Optical Fibre Sensing of High Order Lamb Waves for Structural Health Monitoring

Lamb waves provide arguably the best prospect for achieving a structural health monitoring (SHM) capability with broad diagnostic coverage at sensor densities that are not impractically high. The traditional approach in Lamb wave SHM is to employ a single mode, typically one of the fundamental modes, in a non-dispersive and easily excited regime, which is done largely to simplify the interpretation of the elastic wave dynamics. However, the diagnostic value of an interrogation conducted using only the fundamental modes is limited. In general, higher order modes offer potential for greater sensitivity to structural damage and greater scope for discriminating between different failure mechanisms. This paper reports on experimental work demonstrating an in-situ fibre Bragg grating (FBG) sensing capability for Lamb waves at frequencies of up to 2MHz, an achievement that represents an important step toward developing a more robust and versatile approach to Acousto-Ultrasonic SHM.