Wideband surface acoustic wave (SAW) generation with a spatial chirp-based interdigital transducer was optimized for non-destructive characterization and testing of coatings and thin layers. The use of impulse temporal excitation (Dirac-type negative pulse) leads to a wide band emitter excitation but with significantly limited SAW output amplitudes due to the piezoelectric crystal breakdown voltage. This limitation can be circumvented by applying a temporal chirp excitation corresponding in terms of frequency band and duration to the spatial chirp transducer configuration. This dual temporal-spatial chirp method was studied in the 20 to 125 MHz frequency range and allowed to obtain higher SAW displacement amplitudes with an excitation voltage lower than that of the impulse excitation.

In order to accurately examine structures surfaces, it is interesting to use surface SAW (Surface Acoustic Wave). Such waves are well suited for example to detect early emerging cracks or to test the quality of a coating. In addition, when coatings are thin or when emergent cracks are precocious, it is necessary to excite surface waves beyond 10MHz. Finally, when structures are not flat, it makes sense to have flexible or conformable sensors for their characterization. To address this problem, we propose to develop SAW type of interdigital sensors (or IDT for InterDigital Transducer), based on flexible piezoelectric plates. Initially, in order to optimize these sensors, we modeled the behavior of these sensors and identified the optimum characteristic sizes. In particular, the thickness of the piezoelectric plate and the width of the interdigital electrodes have been studied. Secondly, we made composites based on barium titanate foams in order to have flexible piezoelectric plates and to carry out thereafter sensors. Then, we studied several techniques in order to optimize the interdigitated electrodes deposition on this type of material. One of the difficulties concerns the fineness of these electrodes because the ratio between the length (typically several millimeters) and the width (a few tens of micrometers) of electrodes is very high. Finally, mechanical, electrical and acoustical characterizations of the sensors deposited on aluminum substrates were able to show the quality of our achievement.

This study deals with the optimization of transducers for Rayleigh-type Surface Acoustic Waves (SAW) generation. These transducers are based on Interdigital Transducers (IDT) and are specifically developed to characterize properties of thin layers, coatings, and functional surfaces. In order to characterize these coatings and structures, it is necessary to work with wide bandwidths IDT operating in the high frequencies. Therefore, in this study a spatial chirp-based on an IDT are realized for wideband SAW generation. The use of impulse temporal excitation (Dirac-type negative pulse) leads to a wide band emitter excitation but with significantly limited SAW output amplitudes due to the piezoelectric crystal breakdown voltage. This limitation can be circumvented by applying a temporal chirp excitation corresponding in terms of frequency band and duration to the spatial chirp transducer configuration. This dual temporal-spatial chirp method was studied in the 20 to 125 MHz frequency range and allowed to obtain higher SAW displacement amplitudes with an excitation voltage lower than that of the impulse excitation.

The study of dispersive behavior of guided waves is of great interest in non-destructive testing and development of inspection systems. In this work, a semi-analytical numerical formulation for the computation of dispersion curves and mode shapes in plate-like structures is presented. The formulation is based on a discontinuous Galerkin Finite Element Method. The discretisation of the ordinary differential system in the through-thickness direction yields an eigenvalue problem. The resolution of the latter for a frequency range provides the dispersion curves. The study focuses on Lamb modes in functionally graded material plates with traction-free boundary conditions. The influence of the gradient variations is demonstrated. First, a multilayered approximation consisting of homogeneous laminate is used. Afterwards, the gradients of the parameters are added to the formulation. Numerical examples are presented and compared with those found in the literature. The results are free of spurious modes and show an excellent agreement for all cases, specially for continuously varying properties. The method is very efficient and numerically stable for small and large wave numbers. It was found that a high accuracy is obtained when high-order elements are used on a relatively coarse mesh.