Search

Double layer structures consisting of stainless steel and polymer rods are designed to blur and attenuate Bragg and Bloch-Floquet scattering from a periodically ribbed plate in a given frequency bandwidth. These structures can be considered as ribbed plate-spring-mass systems, the resonance frequencies of which are obtained from random and circular permutations of five basic oscillators. Analytical and finite element methods are used to find their parameters and tank tests have been carried out to ensure the accuracy of the numerical results and validate the relevance of such a model. It has been found that the typical features associated with the periodicity of the ribs are replaced by a “Christmas tree” appearance when the far field pressure backscattered from the model is displayed in the (frequency, aspect angle) plane. An analysis of backscattering patterns is presented and comparisons with Naval Research Laboratory results are made.Double layer structures consisting of stainless steel and polymer rods are designed to blur and attenuate Bragg and Bloch-Floquet scattering from a periodically ribbed plate in a given frequency bandwidth. These structures can be considered as ribbed plate-spring-mass systems, the resonance frequencies of which are obtained from random and circular permutations of five basic oscillators. Analytical and finite element methods are used to find their parameters and tank tests have been carried out to ensure the accuracy of the numerical results and validate the relevance of such a model. It has been found that the typical features associated with the periodicity of the ribs are replaced by a “Christmas tree” appearance when the far field pressure backscattered from the model is displayed in the (frequency, aspect angle) plane. An analysis of backscattering patterns is presented and comparisons with Naval Research Laboratory results are made.

The problem of a plane acoustic wave scattered by a layered cylinder submerged in water is considered. This cylinder consists of a tube made of aluminum with a solid Lucite cylinder concentrically fitt inside it. These two components are coupled by a thin layer of water. A particular investigation is made on the influence of the thickness of the water layer on the presence of the bending wave A on the tube and the Scholte–Stoneley wave on the cylinder. The presence of these waves is examined in the function of the varying water layer thickness: two special cases are discussed. First, for a layer thickness greater than the tube thickness, it is shown that both the A wave on the aluminum tube and the Scholte–Stoneley wave on the Lucite cylinder are generated. Second, for a thickness much smaller than the tube thickness, a different wave is generated, which is a combination of both waves. These two cases are experimentally verified in a setup that employs a short pulse method.

The influence of evenly spaced ribs (internal rings) on the acoustic scattering from a finite cylindrical shell is examined over the dimensionless frequency range 1

Forward acoustic scattering of an immersed solid LINE (cylinder bounded by hemispherical endcaps) in water is investigated in our study. The object is made of stainless steel and its L/2a ratio is equal to 2 (L: Length of the cylinder part and a: the radius is equal to 60 mm). An impulse measurement method is used in the experimentation. Most of results are obtained experimentally, in bistatic configuration. Mobile receiver transducer is located in a distinct position from the emitter. The polar diagram patterns of the scattered pressure shows an important amplitude of pressure in the shadow side of this object. Analysis of this phenomenon is based on theoretical and experimental results obtained for a sphere with the help of elasticity theory. Moreover, this study relies on the grey-level representation of the angular position of the receiver in function of recorded time signals. Thus on forward acoustic time signals, it is possible to identify echoes due to propagation paths of waves on this object.

Recent results [J. D. N. Cheeke et al., J. Acoust. Soc. Am. 104, 3678 (1998)] reported a 20% decrease in group velocity for flexural mode circumferential waves propagating around water-filled thin-walled stainless-steel tubes. In the present work, the full theory of such modes is developed to explain the structure modes which originates from the coupling between the water filler and stainless tube. Calculated values of the group velocity for the first two coupled modes are in excellent agreement with experiment. The results are of interest for the physics of acoustic waves in fluid-loaded structures and have potential application for liquid level detection.

Acoustic backscattering from elastic cylindrical shells of finite lengths, immersed in water, is investigated. These objects, characterized by the ratio of length over diameter (L/2a = 9.76, 4.88, 2.44, a: outer radius), are excited by an obliquely incident plane acoustic wave. In the three cases studied here, the radii ratio b/a (b: inner radius) is fixed at 0.97. The investigated dimensionless frequency range extends over 10 k1aor = 50 (k1 : wave number in water). The first guided wave, T0, is of particular interest here. The influence of the shell's length on the backscattered pressure is experimentally observed in the time-angle and frequency-angle representations. In support of this experimental study, a time-domain representation is used by extending a theoretical model that provides a geometrical description of the helical propagation of the surface waves around the shell [Bao, J. Acoust. Soc. Am. 94, 1461-1466 (1993)]. Theoretical results on cylindrical shells considered as infinitely long, with identical characteristics, are compared with both experimental representations.

