Engineers from McGill University have definitively demonstrated that multi-wall carbon nanotubes (MWCNTs) can improve the mechanical toughness of carbon fibre laminates.

Carbon fibre composites have been in widespread use for decades – in Formula1, such materials form the chassis of every car, and up to 50% of an aircraft’s structure is now composite-based. It is all about their mechanical properties – when compared to metals, composites offer a superior strength-to-weight ratio, so in mass-critical applications, carbon fibre composites are the material of choice.

But the performance of these materials is not defined by the individual fibres – when it comes to determining damage initiation and growth in the composite, it is the properties of the polymer matrix that dominate. The most widely used polymeric resins tend to provide high stiffness but low fracture toughness, which can result in delamination in the final composite. Now, a team from Quebec’s McGill University have a demonstrated that the inclusion of multi-wall carbon nanotubes (MWCNTs) in the matrix significantly improves its fracture toughness, leading to a new generation of tougher carbon fibre composites.

Published in Carbon 79 (2014) 413-423 [DOI: 10.1016/j.carbon.2014.07.084], this work focused on modifying the brittle thermoset resin used in most carbon-based composites. Two different formulations were used – in the first, functionalised MWCNTs were mixed with the resin. The second formulation combined functionalised MWCNTs with a more traditional acrylate-based toughening agent. A technique called Resin Film Infusion (RFI) was then used to flow the MWCNT-filled resin through layers of carbon fibre mats, to produce the laminated composites. RFI is used in the aerospace industry to produce composites impregnated with rubber particles, but McGill researcher Pascal Hubert used it to ensure an even dispersion of aligned carbon nanotubes throughout the resin.

Fracture toughness tests were carried out on the MWCNT-filled resins and on the final laminates. The mechanical properties of the raw polymer resins were only marginally improved by the addition of MWCNTs. But, the final laminated composites exhibited significant improvement in their delamination properties (up to 143% in the case of Mode II fracture toughness). Hubert and his team believe that when the resin flows through the carbon fibre fabric, the fibres act as a sieve, ensuring a more even dispersion of MWCNTs, and improved mechanical properties. The team believe that this work can lead to a new generation of nano-enhanced carbon fibre composites, but further work on scaling up their system is still needed.

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