Experimental studies on the reflection and transmission of bounded acoustic beams at a liquid–solid plate–liquid interface show that, for incidences at or near the Lamb angles, some effects occur which are similar to those reported at a liquid–solid interface for the Rayleigh incidence. Theoretical descriptions of this phenomenon have been proposed that are based upon asymptotic considerations for the reflected beam and numerical considerations for the reflected and the transmitted beams. These previous theories are extended in this work. The reflected and transmitted Gaussian beams are described through a short wave asymptotic analysis in regions which are not restricted to the interfaces. The nonspecular effects which appear in the transmitted field composition are different according to whether the Lamb mode under consideration is symmetric or antisymmetric. The particular case when two Lamb modes are simultaneously excited is also studied. If the two modes have a different nature, the nonspecular effects are added while, for two modes of same nature (symmetric or antisymmetric), this is no longer true.
Ultrasonic echography is one of the methods which are more and more used for non destructive evaluation of materials, according to the requirements of the users who are often increasingly concerned by quantitative evaluation of defects. This requires a better knowledge of the measurement system, as weIl as of the ultrasonic field generated by the transducer inside of the material and this has led us to develop methods for the calculation of this field. We present a method for the determination of the ultrasonic field in a solid medium separated by a fluid-solid plane interface, from the knowledge, by modelization or by measures, of the field in a plane of the fluid medium. The method is based upon a decomposition by a plane waves obtained by spatial Fourier Transform, which permits a simple use of the SNELL-DESCARTES refraction laws at the interface. The method has shown quite good agreement with experimental results. Moreover, this method has permit us to predict undesirable effects in the case of oblique incident beams.
NUMERICAL METHon
The aim of this paper is threefold: to describe the physical phenomenon of the excitation of modal waves such as Lamb waves, in anisotropic multilayered media, by a monochromatic incident beam, using an asymptotic approach; to present a three-dimensional model using the decomposition of the incident beam into monochromatic plane waves (the formalism is applied to the particle displacement vector); to illustrate the phenomenon both numerically and experimentally. Numerical and experimental maps of the reflected field of pressure are presented, and the reradiation of the Lamb wave beam in an oblique plane is theoretically and numerically illustrated.
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