Aerospace boffins in America have come up with yet another use for carbon nanotubes - to build a self-healing layer into composite structures such as aircraft wings.
Structural components made out of polymer composites are just great, as everyone knows. They can be lighter and tougher than metal alloys, allowing aircraft - or bicycles, or whatever - to carry more or be stronger for the same weight. This allows comfier or faster bikes, nonmetallic airliners which potentially carry people using less fuel, advanced fighter jets which can perform amazing manoeuvres and not crack up, better golf clubs, tennis rackets, etc etc.
Just like metal, however, composites can be subject to tiny surface cracks which can rob them of their strength and lead to catastrophic failures if they aren't sorted out. But researchers at Rensselaer Polytechnic Institute in New York state think they have the answer.
The idea is to embed a fine grid of wires in a given composite surface, covered in an epoxy matrix full of nanotubes. By sending pulses down wires, it becomes possible to know as soon as a crack appears because the electrical resistance of the nanotube-laden epoxy will have changed. The fact that the wires are in a transverse grid also furnishes an exact location. That's good, because finding small cracks is a big maintenance issue.
"The beauty of this method is that the carbon nanotubes are everywhere. The sensors are actually an integral part of the structure, which allows you to monitor any part of the structure," according to Rensselaer assistant prof Mikhil Koratkar.
Unlike existing ultrasonic testing, Koratkar's coating can also test materials while they are in use; as when a plane is in flight.
But it gets better. The Rensselaer guys think you should then be able to send a higher-energy current down the wires. This will cause the nanotubes to heat up, melting ingredients in the epoxy so as to fill up and re-bond the crack automatically, as soon as it appears. This process could occur automatically, as soon as a crack was detected, controlled perhaps by an aircraft's computers.
"What's novel about this application is that we're using carbon nanotubes not just to detect the crack, but also to heal the crack," says Koratkar. He reckons that 70 per cent of the material's original strength can be recovered/retained, preventing catastrophic failures. The self-repair coating - in the Rensselaer tests - weighed only one per cent of what the uncoated structure did.
This could be good news for Boeing's new composite Dreamliner, which has had doubts cast on its safety lately. Thus far, even the best metallic aircraft haven't offered the ability to make structural repairs automatically in mid-air.
Not to mention the possibilities for self-healing golf clubs. ®