance the damage tolerance and its detection in advanced fibre composite materials.