Resonance ultrasonic spectroscopy applied to normal and tangential contact modes investigation of a constrained metallic sphere.

This article reports on the analysis of the vibration modes (central eigenfrequencies) observed on a dry steel sphere clamped between two planes, under static compression and subjected to an acoustic excitation. The experimental technique is based on the analysis of the ultrasonic resonance spectrum (RUS); compression or transverse excitations are used. Two types of vibrations are observed. The first occurring in the low frequency range corresponds to modes of movement conditioned by the contacts; the second corresponds to the internal, toroidal and spheroidal deformation modes. In this article we limit our research to the modes observed in the low frequency regime and our objective is to study the influence of the static force on the position of the peaks observed in the frequency spectra. In the case of a longitudinal excitation, only one mode is observed; the transverse excitation makes it possible to detect two modes. The experimental results are compared to theoretical calculations based on theoretical models of Hertz and Hertz-Mindlin, on a development of the JKR model based on adhesion due to Van der Waals forces. In this work, the mechanical coefficients are considered to be independent of time and we do not study the phenomena of hysteresis of these coefficients.

Crossing of acoustic envelope solitary waves in homogeneous elastic rods: Experiments.

In this letter we focused on the problem in which two shear envelope solitary waves traveling in 1-D homogeneous system (elastic PMMA rod of constant cross-section) and incoming in opposite directions, meet one another and head-on collide. The properties of incident acoustic waves after collision are investigated. Two cases are experimentally examined: incoming waves with close amplitudes and quite large difference in magnitude. No phase shift and no variation of polarization are experimentally detected after the collision.

Generation of transversal envelope soliton in polymeric and wooden rods.

This paper is concerned with the probing of the transversal envelope solitons propagation in circular waveguides when a set of requirements (non-linearity and dispersion) are fulfilled in the waveguide and balanced. The basic idea is to analyze the shape of an acoustic pulse after it has traveled one or few trips through samples constituted of a rod and two ended beads. The dispersive behavior is associated to the bounded medium (rod) and the contacts between the elements of the specimens are assumed being described by non-linear Hertz' law type. The experimental data are obviously material dependent and have pointed out the existence of common properties on the formation and propagation properties of the envelope solitons whatever is the material (polymers, carbon fibers and wood) of the rods and spheres. Peculiar behaviors were also observed for specific material (woods) probably caused by the anisotropy of this kind of rod material leading to a double envelope soliton.

On the formation of envelope solitons with tube ended by spherical beads.

In this paper it is suggested the possibility of using a simple device to explore the propagation of short transverse perturbations in specimens of three linearly aligned elements (bead, tube and bead). By using time signal analysis and frequency spectroscopy some properties of the transmitted wave packets are experimentally analyzed in the acoustic domain. It is clearly demonstrated that the shape and the velocity of propagation of the waves do not change with distance. The amplitude may vary depending on the dissipation effect. These properties allow us to identify the wave packets as "envelope solitons". The influence of the elasticity of these materials composing the beads and the tube on the generation of envelope solitons is also investigated.

On the low vibration modes observed in a sphere submitted to a tangential contact force.

In this paper we discuss the existence of translational and rotational displacements of a sphere submitted to a tangential contact force. On the basis of the Bogdanov and Skvortsov's works [A.N. Bogdanov, A.T. Skvortsov, Sov. Phys. Acoust. 38 (1992) 224-226.] the dispersion equation has been established and solved for any value of the frequency in the case of the linear approximation. The present experimental analysis confirms that it exists two branches: an upper branch associated to the translation of the sphere and a lower branch characteristic of the rotation of the bead.

On the splitting of aspherical structure submitted to an acoustic polarized excitation.

We experimentally show how the deviations from spherical symmetry such as the inhomogeneity, the removal mass or the asphericity affect the frequency spectrum of a single elastic sphere. The recorded spectrograms point out the great influence of the polarization of the source on the splitting and the shift of the peaks. Qualitatively, the results are consistent with the data reported in Geophysics although the present study is concerned with a low degree and low- [Formula: see text] modes in contrast to those usually encountered in seismic investigations. From a practical point of view, this study suggests that Geophysics phenomena could be analyzed through spherical beads reproducing the imperfections encountered in the mantle and in the core of the earth.

Experimental study of sound propagation in a chain of spherical beads.

In this paper are described experimental observations which are concerned by the propagation of pulsed ultrasonic waves transmitted through a limited one dimensional periodic granular medium submitted to a static force. This study--which is limited to a time domain analysis--exhibits experimental results which depend on the polarization of the acoustic excitation. In the case of compressional excitation, spherical Rayleigh type surface waves propagate around the beads. In the case of shear excitation, the experimental recordings point out the existence of a very low signal, the frequency of which is equal to the cut-off frequency of the chain. Moreover it is established that the frequency value varies with the radius of the bead, the normal force applied to the beads, and the mechanical properties of the material.

Methods of analysis for backscattering from tissues.

Two techniques for tissue characterization by ultrasonic analysis are described. In the first, using narrow-band pulses, the angular variation I (theta) of the ultrasonic intensity back-scattered by vegetal or animal targets is analysed. We examine how it is possible to estimate the surface roughness and the average spatial periodicity d characteristic of the tissues, by calculating the Fourier transform or the autocorrelation function of I (theta). In the second technique the sample is insonified with a broadband pulse at a fixed angle. A value of the mean periodicity can be obtained directly from the frequency spectrum of the echo for tissues of fairly regular structure.