Réflexion et conversion d'une onde de Lamb à l'extrémité biseautée d'une plaque

Jury composé de : Gérard Maze (Président), Marc Gazalet (Rapporteur), Bertrand Dubus (Rapporteur), Koen Van Den Abeele, Marc Deschamps, Jean Duclos (Directeur de thèse), Hugues Duflo, Alain TinelThis study deals with the Lamb wave interaction on a bevel. The aim is to identify the different reflected modes which exist when a wave is incident on the bevelled end of a plate and to evaluate the respective energy of each mode. In a first time, the study is experimentally realised on a stainless steel plate. Energy balances are performed from the displacement measurements on the surface by laser interferometry. They are corrected to take into account wave diffraction. A theoretical model, based on a modal decomposition, is also done in order to justify the previous results. Computations of the energy reflection coefficients have been realised when the A0, S0 and A1 Lamb modes are incident on plate ends bevelled from 65 to 90°, for a frequency-thickness product included between 1 and 4 MHz mm. The quantitative balances obtained by this method agree with the experimental results. The theoretical computation reveals the importance of “complex modes” at the plate extremity. A finite element simulation confirms their effect on the reflections and allows to evaluate the distance from the bevel where the reflected Lamb waves are alone. Finally, a study is done to know the influence of a bevel on the “edge mode” behaviour - the “edge mode” is a particular phenomenon which appears in a narrow frequency band when the S0 Lamb mode is incident.Ce travail est consacré à l'étude de l'interaction d'ondes de Lamb avec un biseau. L'objectif est d'identifier les différents modes propagatifs issus de la conversion de l'onde incidente sur l'extrémité biseautée d'une plaque et d'évaluer l'énergie transportée par chaque mode. Dans un premier temps, cette étude est réalisée expérimentalement sur une plaque d'acier inoxydable. Les bilans d'énergie sont déduits des mesures des déplacements en surface par interférométrie laser. Ils sont corrigés pour tenir compte de la diffraction des ondes ultrasonores. Un modèle théorique, basé sur une décomposition modale, est ensuite mis en œuvre afin de justifier les résultats précédents. Le calcul des coefficients de réflexion en énergie a été mené dans le cas des modes de Lamb A0, S0 et A1 incidents sur des extrémités biseautées de 65 à 90° pour un produit fréquence-épaisseur FE variant de 1 à 4 MHz.mm. Les bilans quantitatifs obtenus par cette méthode valident les résultats expérimentaux. Le calcul théorique met en évidence l'importance des « modes complexes » existant dans la plaque. Une simulation par éléments finis a montré leur influence sur les réflexions et a permis d'évaluer à quelle distance du biseau les modes de Lamb se propagent seuls. Enfin, une étude est effectuée pour décrire l'influence d'un biseau sur le comportement du « mode de bout », phénomène particulier pouvant se produire à l'extrémité d'une plaque dans une gamme fréquentielle étroite lorsque le mode de Lamb S0 est incident

