BAE Systems is planning tests of a sensor system for military jets capable of listening for cracks in composite and metal structures during flight.

The system, which detects sound waves — or acoustic emissions — produced by cracks within aircraft components during flight, is being developed as part of a European defence programme, Ahmos-2, to evaluate various structural health monitoring techniques.

In the UK BAE is carrying out the work for the MoD, and the company plans to flight test the system on a Hawk jet within the next two to three years, according to Dr Peter Foote, executive scientist at BAE’s Advanced Technology Centre.

Using acoustic emissions to detect fractures is seen as a promising technique for aircraft structural health monitoring, particularly for composite components where damage can be difficult to spot.

Weakened composite areas contain broken fragments of resin, which crunch and grind together during flight to produce short bursts of high-frequency sound. With metal parts the natural flexing motion in flight causes the crack to propagate, creating acoustic emissions. Ceramic piezoelectric sensors fitted to the structure can detect these short bursts of sound. The emissions are typically in the 100Hz to 1MHz range, so other aircraft sounds including engine and aerodynamic noise, usually lower frequency, can be filtered out.

Unlike other technologies the system does not require an optical imager to locate the damaged area. The emissions travel through the structure at a given velocity, and reach each of the sensors dotted around the aircraft at different times, allowing the location of the original burst to be calculated through triangulation.

‘The data is recorded during flight, allowing the system to build up a map of damage. For a commercial plane this data would be downloaded on the ground at a scheduled maintenance check as part of the inspection process.

‘The big potential payoff would be that inaccessible bays could be monitored by the system, providing ground crew with diagnostic information without them having to tear the aircraft apart, as they do now during main checks,’ said Foote.

The Ahmos-2 project, which also includes Qinetiq, UK fibre optics specialist Smart Fibres and EADS, is part of the European Co-operation for the Long-term in Defence (EUCLID) programme. The project follows the first Ahmos programme, in which the firms began developing the different sensor technologies.

BAE has been carrying out ground tests using representative structures, and over the next few months will be performing a design review of the various sub-systems and further laboratory-based tests of the instrumentation.

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