Martine Rousseau

Acoustical validation of the rheological models for a structural bond

The purpose of this study is to characterize by ultrasound a bonded structure metal/adhesive/metal. At first, we investigate the guided modes of the structure for the adhesive layer in its entirety; this is the exact model. Secondly, we assume that the adhesive layer is described by one geometrical interface, with a superficial distribution of longitudinal and transversal springs with or without mass; this is the rheological model. A comparison of the guided modes for the two models allows to determinate the validity limits for the rheological modelling and define the equivalent stiffness coefficients and the mass as a function of the frequency. Some cutoff-frequencies are better evaluated when the springs mass is taken into account.

Rayleigh surface waves and their canonically associated quasi-particles

Inspired by soliton theory and exploiting the conservation law of wave momentum, it is shown that one can associate with the surface Rayleigh wave of macroscopic elasticity a quasi-particle, a ‘surface phonon’, which is in inertial motion for the standard boundary conditions. The ‘mass’ of this ‘particle’ is determined in terms of the wave properties. Different types of alteration in the boundary conditions are shown to result in perturbations of this inertial motion in various ways. The essential tool in the presented derivation is the exploitation of the canonical equations of conservation, which are consequences of the celebrated Noether theorem of field theory. The results obtained may be useful in the mechanics of surface waves at the nanoscale, in particular in treating perturbations of various kinds.

Theoretical analysis for the reflection of a focused ultrasonic beam from a fluid–solid interface

A theoretical study of the reflection of focused acoustic beams from a fluid–solid interface is presented. The incident field is defined by a Gaussian velocity distribution along a plane emitter. The reflected field is described through its pressure field in a region not restricted to the interface. Nonspecular phenomena were exhibited at any angle of incidence by means of short wave asymptotic analysis. In particular, for incidence near the Rayleigh angle, a distortion of a part of the caustic of the reflected beam including lateral and axial displacements of the focal point is observed.

Subwavelength ultrasonic measurement of a very thin fluid layer thickness in a trilayer

We suggest two original methods to determine the thickness of a fluid layer so thin that it cannot be evaluated by classical techniques. These methods are based upon the characteristic behavior of some structure resonances. We recall the main features of these particular resonances and we report the relations between the liquid layer thickness and the structure cut-off frequencies. Then, the experimental devices are presented which give these cut-off frequencies. The thickness, so obtained, is given with a good precision since the error is less than 5%.

Experimental study of focused ultrasonic beams reflected at a fluid-solid interface in the neighborhood of the Rayleigh angle

A technique for measuring the lateral and axial displacements of the focal point of an acoustic focused beam reflected at a water-aluminum interface in the neighborhood of the Rayleigh angle incidence is presented together with a qualitative representation of other nonspecular phenomena, based on the cartography of the reflected beam. A study of the feasibility of the experiment and the choice of the transducer is carried out. The results obtained are in agreement with theoretical predictions.<<ETX>>

Ultrasonic Measurements of Pretreated Aluminium Joints Durability during Hot Cycles

Adhesive bonded joints have been investigated due to their growing importance in aeronautic, aerospace, and automotive industries. This article examines the durability of adhesive bonded 2024 T3 aluminium alloy joints by comparing the stiffnesses of the two interfaces before and after exposure to a hot atmosphere (70 degrees C) for several cycles each of 67 hours. The stiffnesses values are calculated by ultrasonic measurements via an equivalent rheological model where the two aluminium/adhesive interfaces are replaced by an uniform distribution of longitudinal and transversal springs without inertia. Before any ageing, and for all studied specimens, the adhesion was increased by pretreating the metal by phosphoric acid pickling prior to bonding. During the first cycle of ageing, the complete crosslinking of the adhesive is observed. This phenomenon involves an increase of Young modulus of the adhesive. For the three following cycles of ageing, a small degradation of the structural joint is observed, and several hypothesis are examined. These different observations are obtained by a theoretical and experimental study of guided Lamb modes of the structure.

Wave momentum and quasi-particles in physical acoustics

Wave Momentum and Radiative Stresses Continuum Mechanics Pseudomomentum and Eshelby Stress Action, Phonons and Wave Mechanics Transmission-Reflection Problems Dynamic Materials Elastic Surface Waves in Terms of Quasi-Particles Electroelastic Waves in Terms of Quasi-Particles Generalized Elastic Materials Examples of Solitonic Systems

Lamb modes and acoustic microscopy for the characterization of bonded structures

This paper is a contribution to the evaluation of the bonding by means of guided waves, when the quality of the glue is degraded by addition of grease. Experimental dispersion curves are compared to the theoretical ones obtained either from a perfect welded tri-layer model or from a second model using delaminated conditions. It is shown that the second model is more convenient when the quality of the glue is highly degraded. On another hand, the parameters of Jones are extracted from experimental data by inverse problem. It is shown that the values of these parameters strongly diminish and the variation is of logarithmic type when the quality of the glue is progressively degraded. Acoustic microscopy is also performed, and it provides images of the inner structure giving a qualitative explanation of the values of the parameters of Jones.

Vertical mode for an immersed structure composed of two elastic layers coupled by a thin water film

We aim at evaluating the thickness of a thin water film, coupling two elastic layers. By a plane acoustic wave approach, we have shown the existence of a particular eigen-mode, called the vertical mode (V-mode), which strongly depends on the fluid layer. The water film thickness is expressed as a function of the V-mode cut-off frequency. The value of this particular frequency is experimentally obtained by a retrodiffusion and resonance spectra analysis. However, in practice, the acoustic waves propagate as a bounded beam, and it is necessary to show that the measurement accuracy remains compatible with a plane wave approach

The Love surface-acoustic wave seen as a guided non-Newtonian quasi-particle

Following along the line of recent works, the paper introduces the notion of quasi-particles that are associated with surface-acoustic waves of the Love type via the canonical conservation laws of Noether’s theory of invariants. The strange point mechanics (velocity-dependent “mass” and energy) is obtained by integrating the conservation laws of “wave momentum” and energy over an element of volume representative of the wave motion. This original but involved point mechanics obtained for the quasi-particle is neither Newtonian nor Lorentzian because of the existence of surface waves restricted to a bounded interval of velocities that is characteristic of Love waves. The reduction of this description to the cases of an isolated elastic layer of finite thickness with pure shear horizontal motion and of the so-called Murdoch case corresponding to a thin-film approximation (half-space with an upper material boundary equipped with mass and surface elasticity) studied independently is exactly proved.