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A distributed sensing capability for in situ time-domain separation of Lamb waves

A distributed fibre Bragg grating array called the AMAP (Acoustic Mode Assessment Photonic) sensor is applied to spectrally decompose a complicated multi-modal packet of Lamb waves propagating in an aluminium plate. Pass-band filtering of the complex spectrum is then used to achieve a complete time-domain separation of the constituent modes. The use of a compact surface-mounted sensor array to unscramble a multi-modal Lamb wave packet represents a novel development with potentially important implications for ultrasonic methods of structural health monitoring and non-destructive testing, in particular acousto-ultrasonics and acoustic emission. It provides in principle an opportunity to better exploit the significant diagnostic potential of modes above the first cut-off frequency, which traditionally has been difficult because of the time-domain complexity of mixed signals in this regime. More generally, it enables selective time-domain reconstruction of specific modes of interest, eliminating interference from other modes, boundary reflections and noise. An experimental case study involving a high frequency multi-modal Lamb wave inspection is used to validate and demonstrate the approach.