Future aircraft might repair themselves

Dye mixed into the resin could make repaired damage show up as colored patches that could easily be spotted during subsequent ground inspections, so that full repairs could be carried out if necessary. Such dye would not show up in normal lighting conditions, only visible when exposed to ultraviolet light. This way, the system will "complement rather than replace conventional inspection and maintenance routines, which can readily pick up larger-scale damage, caused by a bird strike, for example," Bond said.

These resin-loaded fibers could find use wherever fiber-reinforced polymer composites are used. Such lightweight, high-performance materials are proving increasingly popular not only in aircraft but also in cars, wind turbines and even spacecraft. The new self-repair system could therefore have an impact in all these fields.

Circulating scheme
The scientists are also developing systems where the healing agent is not contained in individual fibers, but can actually move around in a network of tubes, "just like the circulatory systems found in animals and plants," Bond said.

"Such a system could have its healing agent refilled or replaced and could repeatedly heal a structure throughout its lifetime," he added. "Furthermore, it offers potential for developing other biological-type functions in man-made structures, such as controlling temperature or distributing energy sources."

Currently Bond and his colleagues are working with hollow glass fibers loaded with an off-the-shelf resin. They are currently developing a custom-made resin optimized for use in the system.

"Current epoxy systems have to be mixed in precise ratios, such as two parts of one ingredient with one part of another," Bond said. "We'd need the resin to be much more tolerant of different mix ratios, because we can't control the situation in the air."

"And we'd need it to be low viscosity, and fairly stable — it needs to last a long time," he added. "It could be sitting in an aircraft's structure for quite long periods of time, maybe years, and still needs to be reactive when called upon."

The resin would need enough time to ooze into any crack before hardening, yet ideally work quickly enough to have an effect when the aircraft is still flying. Control of the hardening of the resin would get complicated by the cold temperatures found in the high altitudes that planes typically fly at.

"I think such a resin is achievable," Bond said. "It's just that no one's ever been asked to make such a resin."

A working system could be up in the next five years, Bond suggested.

The scientists received funding from the United Kingdom's Engineering and Physical Sciences Research Council.

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