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Evaluation of Lamb wave performance for the health monitoring of composites using bonded piezoelectric transducers

Specific characteristics of Lamb waves, and more particularly their ability to propagate over long distances, make them a promising technique for health monitoring of composite structures using piezoelectric transducers bonded on the surface or embedded inside the material. In order to evaluate the possibilities of the technique, a theoretical dispersion model for the Lamb waves propagation has been realized for a carbon-epoxy composite material, and the control parameters, incidence angle, emission mode and frequency, have been fixed. The technique has then been experimented in a transmission mode with contact conversion mode transducers. The propagation over long distance and the detection of delaminations have been put in evidence. Then, optimized transducers have been manufactured in order to instrument, first, monolithic composite structures using surface bonding, and second, sandwich structures with embedded elements in the bulk. Experimental results show that a large network could be envisaged. This technique presents serious interests compared to conventional ultrasonic techniques, testing at a point, or passive measurements of local stress and strain modifications due to the delamination.