In this paper the evolution of an ultrasonic cavitation bubble field is studied by means of the void rate, the cavitation noise power and the electric power that feeds the ultrasonic transducer. The ultrasonic irradiation is performed in chopped mode. It is observed two kinds of evolution that differ respectively by a slow (A regime) and a fast (B regime) void rate increase. They correspond to cavitation regimes that are identified as respectively a stable cavitation and an inertial cavitation field. The beginning of the fast increase of the void rate is delayed from the irradiation start and this delay depends on the chopping frequency of the irradiation mode.

The aim of this study is to control aeronautical composite structures on production line since they are extremely vulnerable to impact damages due to tool drops for example. To detect such an event on the structure, a piezoelectric wireless sensor network has to be developed. Therefore the feasibility of an innovative technique based on a piezoelectric harvesting device is presented. The system carries out structural health monitoring tasks using piezoelectric transducers bonded onto the structure and is also fully autonomous since the same transducers are used to convert the mechanical vibrations of the structure into electrical power. As vibrations are not available during the production process, the autonomy of the system is provided thanks to a Lamb wave emitter located in the sensor array. After measurements, the system is able to harvest 7?36 milli-watts for a 100 milli-watts mechanical power applied to the structure. This electrical power can be used both by the electronic detector and the WIFI transmitter for the detection of impacts of less than 1 Joule.

Cross-talk in acoustical transducer arrays is an undesirable phenomenon which decreases seriously the performances of these arrays. Indeed, when one element of the array is driven, it generates parasitic displacement fields at the radiating surfaces of the neighboring elements, which changes the directivity of the antenna. To well understand this phenomenon a transducer array similar to those used in medical imaging and NDT applications was modelled by finite element method. The research work investigated systematically the effects of the cross-talk. Firstly, it inspected the acoustical and mechanical cross-talk throughout the propagating medium and the filling material. Secondly, it studied the influence of the matching layer on the acoustical performances of the transducer. It was shown that the filling material and the matching layer are the major factors contributing to this phenomenon. In order to cancel the cross-talk a correction method previously developed by the author has been used. This solution consisted in applying adapted electrical voltages on each neighboring element of the active one in the purpose to reduce the displacement field on their active surface. This method was tested numerically and the obtained results clearly demonstrated its ability to reduce the cross-talk.

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Abstract. Electromagnetic Acoustic Transducers (EMATs) allow non-contact ultrasonic measurements in order to characterize structures for a wide range of applications. Considering non-ferromagnetic metal materials, excitation of elastic waves is due to Lorentz forces that result from an applied magnetic field and induced eddy currents in a near surface region of the sample. EMAT's design is based on a magnet structure associated with a coil leading to multiple configurations, which are able to excite bulk and guided acoustic waves. In this work, we first present a self-developed EMAT system composed of multiple emission and reception channels. In a second part, we propose an original method in order to determine the elastic constants of an isotropic material. To achieve this goal, Rayleigh and shear waves are used and the advantages of this method are clearly highlighted. The results obtained are then compared with conventional measurements achieved with piezoelectric transducers.