No abstract

This work concerned a technique for a health monitoring system based on the generation and sensing of Lamb waves in composite structures by thin surface-bonded piezoceramic transducers. The objective was to develop transducers that are adapted for the damage detection in orthotropic composites. The key problem with the investigated Lamb waves was to select a mode to be sensitive to the damage. A hybrid modeling technique was therefore used to conceive transducers that were adapted to achieve such a feature. This modeling technique enabled studying the influence of the transducer characteristics on the Lamb waves propagating in orthotropic plates. It was demonstrated that a Lamb mode could be generated dominantly to other modes by using a multi-element transducer. The effectiveness of this technique was successfully verified experimentally on composite plates. It was shown that the dominant Lamb mode, obtained by use of dual-element transducers, was an appropriate mode for successfully detecting a damage in composites.

A wavelet technique was used in an active system for the damage detection of aerospace composites. The active system was based on the generation and reception of Lamb waves by embedded piezoceramic transducers. The wavelets were used to decompose the Lamb-wave response into wavelet coefficients. The decomposition performance was improved by utilizing more adapted wavelets, based on the recurrent waveforms of Lamb waves. The changes in the Lamb waves interacting with damage in the plate were successfully characterized by this wavelet technique, through the amplitude change of the wavelet coefficients. The wavelet technique also showed great sensitivity in detecting damage of small sizes. This technique was found to be straightforward for detection of impact damage and evaluation of the damage size.