Active Health Monitoring of Aerospace Composite Structures by Embedded Piezoceramic Transducers

The objectives of the thesis work were to study the interaction between embedded piezoceramic transducers and composite structures as well as determine techniques to simplify the Lamb waves analysis. Firstly, this study considered the design of the embedded piezoceramic transducers. Secondly, the effect of the embedded transducer on the composite strength as well as the influence of the mechanically loaded composite on the characteristics of the embedded transducer were investigated. Finally, to simplify the analysis of such complex Lamb wave responses, two techniques were developed. They were based on the wavelet technique and a modelling technique, respectively. The design of the embedded piezoceramic transducers was improved by reducing the stress concentrations in the composite as well as in all components constituting the piezoceramic transducer, that is, the piezoceramic element, interconnector and conductive adhesive. The numerical analysis showed that the thickness of the interconnector had no significant influence on the stress state of the piezoceramic transducer. It was also found that a compliant conductive adhesive reduced the stress concentration located at the edge of the piezoceramic element. The structural integrity of composites embedded with the improved piezoceramic transducer was investigated. The experiments, performed in tensile and compressive static loading, indicated that the strength of the composite was not significantly reduced by the embedded piezoceramic transducer. Further investigations were conducted to evaluate the performance of the improved piezoceramic transducer used as a Lamb wave generator embedded in composites subjected to mechanical loading. The tests were conducted in tensile and compressive static loading as well as fatigue loading. The study showed a large working range of the embedded piezoceramic transducer. A post processing technique based on the wavelets was further assessed in the detection of damage and in the damage size evaluation. A new wavelet basis was developed specially for processing the Lamb wave response. This method, focused on the wavelet coefficients from the decomposition Lamb wave response, showed promising results in evaluating the damage size. The wavelets offered a sensitive tool to detect small damage, compared to other detection methods, improving the damage detection capabilities. The other technique was devoted to the simplification of the generated Lamb waves by the use of multielement transducers. The transducers were designed using both a normal-mode expansion and a FE-method. This technique allowed reducing the effect of a Lamb wave mode towards another. This technique was successfully implemented in a damage detection system in composites.