Shape of things to comeSelf-repairing aircraft may revolutionize aviation safety

Published 21 May 2008

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

You may recall the incident a few years ago of a plane flying off the coast of California: Undetected cracks in the top of the fuselage caused part of the plane’s top to be sheared off during flight. The plane managed to land with the scared passengers exposed to the elements. A new technique may well prevent this type of accident. 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. This is how it works: If a tiny hole or crack appears in the aircraft (due to wear and tear, fatigue, a stone striking the plane, etc), epoxy resin would “bleed” from embedded vessels near the crack and quickly seal it up, restoring structural integrity. 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. This simple but ingenious technique, similar to the bruising and bleeding/healing processes we see after we cut ourselves, has been developed by aerospace engineers at Bristol University, with funding from the Engineering and Physical Sciences Research Council (EPSRC). It has potential to be applied wherever fiber-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.

The technique’s innovative aspect involves filling the hollow glass fibers contained in FRP composites with resin and hardener. If the fibers break, the resin and hardener ooze out, enabling the composite to recover up to 80-90 percent 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,” 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 aluminum-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 Bond. “We’re also developing systems where the healing agent isn’t contained in individual glass fibers 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 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, 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, nose, wings, tailfin). 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 optimized 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 fibers 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.