SÉRIES DE DEBYE ET INTERFÉRENCES DANS UNE PLAQUE ÉLASTIQUE

Nous présentons l'essentiel des travaux effectués ces dernières années sur les séries de Debye. Effectuer une décomposition en série de Debye consiste d'une manière générale à écrire l'interaction globale d'une onde avec un diffuseur comme une suite d'interactions locales. Appliquée à la plaque élastique, nous montrons alors le lien profond existant entre les résonances et les interférences. Plus précisément, nous montrons que les résonances observées aux croisements des courbes de dispersion des ondes de Lamb symétriques et antisymétriques sont dues aux interférences entre toutes les ondes longitudinales et transversales multiplement réfléchies dans la plaque.We describe the main works of these last years about the Debye's séries. These last ones provide in a general way with an expansion of the total interaction between a wave and a scatterer into some local interactions. Applied to an elastic plate, we explain the close relation between resonances and interferences. More accurately, we show that observed resonances when the dispersion curves of symmetric and antisymmetric Lamb waves are crossing, result from interferences between all the longitudinal and transversal waves refracted into the plate

NOUVELLE ANALYSE DES MODES DE LAMB : UTILISATION DE LA DÉRIYÉE DE LA PHASE DU COEFFICIENT DE RÉLEXION

The Lamb modes of a plate immersed in water are studied by means of the partial derivatives of the phase of its reflection coefficient with respect to the frequency, to the longitudinal and shear celerities and to the Sound velocity in water. Results about the reemission, the longitudinal or shear character and the coupling with the external medium of Lamb waves are obtained from them

DESCRIPTION DES ONDES DE LAMB EN TERMES D'ONDES ÉVANESCENTES ET DE RÉSONANCES

Leaky Lamb modes of an immersed plate are obtained by a direct method using evanescent waves. Results for A1, S1 and S2 waves are presented and compared with these obtained by the phase angular gradient method, which uses the reflection coefficient phase derivative with respect of the incidence angle of an homogeneous wave. The two methods agree and give correct values of wave numbers and celerities of modes.Nous présentons une méthode de calcul direct des modes propres d'une plaque immergée dans un fluide en utilisant la notion d'onde évanescente. Des résultats relatifs aux modes A1, S1 et S2 sont fournis. Ils sont confirmés par ceux obtenus en calculant la dérivée de la phase du coefficient de réflexion d'une onde plane homogène par rapport à l'angle d'incidence

Reflection and transmission at low concentration by a depth-varying random distribution of cylinders in a fluid slab-like region

International audienceThis paper reports a study of the backscattering behavior of a solid layer containing randomly spaced spherical cavities in the long wavelength limit. The motivation for the work arises from a need to model the responses of porous composite materials in ultrasonic NDE procedures. A comparison is made between models based on a summation over discrete scatterers, which show interesting emergent properties, and an integral formulation based on an ensemble average, and with a simple slab effective medium approximation. The similarities and differences between these three models are demonstrated. A simple quantitative criterion is established which sets the maximum frequency at which ensemble average or equivalent homogeneous medium models can represent echo signal generation in a porous layer for given interpore spacing, or equivalently, given pore size and concentration