Réflexion et conversion d'une onde de Lamb à l'extrémité biseautée d'une plaque

Jury composé de : Gérard Maze (Président), Marc Gazalet (Rapporteur), Bertrand Dubus (Rapporteur), Koen Van Den Abeele, Marc Deschamps, Jean Duclos (Directeur de thèse), Hugues Duflo, Alain TinelThis study deals with the Lamb wave interaction on a bevel. The aim is to identify the different reflected modes which exist when a wave is incident on the bevelled end of a plate and to evaluate the respective energy of each mode. In a first time, the study is experimentally realised on a stainless steel plate. Energy balances are performed from the displacement measurements on the surface by laser interferometry. They are corrected to take into account wave diffraction. A theoretical model, based on a modal decomposition, is also done in order to justify the previous results. Computations of the energy reflection coefficients have been realised when the A0, S0 and A1 Lamb modes are incident on plate ends bevelled from 65 to 90°, for a frequency-thickness product included between 1 and 4 MHz mm. The quantitative balances obtained by this method agree with the experimental results. The theoretical computation reveals the importance of “complex modes” at the plate extremity. A finite element simulation confirms their effect on the reflections and allows to evaluate the distance from the bevel where the reflected Lamb waves are alone. Finally, a study is done to know the influence of a bevel on the “edge mode” behaviour - the “edge mode” is a particular phenomenon which appears in a narrow frequency band when the S0 Lamb mode is incident.Ce travail est consacré à l'étude de l'interaction d'ondes de Lamb avec un biseau. L'objectif est d'identifier les différents modes propagatifs issus de la conversion de l'onde incidente sur l'extrémité biseautée d'une plaque et d'évaluer l'énergie transportée par chaque mode. Dans un premier temps, cette étude est réalisée expérimentalement sur une plaque d'acier inoxydable. Les bilans d'énergie sont déduits des mesures des déplacements en surface par interférométrie laser. Ils sont corrigés pour tenir compte de la diffraction des ondes ultrasonores. Un modèle théorique, basé sur une décomposition modale, est ensuite mis en œuvre afin de justifier les résultats précédents. Le calcul des coefficients de réflexion en énergie a été mené dans le cas des modes de Lamb A0, S0 et A1 incidents sur des extrémités biseautées de 65 à 90° pour un produit fréquence-épaisseur FE variant de 1 à 4 MHz.mm. Les bilans quantitatifs obtenus par cette méthode valident les résultats expérimentaux. Le calcul théorique met en évidence l'importance des « modes complexes » existant dans la plaque. Une simulation par éléments finis a montré leur influence sur les réflexions et a permis d'évaluer à quelle distance du biseau les modes de Lamb se propagent seuls. Enfin, une étude est effectuée pour décrire l'influence d'un biseau sur le comportement du « mode de bout », phénomène particulier pouvant se produire à l'extrémité d'une plaque dans une gamme fréquentielle étroite lorsque le mode de Lamb S0 est incident

Transient space-time surface waves characterization using Gabor analysis

Laser ultrasonics allow the observation of transient surface waves along their propagation media and their interaction with encountered objects like cracks, holes, borders. In order to characterize and localize these transient aspects in the Space-Time-Wave number-Frequency domains, the 1D, 2D and 3D Gabor transforms are presented. The Gabor transform enables the identification of several properties of the local wavefronts such as their shape, wavelength, frequency, attenuation, group velocity and the full conversion sequence along propagation. The ability of local properties identification by Gabor transform is illustrated by two experimental studies: Lamb waves generated by an annular source on a circular quartz andLamb wave interaction with a fluid droplet. In both cases, results obtained with Gabor transform enable ones to identify the observed local waves.status: publishe

Monitoring of Engineered Stones Used in Artwork Reproductions: Mechanical Characterization by Laser Vibrometry

Many museums have been producing reproductions for several years to replace artworks weakened by outdoor exhibition. Among these, in order to imitate the original aesthetic, the French consortium Réunion des Musées Nationaux–Grand Palais has chosen to work from large-format marble sculpture molds to complex composite materials based on resins comprising mineral fillers. However, similar to the original works of art, these reproductions age and deteriorate due to constant outdoor exposure. For this reason, current research focuses on the preventive conservation and monitoring of the structural health of these reconstructed objects. The goal of this paper was to study the resin/mineral powder composite materials used to produce cultural heritage reproductions of sculptures. This work is oriented toward a comparison of the mechanical properties of composite materials used in the replacement of cultural heritage sculptures (for instance, in the Garden of the Palace of Versailles or the Rodin Museum). The objectives were to first characterize the physical and mechanical properties of these materials in order to identify the most suitable material for cultural heritage reproduction, and secondly, to propose a method with minimal contact that obtained equivalent information as analyses performed with conventional ultrasonic techniques. These nondestructive evaluation techniques could be used for laboratory and in situ analyses. Samples of different polymer/mineral powder filler compositions were analyzed by compressional, shear and surface waves, generated by a 1 MHz center frequency ultrasonic transducer. Firstly, the measurements made it possible to evaluate the velocities of the bulk acoustic waves and extract the Young’s modulus of each tested material. Secondly, in order to have minimal contact with the analyzed structure, a laser interferometry system was used to detect waves at the surface and follow their propagation. The results clearly showed the possibility of using this technique to extract mechanical characteristics of composite materials, allowing for selection of material for the reproduction of large-format statues. For different types of polymer resins, the ability of ultrasonic analysis to track the impact of rock powder (marble or slate) on the mechanical properties of these synthetic materials was clearly observed, proving that this technique holds promise for monitoring the structural health of large-format artwork

