Système instrumental pour la rhéologie ultrasonore

L'élaboration de fluides complexes (les produits de soins, ménagers ou agroalimentaires), constitue un enjeu économique important. En particulier, la mise au point de nouveaux matériaux à transition de phase s'avère en fait très complexe. Elle nécessite un suivi précis des caractéristiques viscoélastiques à l'aide de systèmes de contrôle non destructifs adaptés. Le dispositif expérimental original développé dans le cadre de cette thèse utilise un capteur piézoélectrique qui a été dimensionné de sorte à optimiser sa sensibilité. Pour extraire les caractéristiques du matériau, un nouveau mode le électrique a été développé. Il tient compte simultanément des évolutions électriques et mécaniques du matériau. Nous validons notre instrumentation et le modèle associé sur deux types matériaux : des matériaux élaborés par le procédé sol-gel (peu conducteurs) et des yaourts en formation (conducteurs).The development of complex fluids for care, domestic or agroalimentary products constitutes an important economic stake. The development of new materials with phase transition is in fact very complex. It requires an accurate follow-up of the viscoelastic characteristics by using a non destructive testing method. The present work describes an adapted electronic set-up. The experimental set-up is based on a piezoelectric quartz crystal resonator in contact with the material. The quartz electrical impedance is used to monitor and to extract the complex shear modulus of the material. A new electrical model of the loaded quartz is developed and validated with well-known viscous conductive fluids. This new experimental set-up is successfully applied to the gelation process monitoring of two typical materials. The first one is a silica gel, the second one is a dairy product made from acidified milkCERGY PONTOISE-BU Neuville (951272102) / SudocSudocFranceF

Laser ultrasonic analysis of normal modes generated by a voltage pulse on an AT quartz sensor

Laser ultrasonic detection is a versatile and highly sensitive tool for the observation of surface waves. In the following study, laser ultrasonic detection is used for the experimental study of spurious normal vibration modes of a disk quartz sensor excited by a voltage pulse. The AT cut crystal (cut of the crystal relative to the the main crystallographic axis is 35.25) is optimal for generating mainly thickness–shear vibrations (central frequency 6 MHz) on the quartz surface. However, resulting from shear-to-longitudinal and shear-to-surface mode conversion, and from the weak coupling with the other crystallographic axes, other modes (thickness-compressional and bending modes) are always present in the plate response. Since the laser vibrometer is sensitive to normal displacements, the laser investigation shows waves that can be considered as unwanted for the AT quartz used as a shear sensor. The scanned three dimensional (3D) amplitude–space–time signals are carefully analysed using their representation in three dual Fourier domains (space–time, wave number–frequency). Results on the transient analysis of the waves, the normal bending modes and the dispersion curves are shown.status: publishe

3D Gabor analysis of transient waves propagating along an AT cut quartz disk

Laser detection methods allow the investigation of ultrasonic transient phenomena in both space and time dimensions. Used for the experimental investigation of surface wave propagation along a 2D surface, laser ultrasonic leads to three dimensional (3D) space–time signal collections. The classical high resolution signal processing methods or 3D Fourier Transforms can be used in order to extract the wave propagation information, however these methods are not adapted for identifying where and when the waves are generated. In order to quantify these transient aspects in the space–time–wave number–frequency domains, the 3D Gabor transform is introduced. The 3D Gabor transform properties are presented. The potential of the 3D Gabor for the identification of the local and transient complex wave numbers is illustrated on the propagation of surface waves on a piezoelectric quartz (AT cut, 6 MHz). In this experimental study, the quartz is excited by a voltage pulse and the quartz surface is scanned by a laser vibrometer. The 3D Gabor analysis shows that the circular electrodes borders generate anti-phase surface waves that propagates outside the electrodes, with a strong energy contribution in the low frequency domain (<1 MHz). The transient analysis also points out, for higher frequencies, where the surface waves are generated and how they propagate with respect of both to the geometry of the electrodes and the crystallographic axis of the quartz. These results confirm the theoretical modal analysis and provide new knowledge about the key role played by the electrodes border. This will allow the optimization of the electrodes shape in order to design low frequency Lamb wave sensors.status: publishe