Fibre optic sensors for lamb wave detection

An increasing interest has been shown in the use of Lamb waves for structural health monitoring. Since they propagate along, rather than normal to (as in the case of C-scanning), the plane of a plate, these waves can be used to interrogate a large area of the sample without the need to scan the source and detection transducers. The conventional way of detecting these waves is by the use of PZT transducers similar to those used to generate the ultrasound. Here we describe and contrast two different methods of using fibre optic sensors for this application. Firstly, a polarimetric method is demonstrated that uses analysis of the birefringence induced in an embedded optical fibre. This is a technique that requires only basic, inexpensive optical equipment and standard telecommunications grade optical fibre. The directional properties of this sensor can be utilised by optimising the relative positions of the acoustic source and the fibre so as to maximise the damage-to-transmitter source signal ratio. Secondly, the use of fibre Bragg gratings for ultrasound detection is described, together with a discussion on the importance of optimising various parameters, such as grating length and strength, fibre recoating material, bonding technique and interrogation method. It is shown that if three FBGs are configured into a rosette formation, the relative amplitudes of the signal in each sensor can be used to obtain the direction from which an acoustic wave is arriving. If an array of two or more rosettes is employed, the location of the source can be determined by calculating the intersection of the two wave propagation directions. This idea is further developed to show that if the damage signals can be effectively separated from the transmitter source signals, then the location of the damage can be similarly obtained. Experimental results are shown that demonstrate the successful practical implementation of all the above concepts