Lighter, stronger airplane in the offing

 Scientists have found a new way to make airplane frames lighter and more damage-resistant by reinforcing materials using carbon nanotubes - atom-thin rolls of carbon that are incredibly strong.

Passenger jets today are made from advanced composite materials such as carbon fibre reinforced plastic - extremely light, durable materials that reduce the weight of the plane by as much as 20 per cent compared to aluminium-bodied planes.

Lightweight airframes translate directly to fuel savings, which is a major point in advanced composites' favour.
However, composite materials are also vulnerable. While aluminium can withstand relatively large impacts before cracking, composites can break apart due to small impacts.

Researchers at Massachusetts Institute of Technology (MIT) in the US have found a way to bond composite layers in such a way that the resulting material is substantially stronger and more resistant to damage than other advanced composites.

They fastened the layers of composite materials together using carbon nanotubes - atom-thin rolls of carbon that, despite their microscopic stature, are incredibly strong.

They embedded tiny "forests" of carbon nanotubes within a glue-like polymer matrix, then pressed the matrix between layers of carbon fibre composites.

The nanotubes, resembling tiny, vertically-aligned stitches, worked themselves within the crevices of each layer, serving as a scaffold to hold the composite layers together.

In experiments to test the material's strength, the team found that, compared with existing composite materials, the stitched composites were 30 per cent stronger, withstanding greater forces before breaking apart.

The improvement may lead to stronger, lighter airplane parts - particularly those that require nails or bolts, which can crack conventional composites, said Roberto Guzman, who led the work as a postdoctoral fellow at MIT.

"More work needs to be done, but we are really positive that this will lead to stronger, lighter planes," said Guzman, who is now at the IMDEA Materials Institute in Spain.

Today's composite materials are composed of layers, or plies, of horizontal carbon fibres, held together by a polymer glue, which is "a very, very weak, problematic area," said Brian Wardle, professor at MIT.

Attempts to strengthen this glue region include Z-pinning and 3-D weaving - methods that involve pinning or weaving bundles of carbon fibres through composite layers, similar to pushing nails through plywood, or thread through fabric.

"A stitch or nail is thousands of times bigger than carbon fibres. So when you drive them through the composite, you break thousands of carbon fibres and damage the composite," Wardle said.

Carbon nanotubes are about 10 nanometres in diameter - nearly a million times smaller than the carbon fibres.

"We're able to put these nanotubes in without disturbing the larger carbon fibres, and that's what maintains the composite's strength," Wardle said.The research appears in the journal Composites Science and Technology.