G. Quentin

Backscattering of acoustic waves by randomly rough surfaces of elastic solids immersed in water

In this paper we study the angular variations of the mean total intensity of ultrasonic waves backscattered by randomly rough surfaces of elastic solids immersed in water. The potential method constitutes the starting point of the theoretical approach. The calculations have been done for a normal distribution of heights and a Gaussian autocorrelation function. The roughness of the samples varies from 8 to 110  μm. The experiments have been done at discrete frequencies up to 10 MHz such that the roughness parameter g( = k21h2γ2) has values ranging from 10−2 to 90. A comparison of the experimental curves and theoretical calculations shows good agreement for incident angles of 40° or less. Another significant feature of this experiment is the increase of the backscattered intensity at the Rayleigh angle for k1 h<1.

Measurement of the periodicity of internal surfaces by ultrasonic testing

When an ultrasonic wave insonifies a corrugated surface it is diffracted in agreement with the optical laws. Diffraction peaks are observed and their positions are directly related to the periodicity of the grating. This property has been exploited to estimate the periodicity of an internal periodic surface. Experiments have been performed at 5 and 16 MHz for gratings engraved on different samples (plates and half cylinders) of various materials (copper, Perspex).

Ray theory applied to the internal caustic identification of liquid cavities acoustically insonified

The acoustic field in water‐filled cylindrical cavities machined inside a duraluminum matrix is studied. The internal acoustic beam focused near the cusp points within the liquid cavity. An optical process (pulsed schlieren technique) is used to visualize the acoustic wave front as the wave propagates in the fluid cylinder. The axial positions of the p‐ray cusp caustics are compared with the theoretical values calculated by the ray theory [J. A. Lock and E. A. Hovenac, J. Opt. Soc. Am. A 8, 1541–1552 (1991)].

Experimental investigation of reflection coefficients for lossy liquid-solid-liquid systems

Abstract The reflection coefficient for a bounded ultrasonic beam incident on a liquid-solid-liquid system (LSL) is studied. Systematic measurements were carried out to investigate the variations of the amplitude of the reflection modulus ∣RLSL∣ with incident angle and parameter Fd (frequency-thickness of the plate). The experimental plots confirm the theoretical works of Claeys et al1 and Bogy and Gracewski2. It is shown that the attenuation of the media has a great influence on the variations of this coefficient as has been already mentioned in the literature, for liquid-solid configurations.

On the dispersion of Scholte wave propagating on a layered medium

The phase velocity of the Scholte wave is experimentally studied in the case of plane multilayered systems immersed in water. The ‘‘liquid wedge’’ technique is used for its generation and the investigation is limited to the case where the layers have a high density and a high shear wave velocity with respect to the fluid (water). The variations of the difference between the phase velocity of the Scholte wave and the sound velocity in water is obtained by recording the ‘‘penetration factor’’ of the investigated wave into the liquid, the amplitude of which decays exponentially as exp(−k‘z) from the surface; z measures the coordinate perpendicular to the interface and k‘ designates the ‘‘penetration factor.’’ The agreement between theoretical and experimental results is reasonably good. The method of investigation limits our study on the dispersion of the Scholte wave to the ke values>1.8 (k designates the wave vector in the water and e the thickness of the layer).

Prompt Observation of Elastic Resonance Spectra Via the Use of Short Ultrasonic Pulses

It has been shown by L.R. Dragonette, S.K. Numrich and L.J. Frank [1,2] that the echo scattered by an elastic body can be considered as t he superposition of the elastic response of the target superimposed to the response o f a rigid body having the same shape. T his result has b eenused to analyse t he insonification of long cylindrical wires by short ultrasonic pulses. We show that the F ourier transform o f the elastic contribution alone g ives directly the resonance spectrum of the cylinder. This method has been checked on wires of different materials the v alues of ka involved were less than 50. The positions of the resonances in ka are in good agreement with theoretical predictions 131 as it will be verified by Regges trajectories [41.

BACKSCATTERING OF A BOUNDED ULTRASONIC BEAM AT RAYLEIGH ANGLE FROM PLANE AND CURVED LIQUID-SOLID INTERFACES

Our experimental investigations on different plane interfaces suggest that the amplitude of the backscattered wave increases with the dimensionless parameters δs / λ where δs is the Schoch displacement characteristic of the specularly reflected beam. For cylindrical samples, similar variations are observed. We show also that a back surface wave leaking at the Rayleigh angle exists for any angle of incidence.

Detection of Leaky-Rayleigh Waves at Air-Solid Interfaces by Laser Interferometry

Leaky-Rayleigh waves have been studied extensively at liquid-solid interface in the last 30 years [1–4]. Both leaky-Rayleigh and leaky-Lamb waves have found many application in Nondestructive Evaluation, e.g. surface defect characterization, etc [5,6]. Because of the difficulties associated with ultrasonic wave generation and detection in air in the megaHerz region air coupled non destructive evaluation has only recently been explored [7,8].

Theoretical and experimental study of surface waves on solid–fluid interfaces when the fluid sound velocity is larger than the shear wave velocity in the solid

Two surface waves can propagate on a solid–fluid interface: the Rayleigh and the Stoneley waves. The problem is well known when the fluid sound velocity cF is lower than the velocities in the solid (cS and cL). The Scholte–Stoneley dispersion equation has been studied when cF is greater than the shear wave velocity cS. The numerical results obtained for PVC–water and PlexiglasR–water show that there might exist two surface waves whose velocities are lower than cS. The two corresponding roots of the dispersion equation tend to the Rayleigh root of the solid–vacuum interface as the density of the fluid tends to zero. The structure of the two waves has been studied using the formalism of the evanescent plane waves. They are theoretically propagating without attenuation along the interface and exponentially decaying along the direction normal to the interface inside both media. These two waves have been experimentally observed. They exponentially decay along the direction normal to the interface inside both media. The experimental arrival times are in very good agreement with the theoretical ones.

A Laser Interferometric Method for Small- and Finite-Amplitude Ultrasonic Waves’ Detection in Transparent Media

The acousto-optic interaction affords a convenient way of optically probing ultrasonic waves in medical diagnosis and nondestructive evaluation. The effects of ultrasonic waves on the light transmitting through transparent media arise from the refractive index variations produced by ultrasonic waves. The index variations may be detected by optical deflection, diffraction or interference methods [1–4]. In Raman-Nath regime, the acoustic waves act as a moving phase grating and diffract the light into different orders. Schlieren visualisation derived from this mechanism has been extensively used to ultrasonic measurements in liquids. In solid media, the acousto-optic effects become more complicated because of the induced optical birefringence. The usual photoelastic method consists in detecting the change in the polarization state of the light caused by ultrasonic waves [5]. Both of the methods are only amplitude-sensitive to ultrasonic waves.