Toughening carbon fibre composites at cryogenic temperatures using low-thermal expansion nanoparticles

Matrix cracking of carbon fibre reinforced polymer composites at super cold temperatures, such as liquid hydrogen temperature, is a major issue for lightweight fuel storage, because the microcracks induced by the high thermal residual stresses in the matrix can cause fuel leaks and degrade the structural integrity of the storage vessel. Herein, we report a new method of toughening carbon fibre composites using nanomaterials of lowthermal expansion, i.e., nano-silica (nSiO(2)) and nano-cupric oxide (nCuO), at cryogenic liquid nitrogen temperature (similar to 196 degrees C). In addition to their low coefficients of thermal expansion, these two nanoparticles are sufficiently small to avoid the filtering effect of carbon fibres during resin infusion process. The surfaces of nCuO are functionalized by a polydopamine coating to enhance their bonding with the epoxy resin and the crosslinking density of the epoxy resin. Results from tension and fracture toughness tests of an epoxy modified with these nanoparticles reveal that PDA-coated nCuO are more effective than their un-coated counterpart and nSiO(2) in increasing the mechanical and fracture properties of epoxy nanocomposites at both room and cryogenic temperatures, tripling the fracture toughness values. More importantly, PDA-coated nCuO demonstrate significant improvements in the initiation and propagation fracture toughness of angle-ply carbon fibre composite ([+/- 35 degrees](8)) by 113% and 46% respectively at the cryogenic temperature. The underlying toughening mechanisms are identified using scanning electron microscope as being fiber peel-off, debonding, and striation in the matrix. These exceptional improvements stem from the higher interfacial residual thermal stress at cryogenic temperature due to their low thermal expansion properties, which in turn promotes crack branching that increases the energy dissipation of the matrix.