A self-healing aircraft could be available in the near future, thanks to an epoxy resin developed by Bristol University aerospace engineers that ‘bleeds’ from embedded vessels near the holes or cracks and quickly seals them up, restoring structural integrity.
As well as the obvious safety benefits, this breakthrough could make it possible to design lighter aeroplanes in the future. This would lead to fuel savings, cutting costs for airlines and passengers and reducing carbon emissions too.
By mixing dye into the resin, any ‘self-mends’ could be made to show as colored patches that could easily be pinpointed during subsequent ground inspections, and a full repair carried out if necessary. The dye mixed with the resin would be ultra-violet fluorescent and so would not show up in normal lighting conditions.
This simple but ingenious technique also 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 therefore have an impact in all these fields.
In aircraft, FRP composites can be used in any part of the primary structure (fuselage, nose, wings, tailfin).
The technique’s innovative aspect involves filling the hollow glass fibers 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% of its original strength – comfortably allowing a plane to function at its normal operational load. The resin used in the self-repair system is an off-the-shelf, Araldite-like substance. The team are currently developing a custom-made resin optimised for use in the system.
“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 has 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.”
By further improving the already excellent safety characteristics of 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,” says Ian 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 around four years.
The 3-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.
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.