Composites whisper their problems

Tom Shelley reports on a technique that would allow composites to be used more confidently in safety critical commercial applications

By listening to tiny sounds occurring at the very beginning of composite damage, it is possible to detect damage long before it turns into clearly visible cracks or catastrophic failure.

Once detected at an early stage, such damage could be repaired before it resulted in serious consequences. Composites could then be used in more safety critical applications, with users confident that failure could not happen unexpectedly.

Acoustic Emission (AE) is the detection of very small, high frequency vibrations, arising from the onset of micro damage. Generally, such damage is well distributed throughout the composite and progresses with increasing load. It only coalesces to form a visible macroscopic fracture, shortly before complete failure.

Damage starts with matrix cracking, followed by fibre pullout, delamination and fibre breakage. In each case, the release of localised strain energy leads to elastic waves. The resulting surface responses from these elastic waves can be detected using piezoelectric sensors with bandwidth in the range, 20 kHz to 1 MHz. Surface response, however, is affected by wave propagation and the characteristics of the transducers.

Part of the research project under the direction of Processor Barry Jones, in the Brunel Centre for Manufacturing Metrology, is devoted to relating the responses of different types of transducers to the damage that is actually occurring. An important part of the project is the development of a Finite Element Analysis tool based on ANSYS, to study how the waves propagate and generally find out what is going on. The overall project is known as AESAD, Acoustic Emission traceable Sensing And signature Diagnostics. It is an Intersect flagship project, and has already resulted in three patent applications, and 12 post graduate degrees. NPL is using knowledge from ASEAD to develop traceable standards for acoustic emission, and the project team is undertaking research and consultancy with British Airways London Eye. The Brunel team is collaborating with 12 industrial potential users, including Corus, Rolls Royce, Unilever Research, Glaxo SmithKline and Process Analysis and Automation.

When the work is complete, it is expected to be possible to use commercial sensors to accurately locate and assess damage. Designers and users of both commercial and military aircraft are more than interested. The RAF, apparently, is particularly interested in being able to detect damage occurring as a result of stores loading and maintenance. Glaxo SmithKline is interested in monitoring the mechanical properties of their pills.

Professor Barry Jones

* Initial damage in stressed composites can be detected using surface acoustic transducers working in the range 20kHz to 1 MHz

* Damage can be 'heard' long before it becomes clearly visible

* Damage only becomes clearly visible shortly before failure

Tom Shelley

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