Lamb wave sensor for viscous fluids characterization

International audienceThis paper is a study of a new sensor for fluid characterization. This sensor is composed of a stainless steel plate in contact with a viscous material. The aim is to characterize the material viscosity by using reflected Lamb waves at the boundary interface. In order to identify the effects on the Lamb reflected modes by the viscous material, a complete study of the propagation wave in the alone plate is first presented. The propagation modes of the loaded plate are then investigated. By monitoring the mechanical impedance, the viscosity of the material in contact is extracted. In order to validate the experimental set-up, the mechanical impedance variation is measured for different water-glycerol mixtures. Results are in good agreement with those obtained by other techniques in the literature

Mesoscopic Monitoring of Human Skin Explants Viscoelastic Properties

The investigation of the mechanical properties of skin is of great interest for monitoring physiological and pathological changes in the cutaneous barrier function for dermatological and cosmetic issues. Skin constitutes a complex tissue because of its multi-layered organisation. From a rheological point of view, it can be considered to be a soft tissue with viscoelastic properties. In order to characterise ex vivo mechanical properties of skin on the mesoscopic scale, a biosensor including a thickness shear mode transducer (TSM) in contact with a skin explant was used. A specific experimental set-up was developed to monitor continuously and in real-time human skin explants, including the dermis and the epidermis. These were kept alive for up to 8 days. Skin viscoelastic evolutions can be quantified with a multi-frequency impedance measurement (from 5 MHz to 45 MHz) combined with a dedicated fractional calculus model. Two relevant parameters for the non-destructive mesoscopic characterisation of skin explants were extracted: the structural parameter αapp and the apparent viscosity ηapp. In this study, the validity of the biosensor, including repeatability and viability, was controlled. A typical signature of the viscoelastic evolutions of the different cutaneous layers was identified. Finally, monitoring was carried out on stripped explants mimicking a weakened barrier function

Viscoelastic material monitoring using an acoustical cylindrical spectrometer

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Investigation de Gels d’Agar chargés de Poudre de Marbre par Rhéologie Ultrasonore Multi-Échelle

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Electrodes geometry and surface waves generation on a quartz disk: experimental study

International audienceThe elaboration of new soft hybrid materials requires adapted smart sensors. Among the ultrasonic techniques, the use of an AT cut quartz is a solution that allows the complete tracking of the mechanical properties of the material in contact. In order to get a RF wireless excitation of such a sensor in a wide frequency bandwidth, the tuning of the electrodes geometry is determinant as it must be based on closed loops. The aim of the present experimental study is to analyse the role played by the electrodes shape upon the generation of the surface waves. Several electrodes excitation geometries are presented in order to optimize the surface wave focusing at the center of the sensor. The quartz disk is excited by a voltage pulse and the quartz surface is scanned by a laser vibrometer (JOPCS, 195, 1-8 (2009)). The surface waves and the transient aspects linked to the electrodes shape borders are analysed by using the 3D Gabor transform (Ultrasonics, 43, 1173-1177 (2006)). The observed waves and their generation sources are presented, revealing new insight about the modelisation of the sensor electrodes. A comparison with classical electrodes shapes is carried out