Detection of ultrasonic Lamb waves in composite plates using optical-fibres

This paper presents experimental results for a new condition monitoring system that uses a combination of piezoelectric and fibre-optic technologies as the active and passive elements respectively. Composite plate samples have been manufactured containing two groups of optical-fibres (typically four strands per group) embedded across the length of the plate. Polyimide coated 4/125 μm single-mode glass fibre has been utilised and connected to a portable 633 nm Mach-Zehnder fibre interferometer for signal demodulation and subsequent data acquisition. For this present work, 1-3 connectivity piezocomposite transducers have been used to generate a known Lamb wave within carbon fibre reinforced composite plate (CFRP) structures. The S₀ mode was selected due to the relatively large in-plane motion and low dispersion characteristics. Results using both externally surface bonded fibres and embedded fibres are presented, for a range of defective plates. Defects ranging from 1 mm diameter holes through to significant delaminations have been detected at an ultrasonic operating frequency of 285 kHz