A stitch in time

Lou Reade looks at how carbon nanotubes (CNTs) could be used to ‘stitch' a whole host of composite structures together to make them stronger. 
When it is finally finished, Boeing's 787 Dreamliner will contain a larger proportion of carbon fibre composites than any previously built commercial aeroplane. And the industry's ongoing quest to save weight will ensure that its descendants use even more composites. But, like all materials, composites have their limitations: because they are constructed in layers (or plies), there is a tendency for these to peel apart (or delaminate) under high stress – and as the glue between layers fails. Now US researchers think they have found a way to overcome this by using carbon nanotubes (CNTs). Adding small amounts of these microscopic particles to the composite would act like billions of tiny reinforcements to hold the plies of the composite together and prevent crack propagation. The CNTs are around 1,000 times smaller than the carbon fibres that they are nestling between. The researchers call the technique ‘nanostitching'. “Many issues with the through-thickness direction of composites cause us to overdesign the composite structures,” says research leader Brian Wardle, of the department of aeronautics and astronautics at Massachusetts Institute of Technology. “We're using thicker structures than we need to. This technique could change the way we design our aerospace structures: we wouldn't have to account for this well-known weakness because it would be fixed.” One conventional way of keeping the plies together is to drill holes in the composite, then insert Z-pins or conventional stitching. This can have a tenfold improvement on toughness, but there is a downside: toughness in the perpendicular (‘in plane') direction suffers – by as much as 50% – because the carbon fibres within the composite have been broken. Wardle's idea is to replicate the effect of the pins – but on a molecular scale. The CNTs must be aligned so that they are perpendicular to the plies. Because the CNTs are so small, they fit between the carbon fibres within the plies, keeping them intact. Wardle says that CNTs are four times more effective than Z-pins at holding the plies together – and have no detrimental effect on in-plane properties. “If you had a choice, you would always choose a nanoscale fibre to do stitching,” he says.
Wardle – who, before his academic post, spent considerable time working in industry – says he has already attracted venture capital funding to set up a company to commercialise the technology. “There's a proposal to set up a company this fall [autumn],” he says. “I could foresee a product a year after that.” However, he is philosophical about the time it might take for the material to become widely accepted: “New materials in aerospace take 20 to 30 years to become accepted,” he says. “It's unlikely we'd see anything different with this material.” However, the work has now begun. The team has prepared samples of aerospace-grade materials – 24-ply epoxy laminates – for fracture testing. The system relies on three separate elements: carbon fibres; the aerospace-grade polymer resin; and vertically aligned CNTs in the inter-laminar region. The polymer glue between the plies is heated until it becomes more like a liquid, which draws the CNT's up into the glue. The CNTs then effectively stick the glue to the composite. The nanotubes may sound like a magic ingredient, but Wardle says they are not particularly difficult to prepare. “If you get the recipe right, then carbon nanotubes align themselves,” he says. “We then transfer them to the pre-preg. There are 10-100 billion aligned CNTs per square centimetre.” Much of the underlying work is theoretical, and was based on fracture mechanics modelling carried out by Wardle and several colleagues. The model predicted that reducing the diameter of stitching fibres from microns – as is currently practised – to nanometers would dramatically increase the laminate's resistance to fracture. Part of this was due to the way in which multi-walled CNTs effectively ‘telescope' out to keep the plies together. Nanoparticles such as CNTs are finding increasing use in industry because of the property improvements that they can bring to materials. In conventional thermoplastics, for example, they have been used as an additive to improve the gas barrier of food packaging, as flame retardants and to improve mechanical properties. Their key advantage is that the physical improvement can be achieved with very small amounts of material. This ensures that the properties of the polymer matrix – whether it is a thermoplastic or composite – are unaffected. For Wardle's material, he estimates that 1% by weight of CNTs would have the desired effect. A critical parameter of the CNT fibres is their aspect ratio – that is, the ratio of their length to their diameter. The longer and thinner the CNT, the greater the effect it will have. In a paper on the subject, Wardle says that a 1% volume fraction of CNTs – which are 20 microns long and 8nm in diameter – could increase toughness by a factor of more than 25.
This value – which, it must be stressed, is theoretical – is an order of magnitude greater than current experimental data. But it shows the extent to which such ‘real' systems might be improved. The next level Wardle has concentrated on nanostitching's potential for improving the toughness of aeroplane skins – but says that it could improve a wide range of composite structures. “You'd be able to use composites in a different way and really take advantage of their benefits,” he says. “We've been able to demonstrate that you can put these into nearly all composites that are currently used in aerospace structures. Down the line, we think that nanostitching could be incorporated into most composites applications.” The fact that composites could be made stronger and thinner means that the materials might be able to replace yet more components that are currently made from metals like aluminium, he adds. Another potential advantage is that CNTs imbue materials with electrical conductivity, which may give the aeroplane skins more protection against lightning damage. Wardle's work is supported by a consortium of aerospace companies and materials suppliers. The Nano-Engineered Composite Aerospace Structures (NECST) consortium includes Airbus, Boeing, Lockheed Martin, Saab, Textron and carbon fibre supplier Toho Tenax as members. Mike Kinsella, aerospace marketing manager at Toho Tenax, says of the project: “We don't normally support university research. This is the only project in North America that we're sponsoring.” He says that the technique addresses the problem of composite susceptibility in a different way – by focusing on weakness rather than strengths. “Taking out weak points – rather than making strong points even stronger – is an effective way of increasing performance, and that's what this would do,” he says. “Nanostitching is an interesting approach with a lot of potential, but it's quite a long way off: it will require some patience.” www.mit.edu