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Effet non linéaire d'auto-démodulation d'amplitude dans les milieux granulaires: théories et expériences

Abstract : Nonlinear acoustics methods have demonstrated a high sensitivity to the presence of structural defects in solids and are now used with the purpose of nondestructive testing for damaged materials.

In this thesis, operation of the parametric antenna in unconsolidated granular materials is studied. The parametric antenna, widely used in underwater acoustics, involves a nonlinear acoustic effect that can be interpreted as an amplitude self-demodulation or as a difference frequency generation phenomenon : from the emission of intense waves, composed of several neighbouring high frequencies (around 100 kHz), waves of lower frequencies (1-10 kHz) are generated by nonlinear effect in the medium of propagation. The main interests of this method are i) the possible high directivity of the low frequency radiation, useful for selective testing, ii) the energy conversion towards low frequency waves which propagate easier than high frequency waves in highly absorbing or scattering media.

First, two models of parametric antenna are developed to take into account absorption, scattering, or dispersion phenomena, as well as evanescent feature of the waves. Experiments in disordered packings of glass beads, in qualitative agreement with theoretical predictions are performed and interpreted.

Second, parametric antenna with shear waves is operated for the first time, and produce a low frequency demodulated longitudinal signal. This nonlinear dilatancy phenomenon is tested in different experimental configurations. The demodulated longitudinal signal amplitude as a function of the shear waves amplitude is initially quadratic and then exhibit a transition to a 3/2 power law dependence. It is demonstrated that the clapping phenomenon, i.e. the opening and closing of the contacts under acoustic action, is responsible for this transition. The localization of this transition enables to characterize qualitatively the distribution of the contacts static strains in the medium bulk, especially in the weak contacts region.

Finally, the rotation of the shear waves polarization is a convenient mean to characterize the anisotropy of the nonlinear parameter of the medium. Such anisotropy is present in granular media when, for instance, an uniaxial static force is applied on the whole material, and creates a network of force chains that are preferentially oriented along the apply static force.
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https://tel.archives-ouvertes.fr/tel-00004227
Contributor : Vincent Tournat <>
Submitted on : Tuesday, January 20, 2004 - 12:54:09 PM
Last modification on : Tuesday, March 31, 2020 - 3:20:58 PM
Long-term archiving on: : Friday, April 2, 2010 - 7:10:08 PM

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  • HAL Id : tel-00004227, version 1

Citation

Vincent Tournat. Effet non linéaire d'auto-démodulation d'amplitude dans les milieux granulaires: théories et expériences. Acoustique [physics.class-ph]. Université du Maine, 2003. Français. ⟨tel-00004227⟩

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