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Sounding out Danger in Aircraft

26 March 2004

Potentially dangerous cracks in composite and metal aircraft components, including wheels and turbine blades, could be quickly detected using an ultrasound device being developed in the UK.

The technology, which could also be used to detect faults in components within structures such as ships and nuclear power plants, is being developed at the Research Centre in Nondestructive Evaluation, based at Imperial College, with support from the EPSRC, Rolls-Royce, Airbus, BNFL and the Defence Science and Technology Laboratory (DSTL).

The technology uses a pulse of high-power ultrasound, in the 20-100kHz frequency range, which causes the component to vibrate. If there is a crack within the structure, this vibration causes the surfaces of the crack to rub against each other, generating heat. This heat can then be detected using a thermal imaging camera.

Conventional tests involve disassembly of the aircraft and subjecting individual components to standard ultrasound examination - a more time-consuming process. The device can be used to detect cracks in metal and delaminations in composite components, either in service or for quality control as they come off the production line. Airbus in particular is interested in the technology for its ability to spot delaminations within composite components, according to Prof Peter Cawley, group leader for non-destructive testing at Imperial College.

These delaminations are created when the layers of carbon-fibre peel away from each other as a result of impact-related stresses, or when water penetrates the layers through repeated use in hot and humid conditions.

Such separations may not be visible to the naked eye, and as a result manufacturers and research bodies have long been searching for a device to quickly inspect the growing number of composite components within aircraft. The technology is simple to use, and could analyse larger components such as wings, by testing a section at a time, said Cawley. 'It is very much quicker to use [than existing systems] as it can test large areas in one go, and you do not have to worry about angling the beam in a certain way, as you do with other ultrasound devices.'

BNFL is interested in the technology's potential for inspecting difficult-to-reach sections of its power plants, where minimum human access is permitted, while DSTL wants to use it to analyse military aircraft and ship components, he said. The technique was originally developed by researchers in the US, but no one has yet been able to develop the method into a practical system suitable for regular use in inspection regimes, which the team now hopes to do.

But although the technology is still at an early stage, and it is likely to be two to three years before the researchers are ready to begin producing a prototype, the team is already considering taking the idea a step further.

By opening and closing the crack within the metal or composite component, causing the rubbing effect that generates heat, the ultrasound signal itself is distorted. This distortion can then be detected, allowing the crack to be identified. Such a system would be simpler to use than a thermal imaging camera, and could also be more effective, said Cawley.

'Our current findings are that we can detect cracks at lower ultrasonic amplitude than is required to use the thermal imaging system. So therefore it is a more sensitive test, and there is less concern about the potential damage to the component,' he said. The research centre is made up of a consortium of universities, led by Imperial College and Strathclyde University, and also includes Bristol, Bath, Nottingham and Warwick universities.






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