Glowing report

2019-03-07 10:06:20

By Sharon Ann Holgate WHEN an aircraft collides with a bird, it can cause potentially catastrophic damage. But in planes made of carbon composites, such damage can be impossible to spot. So a team at Britain’s Defence Evaluation and Research Agency plans to use crystals that glow when fractured to warn of such unseen damage. “A one kilogram wrench dropped from one metre is enough to cause serious damage to a carbon composite structure,” says Glyn Davies of Imperial College, London. Unlike metal structures, which get obvious dents, the effects of these impacts on the carbon composites used in the latest military jets can be invisible. “Sixty per cent of the materials’ compressive strength could be lost due to impact damage, but the world can’t see it. The composite just springs back to shape, hiding the damage inside,” says Davies. This makes visual inspections for damage unreliable, so there’s a clear need for something that tells you if an aircraft has been hit, and where, he says. Ultrasound and X-ray techniques are both used, but these are slow and costly. Getting an aircraft back into the air quickly, or keeping it on the ground if it is badly damaged, can be vital. So researchers having been trying to develop smart materials that sense any damage as it happens. The DERA team, based at centres in Malvern and in Farnborough, has proposed measuring both the location and the extent of impact damage using “triboluminescent” materials that emit light if fractured. This is an effect you can see when someone crunches mints in a darkened room: tiny flashes of light can sometimes be glimpsed between the teeth. The flashes occur when positive and negative charges inside the crystals recombine. Some crystals with no rotational symmetry and some symmetrical ones with added impurities have proven good emitters (New Scientist, 24 May 1997, p 19). A feasibility study has showed that a variety of highly triboluminescent (TL) materials, ranging from a derivative of aspirin to metallo-organic compounds, could be included in carbon composite structures (Smart Materials and Structures, vol 8, p 504). The tests also show that the worse the damage, the more light the materials emit, and that light is emitted only if the crystals are fractured, so there’s no possibility of false alarms. The idea is to embed these TL materials in an aircraft’s structure and monitor their output using optical fibres linked to a detector (see Diagram). If the plane is struck, the number of crystals that flash and the intensity of the light they emit would give a real-time picture of the extent of the damage.FIG-22042301.jpg To locate the damage, crystals that emit light at different wavelengths could be embedded in different places. Alternatively, the colour of the light produced by crystals in different areas could be altered using conventional fluorescent laser dyes. “We plan to investigate the most efficient ways of coupling the light output from the sensors to the detector, and to develop a representative demonstration,