'Bleeding composites' could revolutionise aviation safety

A technique that mimics healing processes found in nature could enable damaged aircraft to mend themselves automatically, even during a flight.

As well as enhancing safety, the use of 'self-healing' composite materials could make it possible to design lighter aeroplanes that use less fuel and have lower carbon emissions. The technology could be available commercially within four years, with potential applications in many other industries, too.

The technique's innovative aspect involves filling the hollow glass fibres contained in fibre-reinforced polymer (FRP) composites with resin and hardener. If the fibres break, the resin and hardener ooze out, sealing up small holes and cracks and enabling the composite to recover up to 80-90 per cent of its original strength. This is more than enough to let a plane function at its normal operational load.

By mixing ultra-violet fluorescent dye into the resin, any 'self-mends' could be made to show as coloured patches that could easily be pinpointed during subsequent ground inspections and a full repair carried out if necessary.

Aerospace engineers at Bristol University developed the technique with funding from the Engineering and Physical Sciences Research Council. It has potential to be applied wherever FRP composites are used. These lightweight, high-performance materials are proving increasingly popular not only in aircraft but also in car, wind turbine and even spacecraft manufacture.

"This approach can deal with small-scale damage that's not obvious to the naked eye, but which might lead to serious failures in structural integrity if it escapes attention," says Dr Ian Bond, who led the project. "It's intended to complement rather than replace conventional inspection and maintenance routines, which can readily pick up larger-scale damage, caused by a bird strike, for example."

"This project represents just the first step", says Bond. "We're also developing systems where the healing agent isn't contained in individual glass fibres but actually moves around as part of a fully integrated vascular network, just like the circulatory systems found in animals and plants. Such a system could have its healing agent refilled or replaced and could repeatedly heal a structure throughout its lifetime. Furthermore, it offers potential for developing other biological-type functions in man-made structures, such as controlling temperature or distributing energy sources."

The team is working with industrial partner Hexcel Composites Ltd, a manufacturer of composites for aerospace and other industrial applications.

The resin used in the self-repair system is a commercially available product. The researchers are currently developing a custom-made resin optimised for use in the system.

Image: Fractured fibre-reinforced polymer under UV illumination, showing how the 'healing agent' bleeds into the damage

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