The design of embedded transducers for structural health monitoring applications

This paper describes a theoretical and experimental investigation concerning embedded piezoelectric transducers employed principally for condition monitoring of engineering composites. Both interdigital transducers (IDTs) and plate transducers are investigated with the aim of assessing their efficiency as uni-modal Lamb wave transmitters. The IDT configuration comprises a piezocomposite layer sandwiched between two flexible printed circuit boards, where the interdigital electrode spacing corresponds to the wavelength of the desired Lamb wave mode. The alternative configuration comprises a thin piezoceramic plate for which the lateral dimensions are chosen to efficiently couple energy into the desired mode. For both types of transducer, finite element models have been successfully employed to establish the design requirements for generating the zero order symmetrical mode (So) without simultaneously generating the zero order anti-symmetrical mode (Ao), which exhibits strong velocity dispersion. In this investigation the Ao mode is regarded as coherent noise. Generation of a pure So mode is shown to require positioning of the transducer at a depth which is exactly half way between the top and bottom faces of the plate-like structure within which it is embedded. For structural monitoring, the plate-type transducer is shown to be more suitable than the IDT. A scanning laser vibrometer was used to verify many of the theoretical findings.