Aircraft FRP Composites

Self-repairing aircraft could revolutionise aviation safety

Press release issued 19 May 2008 by Bristol University

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

As well as the obvious safety benefits, this breakthrough, developed by aerospace engineers at Bristol University, could make it possible to design lighter aeroplanes in future. This would lead to fuel savings, cutting costs for airlines and passengers and reducing carbon emissions.

The technique works like this. If a tiny hole or crack appeared in an aircraft (for example, due to wear and tear, fatigue or a stone striking the plane), epoxy resin would ‘bleed’ from embedded vessels nearby and quickly seal it up, restoring structural integrity. By mixing 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.

This simple but ingenious technique, similar to the bruising and bleeding/healing processes we see after we cut ourselves, has been funded by the Engineering and Physical Sciences Research Council (EPSRC). It has potential to be applied wherever fibre-reinforced polymer (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. The new self-repair system could have an impact in all these fields.

The technique’s innovative aspect involves filling the hollow glass fibres contained in FRP composites with resin and hardener. If the fibres break, the resin and hardener ooze out, enabling the composite to recover up to 80-90 per cent of its original strength – comfortably allowing a plane to function at its normal operational load.

“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,” said Dr Ian Bond, who has led the project. “It’s intended to complement rather than replace conventional aircraft inspection and maintenance routines, which can readily pick up larger-scale damage, caused by a bird strike, for example.”

By further improving the already excellent safety characteristics of aerospace FRP composites, the self-healing system could encourage even more rapid uptake of these materials in the aerospace sector. A key benefit would be that aircraft designs including more FRP composites would be significantly lighter than the primarily aluminium-based models currently in service. Even a small reduction in weight equates to substantial fuel savings over an aircraft’s lifetime.

“This project represents just the first step”, said Dr 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 new self-repair technique developed by the current EPSRC-funded project could be available for commercial use within about four years.   

Aerospace resins.

The three-year research project ‘Bleeding Composites: Damage Detection and Repair Using a Biomimetic Approach’ concluded at the end of April 2008. It has received total EPSRC funding of just under £171,000.

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

In aircraft, FRP composites can be used in any part of the primary structure (fuselage, aircraft nose cones, wings, aircraf tailfins).

The aerospace resins used in the self-repair system are off-the-shelf, Araldite-like substances. The team are currently developing a custom-made resin optimised for use in the system.

The dye mixed with the resin would be ultra-violet fluorescent and so would not show up in normal lighting conditions.

A similar technique developed at the University of Illinois involves the addition of microcapsules containing dicyclopentadiene, rather than epoxy resin contained in the glass fibres themselves. Such a system sees the rapid reaction of a liquid with a solid catalyst. The resulting plastic gives similar properties to the epoxy. However, the catalyst is based on ruthenium, an expensive and rare metal. The even distribution of capsules and catalyst within an FRP has also proven to be difficult.

The Engineering and Physical Sciences Research Council (EPSRC) is the UK’s main agency for funding research in engineering and the physical sciences. The EPSRC invests around £740 million a year in research and postgraduate training, to help the nation handle the next generation of technological change.

The areas covered range from information technology to structural engineering, and mathematics to materials science. This research forms the basis for future economic development in the UK and improvements for everyone’s health, lifestyle and culture.

EPSRC also actively promotes public awareness of science and engineering. EPSRC works alongside other Research Councils with responsibility for other areas of research. The Research Councils work collectively on issues of common concern via Research Councils UK.