06 Décembre – Thesis defense - Paul Zabbal

14 h Room 33 - CEA Saclay / building Digiteo Labs

Conception of a non-destructive ultrasonic inspection technique of a bonded structure using a reverberant cavity with time reversal process.

Bonding is expanding in the industry to replace former assembly techniques, as it allows lighter structures, better distribution of stresses and the assembly of different materials such as composites. However, one of the obstacles to this continued growth is the possibility of non-destructive evaluation of the adhesion quality of a bonded structure. In this thesis we are interested in developing a method for controlling the adhesion of bonded samples representative of an industrial case, namely metal samples with a thin adhesive. The defects were inserted into the samples at the interface between one of the adherents and the adhesive, creating an adhesive defect, which constitutes the most critical defects for the industry. After showing the limitations of conventional linear methods, algorithms for optimizing dispersion curve imaging have been developed. However, despite these improvements, guided waves have not been sufficiently sensitive to weak interfaces in this configuration, for thin adhesives and when an uncertainty of material thickness is accepted. Thus, control methods exploiting the non-linear interaction of contact defects subjected to a high amplitude ultrasonic wave are studied because they are subjected to less constraints. To be able to measure these non-linearities, it is necessary to generate ultrasound with sufficient energy to sollicitate the bonded assembly, which generally involves the use of electronic power devices that are themselves intrinsically non-linear. To overcome these technological limitations, we have developed a dedicated device, using excited translators with low voltages, bonded to a solid reverberant object, all forming a full-fledged sensor. Thanks to the emission of complex signals on the translators, the acoustic energy is concentrated at the output of the device, in order to generate waves of high amplitudes on the surface of a sample to be controlled. We present inspection results for bonded samples with adhesion defects: teflon spray, fingerprints, etc. These ultrasonic measurements are then correlated with mechanical tests to validate the ultrasonic method.

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