J. Ripoche

Resonances of plates and cylinders: Guided waves

The study of the normal diffusion of an ultrasonic plane wave by cylinders and plates imbedded in the water shows resonances which are the natural modes of vibration. When a natural mode of an elastic target is excited, the energy which is stored during the forced excitation is emitted after the end of the forced excitation. The observation of backscattered spectra obtained by the Resonance Isolation and Identification Method (RIIM) from an aluminum cylinder shows supplementary resonances. The directivity pattern of the transducer is the cause of these supplementary resonances. The behavior of these resonances is analogous to the resonances of the plate. This leads us to study the natural modes of the cylinder. All the resonances which are experimentally detected may be considered as normal modes of the target. The results obtained on plates and cylinders have a common point: the generation of a guided wave by the excitation of a resonance.

Influence des ondes de “galerie a echo” sur la diffusion d'une onde ultrasonore plane par un cylindre☆

Abstract An experimental method which allows measurement of the frequency ( ka ) of circumferential wave resonances which propagate in aluminium cylinders is described. It is also possible to deduce the number of wavelenghts on the circumference or the vibration mode of circumferential waves. This study allows a better understanding of the complex structures of the form function in the far field. It specifies the theoretical results which show the effect of each circumferential wave in the angular diagram.

Influence of the free modes of vibration on the acoustic scattering of a circular cylindrical shell

The scattering of an acoustic wave by an elastic circular cylindrical shell immersed in water depends on the natural modes of the target. Two groups of natural modes may be considered. The first group is related to circumferential waves which propagate around the tube. These resonances are provided by the resonance scattering theory. The second group is related to waves whose direction of propagation is parallel to the axis of the tube. The calculation of the natural modes and the experimental data obtained by the ‘‘method of isolation and identification of the resonances’’ agree very well.

A new acoustic spectroscopy: Resonance spectroscopy by the MIIR

Recent and remarkable advances in the experimental study of acoustic scattering from targets immersed in water are leading to a new spectroscopy: resonance acoustic spectroscopy. The discovery and improvement of an intriguing method, the Method of Isolation and Identification of Resonances (MIIR), has made possible experimental determination of the eigenfrequency spectra of aluminum-elastic cylinders and cylindrical shells. This method gives a quasilinear “resonance spectra.” In addition, it shows the importance of circumferential waves which generate standing waves. They allow us to explain the “reradiation” of targets after the end of insonification. The MIIR has numerous applications, especially in “underwater acoustics” and “nondestructive testing.”

Methode impulsionnelle numerisee (MIN) pour l'isolement et l'identification des resonances de tubes immerges

Abstract The method of isolation and identification of resonances (MIIR) allows a direct verification of “resonance scattering theory” (RST). Indeed it provides the resonance spectra of aluminum elastic cylindrical shells, insonified by plane acoustic waves and it also allows the experimental determination of the mode number of each resonance of the target. At this day, when the shells are insonified by a short pulse it is only possible to obtain the resonance spectrum with a fast Fourier transform of the backscattered response. This paper presents an impulse method which allows the isolation of resonances and besides the identification of each eigenmode of the target.

Acoustic scattering from an immersed plane multilayer : application to the inverse problem

The authors deal with acoustic scattering from a plane multilayered structure. This structure is composed of a first plastic elastic layer, a thin water layer, and a second aluminum elastic layer. A pulse excitation is used to obtain the scattered spectra and the resonance spectra at normal and oblique incidence. These spectra give information about the structure. The spectra of resonances due to the guided waves provide information about the resonant character of the two elastic solids. The farther the guided waves propagate in a layer, the more resonant the layer is. These guided waves are Lamb waves of the solid layers. The backscattered signal is formed by a series of echoes which arise from the different layers of the structure. With simple experiments, involving in particular a temporal filtering of the reflected signal, it is shown that it is possible to obtain three of the four parameters which characterize the solid layers: the phase velocities of the longitudinal waves, thicknesses, and densitie...

Analysis of resonances related to Scholte–Stoneley waves around circular cylindrical shells

It is shown experimentally and theoretically that waves propagate around circular cylindrical shells with a phase velocity that is less than the phase velocity of sound in water. These waves are called Scholte–Stoneley waves. First this paper presents results on air‐filled aluminum pipes of different ratios of the internal radius b to the external radius a. The frequency resonances and the orders n of the resonances increase with the b/a ratio. The frequency range of the observed resonances is fixed for a given pipe. This is related to the evolution of the resonance width with frequency. The frequency resonances and their width are calculated with the use of the resonance scattering theory. The so obtained results agree quite well with the experimental ones. The Scholte–Stoneley wave, which propagates around the shell, is compared to the antisymmetric Lamb wave, which appears on a thin plate when the plate is imbedded in water. Second, the influence of the internal fluid on the Scholte–Stoneley resonances...

Identification of the resonances of a cylindrical shell stiffened by an internal lengthwise rib

A theoretical and experimental study of the acoustic response of a submerged stiffened cylindrical shell is presented. The internal rib is modeled as a clamped‐free plate mounted inside the shell perpendicular to the shell surface. The stiffened shell is excited by a normally incident acoustic pressure wave. Wave propagation around the circumference of the shell and associated sound radiation are discussed. From the directivity of the monostatic scattering, the resonances in the scattered sound pressure field can be separated into three different types. A mechanical admittance is used to help identify the different types of resonances excited in the fluid‐loaded stiffened shell. Each type of resonance is shown to be associated with a particular type of interaction between the shell and the rib in terms of the components of the coupling forces: i.e., the normal force, the transverse force, and the coupling moment. For kR ranging from 16 to 35, the normal coupling force is shown to control the symmetric fle...

Acoustic scattering from finite cylindrical elastic objects

The acoustic scattering from infinite elastic cylinders or spheres is well known. It is possible to characterize these targets by their resonance spectra. The resonances are established by the generation of surface waves that propagate around the circumference of the targets. The resonances originate from the phase matching of repeatedly circumnavigating surface waves. Experimentally, it is possible to characterize a target with a complicated shape, but it is not easy to explain the spectra theoretically because the geometry is not separable and the usual analytical methods to calculate the far‐field pressure cannot be used. In this paper, resonance spectra and angular diagrams obtained from a target consisting of a finite cylinder with hemispherical endcaps are obtained experimentally. To explain the resonance spectra, an integral phase matching condition is used. Upon incidence normal to the cylinder axis, resonances due to the phase matching of surface waves traveling along a circumference or along a m...

Acoustic scattering from a cylindrical shell bounded by hemispherical endcaps. Resonance interpretation with surface waves propagating in cylindrical and spherical shells

The studies of the acoustic scattering from infinite cylinders or spheres have shown the strong influence of the propagation of surface waves. Two types of surface waves are distinguished: the Rayleigh or Whispering Gallery waves and the Scholte–Stoneley wave. In this paper, theoretical and experimental results obtained on objects constituted by a cylindrical shell bounded by two hemispherical shells are presented, when these objects are insonified in the direction parallel to their main axis. To explain the experimental results, the stationary condition is written in the meridian plane of these objects. To apply this condition, the phase velocities of the waves propagating in the cylindrical and in the spherical parts must be known. First, the phase velocity of waves propagating along the length of cylindrical shells is theoretically and experimentally studied. Then an experimental study of the phase velocities of the waves propagating on spherical shells is carried out. Finally, the results of these two...