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International audience; A reliable physics-based theory that directly relates material elastic nonlinearity to damage and other features that may be responsible for the elastic nonlinear response (inter-grain soft bonds, dislocations) does not yet exist. Dynamic acousto-elasticity has significant implication to development of a physics based theory. The method provides the means to dynamically study the wavespeed -strain and attenuation-strain behaviors through the full wave cycle in contrast to most methods that measure average response. The method relies on exciting a sample with a low frequency vibration (several kHz) of strain amplitude ranging from 10-7 to 10-5 in order to cycle it through stress-strain multiple times. Simultaneously, a high frequency ultrasonic source (2 MHz) applies pulses with high repetition rate to read variations in elasticity and dissipation. By integrating the velocity-stress data, we extract the dynamic stress-strain behavior. Thus the low frequency amplitude-dependent anelastic energy dissipation can be assessed. We report preliminary results in 11 different room-dry rock samples, including a crystalline rock (granite), sandstones and limestones. The behaviors of the 6 investigated sandstones show a striking similarity, although they have different microstructures. We also show how acoustical conditioning can modify nonlinear elastodynamics, bringing temporarily the material in a metastable state.

In the context of license renewal in the field of nuclear energy, maintaining in service and re-qualifying existing concrete structures for ten years is a great challenge. The integrity of concrete in the concrete pedestal and biological shield wall in nuclear plants remains unknown. These structures have been subjected to radiation and medium temperature for a long period of time. This paper aims at providing some quantitative information related to the degree of micro-cracking of concrete and cement based materials in the presence of thermal damage. We develop a methodology based on linear resonant ultrasound spectroscopy, numerical simulations and nonlinear resonant ultrasound spectroscopy to get quantitative values of nonlinearity. We show the high sensitivity of derived nonlinearity to thermal damage and its correlation with the evolution of concrete microstructure.

The Time Reversed Elastic Nonlinearity Diagnostic (TREND) has been shown to have the ability to image near surface nonlinear scatterers (e.g., cracks and disbonds) using the normal (out of plane) component of motion. This technique has been advanced with the addition of three-component vector wave field detection using laser vibrometry. This advancement provides the ability of TREND to detect and image features of various orientations, increasing its capability and decreasing the likelihood of overlooking features due to the inability to excite them. Furthermore, the ability to selectively focus each component of motion provides the means to investigate the mechanism of the nonlinear wave generation at the nonlinear scattering sites. Experimental data is presented from samples containing known defects (location and orientation) confirming the sensitivity of three-component time reversal imaging in both linear and nonlinear ultrasonics to feature orientation.

This work deals with the acoustic scattering by a random distribution of cylindrical shells in a fluid. This studyhas three parts. The first one is about a limited number of shells. A resonant coupling is shown. This coupleinduces a split of the A-wave (Scholte-Stoneley wave) resonances. These splits are experimentally validated.The second part presents two effective media theories: Waterman and Truell's and the Fikioris and Waterman'sone. They consist in the analytic averaging of the scattered field with the positions of shells. The average waveobtained is called the effective or coherent wave. The major difference between these theories is that Fikioris andWaterman take into account the size of the shells whereas, in the Waterman and Truell theory, the radius of theshells tends towards zero. These two theories allow the computation of an effective wave number. The influenceof the resonances of one shell is seen on the effective phase velocity and on the effective attenuation. The resultsof these two theories are compared with the ones of a third theory: the one of Foldy. All these theories givesimilar results for weak concentrations, but, for strong concentrations, the results of the theory of Fikioris andWaterman differ from the others. The study of a slab-like region with randomly placed shells leads to thedetermination of a reflection and a transmission coefficient. These are similar to the ones obtained byconsidering a fluid slab. The analogy between the inhomogeneous medium and a viscous fluid is possible. Theexperimental study of such a medium gives good results for the effective attenuation. The last part deals withinhomogeneous media with the shells replaced by a set of two shells close enough to produce resonant coupling.In this case, an effective wave number is obtained with the Fikioris and Waterman theory. The reflection andtransmission coefficients can be expressed as the ones of a fictitious fluid-like slab for which the phase velocityand the attenuation would be dependent of the direction of propagation.; diffuseurs élastiques aléatoirement répartis. Cette étude se compose de trois parties. La première parties'intéresse à un nombre restreint de diffuseurs. Elle a permis de mettre en évidence un phénomène de couplagerésonant entre des diffuseurs cylindriques proches se manifestant par une démultiplication des fréquences derésonances associées à l'onde A (onde de Scholte-Stoneley). Ces démultiplications ont été validéesexpérimentalement. La seconde partie présente deux théories de milieu effectif : celle de Waterman et Truell etcelle de Fikioris et Waterman. La philosophie de ces deux théories est de déterminer l'onde moyenne résultantd'un moyennage sur les positions des diffuseurs. C'est cette onde que l'on appelle onde cohérente. Ces théoriesaboutissent au calcul d'un nombre d'onde effectif caractérisant l'onde cohérente. La différence essentielle entreces deux théories réside dans la prise en compte de la taille des diffuseurs dans celle de Fikioris et Waterman.Les résultats des ces deux théories sont comparés entre eux et avec ceux obtenus par une troisième théorie, cellede Foldy. On note l'influence des fréquences de résonance d'un diffuseur seul sur le nombre d'onde effectif. Lestrois théories donnent des résultats similaires pour de faibles concentrations. Dans le cas de fortes concentrations,les résultats issus de la théorie de Fikioris et Waterman se distinguent de ceux obtenus par les deux autresthéories. Lorsque l'on considère une couche de milieu multi-diffuseur, on peut déterminer des coefficients deréflexion et de transmission s'exprimant formellement comme ceux d'une plaque fluide. Une analogie entre unmilieu multi-diffuseur et un milieu fluide visqueux est possible. L'étude expérimentale menée sur un tel systèmemontre un bon accord avec la théorie en ce qui concerne l'atténuation. La dernière partie de ce documentconsiste en l'étude d'un milieu multi-diffuseur dans lequel les diffuseurs sont anisotropes. Les diffuseurs dumilieu présenté sont constitués de deux diffuseurs cylindriques suffisamment proches pour qu'il y ait couplagerésonant. On peut alors déterminer un nombre d'onde effectif à l'aide de la théorie de Fikioris et Waterman, ainsique des coefficient de réflexion et de transmission. Ces derniers se mettent sous la même forme que ceuxassociés à un milieu fictif de type fluide pour lequel la vitesse de phase et l'atténuation varieraient en fonction dela direction de propagation.