18 Décembre – Thesis defense - Valentin Serey

14 h Amphi Jean-Paul Dom - Laboratory IMS / building A31 (Talence campus)

Modal selectivity of ultrasonic waves in waveguides of finite cross-section using integrated piezoelectric elements for SHM.

SHM systems (Structural Health Monitoring) based on ultrasonic guided waves propagation are used for large structures,     extit{e.g.} in Aerospace or Civil Engineering. Lamb or SH waves are usually employed as they propagate over long distances in plate-like structures while probing the entire thickness. However less conventional modes propagate in waveguides with finite cross-section, such as bars, rails or pipes. The number of modes can be very high even at low frequency in this type of guide, and it is important to carefully select a specific mode. Current methods for modal selectivity, based on the use of several emitters, usually consider identical PZT elements (same sensitivity, same frequency response, etc.) and do not account for real experimental conditions and possible differences (variable coupling between transducers, flawed alignment, variable electronic response, etc.). This work presents a global methodology for modal selectivity in waveguides with finite cross-section, using several piezoelectric elements attached to their surface. This selectivity is based on experimental measurements, with a 3D laser vibrometer, of the amplitudes of the modes generated by each emitter. An optimization process allows to inverse the problem in order to maximize the amplitude of the desired mode, then generated by exciting all the emitters at once. This process requires knowing dispersion curves as well as the displacements of the various modes, calculated with SAFE 2D method. The methodology is tested through numerical simulations and experiments on an aluminium rectangular bar instrumented with 8 PZT elements on top. The method efficiency to generate different pure modes, and to detect and locate calibrated defects, is demonstrated for the aluminium bar. Its potential for SHM application of more complex structures is studied, like a rail or an adhesively bonded composite structure.

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