Contrôle de santé des matériaux et structures par analyse de la coda ultrasonore

Abstract : Ultrasonic coda is formed by multiple scattered ultrasonic waves (i.e. coda waves) propagating in a complex medium, the complexity of which comes either from the material’s micro-structure or the geometry of the media. Coda waves can probe the medium entirely and repeatedly because of their long and complex propagation paths. As a result, coda waves show a high sensitivity to the perturbation of the propagation medium, such as a variation in propagation velocity. By using Coda Wave interferometry (CWI), velocity variations can be determined from ultrasonic records with high accuracy (relative resolution of 0.001%). As in certain cases, variation in propagation velocity can be related directly to nonlinear elastic properties, which can be further connected to the damage state, CWI is considered a promising method for non destructive testing and evaluation (NDT&E) of material/structure. An experimental test is presented as an example of the CWI use on concrete for NDT&E purpose. For solving the experimental repeatability issue revealed in this test, an experimental protocol is designed to reduce the thermal bias in CWI results (i.e. the velocity variations caused by undesirable temperature fluctuations). This bias control protocol has been experimentally validated in a study of the acoustoelastic effect on concrete. Results showed that this protocol can remarkably improve the reliability of CWI results, and thus improves the experimental repeatability. Together with this bias control protocol, the CWI is then used for studying the behaviors of concrete under uni-axial load in direct tension. Both elastic (the acoustoealstic effect) and inelastic (the Kaiser effect) behaviors are observed via the CWI results. Effective value of acoustoelastic coefficient, which describes the importance of elastic nonlinearity of material, is then determined from the CWI results and used to detect an early-stage damage artificially induced to the concrete sample. The CWI is then used in a nonlinear modulation test for a global detection of defects (cracks) in an initially linear medium (glass). A high-amplitude acoustic excitation (the pump wave) and the coda waves (the probe wave) are simultaneously sent into glass samples. Due to the nonlinearity of the defects, acoustic mixing effect occurred. The observation of such effect, the pumpamplitude dependence of propagation velocity of coda waves, is the indication of damage. The use of a broadband pump wave makes possible to 1) detect the presence of defects globally for the entire structure without blind zone and 2) evaluate effective level of material's elastic nonlinearity, hence the assessment of damage level.
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Yuxiang Zhang. Contrôle de santé des matériaux et structures par analyse de la coda ultrasonore. Traitement du signal et de l'image [eess.SP]. Université Nantes Angers Le Mans, 2013. Français. ⟨tel-00971691⟩

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