Composite components proposed for future structural applications include very thick sections that may be subjected to extremely high static and cyclic surface loads. There is a concern that minute material imperfections close to the outer surface region resulting from fabrication or surface finishing processes of these components may initiate delamination and eventual separation of the outer fibers during operation or cyclic testing. Conventional ultrasonic C-scan immersion techniques, commonly used for inspection of composite parts, are not sufficiently sensitive to identify small defects in thick composite structures which may be as large as 10 cm in thickness. Novel approaches based on ultrasonic Rayleigh waves have been successfuly applied to the outer fiber surface of a thick composite structure after repeated cyclic testing. The aim is to identify minute manufacturing imperfections close to the surface region and to determine if such flaws have caused cracks or delamination during the cyclic tests. In this paper, first the ultrasonic Rayleigh wave methods of flaw detection and ways of optimizing detection sensitivity and resolution will be described. Then, results of the optimization and sensitivity tests on carbon fibre composite specimens containing small simulated flaws 0.1 to 2 mm below the surface will be provided. Finally, the results of the application of the Rayleigh wave techniques on an actual thick composite section will be presented.
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