Repairs on the fly

‘Bleeding’ composites that repair themselves in mid-flight while alerting maintenance crews to the site of damage could be used on spacecraft and unmanned aerial vehicles within the next five to 10 years.

Researchers at Bristol University (with funding from ESA and the EPSRC) are developing composites with hollow fibres that mimic blood vessels by releasing uncured resin if fractured in a collision with a foreign object.

As the fibre breaks, the stored resin and a hardener combine and leak into the damaged areas. The resin does not permanently repair the damage, as it is unable to fix the fibres themselves, but would strengthen the area until the end of the flight and prevent the crack getting worse, said Dr. Ian Bond, a member of the structures and materials research group at Bristol.

‘It wouldn’t be a permanent repair, but it would get you home. ESA is interested in the approach for space applications, as once you launch something into space you can’t fix it easily,’ Bond said.

The fibres would also release an ultraviolet fluorescent dye, making the damaged area easier to spot, said Bond. ‘If there is an impact on the aircraft skin you get an initial dent, but then the dent disappears when the material is stressed again, so you don’t know where the craft has been hit, while the rear face of the structure could be badly damaged. The dye leaves a fluorescent sign that can be inspected with a UV lamp,’ he said.

The size of this patch of dye can also provide an indication of the size and energy of the impact, he added. ‘It’s a quick and dirty method of spotting where and how hard you have been hit.’

Hollow glass fibres would also offer some impact protection when used as a surface covering over conventional glass fibre and epoxy resin. The hollow fibres are crushed on impact, thereby absorbing energy and helping to protect the material underneath.

Researchers at the university have tested a series of structures, some produced with and some without the uncured resin and dye within the hollow fibre plies. When damaged, the bleeding dye was shown to highlight the area, while the stored resin restored 90 per cent of the mechanical strength of the component.

While the technology could also be used in military or unmanned aerial vehicles, safety certification issues mean it is unlikely to be used on civil aircraft in the near future. ‘For civil aircraft the design philosophy is that you assume damage from day one and have to prove that the damage will not grow, so there won’t be such a drive for self-repairing systems immediately,’ said Bond.

The researchers still have problems to overcome before the technology can be developed into a commercial system, in particular tackling multiple impacts on the same spot. At present, the system will not self-repair itself if it is hit in the same place twice.

Bond is also investigating the use of the fibres to create self-cooling structures. Passing a coolant through micro-capillaries would reduce the temperature of the structure, which could be particularly important for aircraft travelling at high speeds, where the skin can get extremely hot.