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Smart composite detects and repairs airplane cracks
07 October 2007
From New Scientist Print Edition.
Paul Marks

A smart composite material that senses cracks as they develop and then repairs the damage could make the next generation of aeroplanes safer.

Composites are widely used in bicycles, fishing rods, racing cars and aircraft, and the first all-composite aircraft - the Boeing 787 and Airbus A350 - are on the way.

A polymer resin typically makes up most of the bulk of a composite, while fibres of a stronger material, such as carbon or glass, are embedded in it to add strength. By varying the type and amounts of resin and fibre, composites are tuned to produce different combinations of lightness and strength.

These materials can weaken through delamination, in which the fibres begin to part from the resin. This can be caused by the material experiencing a hard impact, absorbing liquids, or simply getting old. Repeated loading of a composite - caused by the pressurisation and depressurisation of an aircraft fuselage, for example - can turn small delaminations into dangerous cracks.

Now Nikhil Koratkar, an engineer at Rensselaer Polytechnic Institute in Troy, New York, has used the fact that cracks can alter a composite's electrical resistance to develop a self-repairing composite. He made a composite of epoxy resin and carbon fibre containing carbon nanotubes - which increase its electrical conductivity - and sandwiched it between two grids of wires. By applying voltages across the composite at each grid point, Koratkar was able to measure its resistance at these locations. He then made a crack in the composite using a razor blade and scanned each grid point for a second time. The damage increased the resistance at the points nearer the crack, he found. In planes, grids of wires could be snaked around key parts of infrastructure to pinpoint developing cracks.

Koratkar has gone a step further by also repairing the cracks. He created a second composite containing a powder with a low melting point, and introduced a similar crack into it. When he passed a current through the sample, it heated up and after 15 seconds the powder melted, flowed into the crack and hardened as it cooled, restoring about half of the original strength of the sample.

Philip Irving, an expert in damage tolerance at Cranfield University in the UK is impressed with the repair time, but says heating could weaken a composite. "The top surface of a wing may buckle," he warns.

From issue 2624 of New Scientist magazine, 07 October 2007, page 32

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