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Technology update
Light-emitting crystals reveal damage
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DERA scientists are using triboluminiscent crystals to help detect damage to composite materials.
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The use of carbon composites on aircraft bring many benefits, including weight reduction, but a tradeoff has always been detection of accident damage and the extent of that damage. In the UK, the Defence Evaluation and Research Agency (DERA) is working on a potential solution using "smart" techniques with particular focus on investigating impact damage such as the effects of birdstrike. "The impact of a bird striking an aircraft can cause catastrophic results if the damage is not discovered," said DERA, which is currently investigating new detection methods using triboluminescent (TL) crystals. "Unlike metal, the carbon composites used to cover modern military aircraft may spring back to their original shape, unmarked, after impact. However, there could be internal damage and up to 60% of the material's compressive strength could be lost."
The DERA work involves the use of embedded, 5.0-µm diameter crystals beneath the carbon composite covering. These give a flash of light when broken, thus highlighting an area that has sustained damage but which might otherwise be invisible. TL crystals, though, even provide gradation of the light to indicate how severe damage may be. According to DERA, the output from the TL crystals is monitored using optical fibers linked to a detector, and the intensity of the light emissions indicates the severity of the damage. Different wavelengths or colors of the crystals' light emission can be used in specific places on the aircraft body to indicate the location of the damage.
The TL crystals project is being headed up by Grant Bourhill and Ian Sage of DERA's Electronics Sector. They stated that conventional methods of X-ray and ultrasonic tests were unable to detect a very small fracture in a sample of high-performance resin similar to that used on aircraft and analyzed by the Structural Department of DERA. "However, we had hard evidence through the TL results that the sample had sustained damage," said Bourhill. "We now know that the TL crystals are a good fracture indicator - however small the fracture."
As well as showing birdstrike damage, the TL crystals technique could be used if damage were caused to an aircraft during routine servicing - a tool being accidentally dropped on a surface during repair, or a fuel nozzle striking the surface during refueling. Severe weather conditions, such as hailstones, can also cause damage.
For use on a production aircraft, the TL crystals could be applied to its skin in a similar way to dope on a fabric surface. It would be the effect of impact and the subsequent cracking of the surface that would cause the telltale flash of light. High-energy impacts would result in a brighter light emission. Crystals being investigated by DERA as possibly being suitable for use on aircraft include metal-organic types containing manganese, europium, or terbium. Organic crystals are also under test, with emission color "tuned" to test requirements. If the crystals are stressed rather than broken by impact they do not emit light, says DERA, and it is possible also to "tune" the crystals to emit light if serious damage has occurred. In fact, this could be taken to a point where it would alert an aircrew member that a critical event had occurred.
Stuart Birch
Aerospace Engineering April 2000
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