Detection and localization of unbounded contacts in industrial structures are crucial for user safety. However, most structural health monitoring techniques are either invasive, power-consuming, or rely on time-varying baseline comparison. A passive acoustic method is proposed to localize unbounded contacts in plate-like structures, using the acoustic emissions by the contacts when they are excited by ambient noise. The technique consists of computing the correlation matrix of the signals measured by a set of receivers and applying to this matrix a beamforming algorithm accounting for flexural wave dispersion. To validate the technique, an experimental setup is developed in which three idealized unbounded contacts are created on a thin plate excited by a shaker. How the quality of the defect localization depends on the defect type, receiver number, and the characteristics of the noise is investigated. Finally, it is shown that the localization of unbounded contacts is possible using either an acoustic ambient noise source or a more realistic jet engine noise.
Modal analysis is a major issue in the industry to identify resonances in mechanical parts. Indeed, resonances can induce high vibration levels that are potentially destructive. Active modal analysis methods require on the one hand to excite the controlled part and on the other hand to record the induced displacements/acceleration/stresses. Conventional methods, called SIMO, involve a mechanical excitation source associated with several receivers. More rarely, the analysis is performed with several sources and one receiver (MISO). Finally, for several years, MIMO techniques with several sources and several receivers are commonly implemented. However, for some parts, it may be difficult to implement an array of mechanical sources. That is why, here, we propose to carry out the modal analysis with a set of 8 loudspeakers as sources, and the measurements are performed at a large number of measurement points using a LASER vibrometer. The analysis of the singular value decomposition of the contactless transmission matrix allows to identify superposed modes. We will discuss the advantages and disadvantages of such a configuration. This method based on acousto-elastic coupling is successfully applied to the modal analysis of two axi-symmetric parts: a pinion and a rotating wheel. Finally, we will see that the same device can be used to control vibration fields.
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