The acoustic scattering from a fluid-loaded stiffened cylindrical shell is described by using elasticity theory. The cylindrical shell is reinforced by a thin internal plate which is diametrically attached along the tube. In this model, cylindrical shell displacements and constraints expressed from elasticity theory are coupled to those of the plate at the junctions, where plate vibrations are described by using plate theory. The present model is first validated at low frequency range (k1a approximately 5-40) by comparison with a previous model based on the Timoshenko-Mindlin thin shell theory and by experimental results. Theoretical and experimental resonance spectra are then analyzed in a high frequency range (k1a approximately 120-200). Only resonances due to the S0 wave are clearly observed in this frequency range, and their modes of propagation are identified. Furthermore, A0 wave propagation is detected, because of the presence of the reflection of this wave at the shell-plate junctions.

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...

The Lamb waves are guided modes which propagate in a plane layer. The Ai antisymmetric and Si symmetric (i=1,2,...) Lamb waves have an infinite phase velocity in low frequencies (cutoff frequencies), and the phase velocity limit, in high frequencies, is equal to the velocity of the shear wave in the isotropic solid material. The thickness resonance of an elastic plate normally insonified by an ultrasonic plane wave is related to the cutoff frequencies. From these resonance frequencies, the plate thickness can be easily deduced. In this presentation, a focused ultrasonic transducer with a large bandpass to characterize an electrical solder point between two steel sheets is used. This solder is abundantly used to manufacture cars. An experimental ultrasonic pulse method (pulse‐MIR) allows us to plot acoustic spectra. The solder is placed in the focal spot of the beam. The transducer can be moved in a parallel direction to the surface of the sheet steel. Many time signals are recorded along a diametrical lin...

The scattering of elliptical cylinders of infinite length insonified by an incident beam perpendicular to their axis is investigated. The method of isolation and identification of resonances (MIIR) allows one to plot resonance spectra. These spectra are obtained from three targets characterized by their major radius/minor radius ratio equal to 4/3, 4/2, 4/1. The resonance spectra obtained from a circular cylinder excited in the same conditions show peaks related to circumferential waves which, for particular frequencies, constitute standing waves. In this case, the phase velocity depends only on the frequency. Contrary to the circular cylinder, resonance spectra obtained by an experimental monostatic method depend on the azimuthal position of the transducer. Some resonances vanish at certain positions. In this case, the phase velocity and the coupling coefficient are a function of the curvature radius and the frequency. The wavelength is not identical on the circumference of the elliptical cylinder. To explain the experimental results, a phase‐matching model is developed to determine the resonance frequencies [H. Uberall et al., J. Acoust. Soc. Am. 81, 312–315 (1987)]. This method allows one to represent the vibration state on the circumference. An integral calculus applied on the latter result gives the far‐field pressure.

An experimental and theoretical study of acoustic scattering from a fluid‐loaded circular cylindrical shell with an axially attached internal plate is presented. The normally insonified stainless‐steel shell has an outer to inner radius ratio b/a=0.98. A stainless‐steel plate of the same thickness is diametrically soldered, inside the shell, at both edges along the axis. The object is closed by two flat circular endcaps and is vertically immersed in a water tank (6 m×4 m×3 m). The aim of the study is to separate the effects of the shell and those of the plate and to interpret different sound generation mechanisms of the structure. The numerical simulation is done by the modal expansion method using the unstiffened cylindrical shell eigenmodes. The shell is taken as infinitely long. The internal plate is considered as a line constraint with the frequency‐dependent impedance. The comparison between theoretical and experimental results is achieved by an analysis of backscattered spectra and of arrival times ...

Acoustic scattering from an absorptive cylinder (Lucite) of infinite length excited by a normal plane acoustic wave is investigated in low frequencies. This one is assumed to be homogeneous and isotropic in its viscoelastic properties. The experimental results are obtained with a method of isolation and identification of resonances (MIIR). The resonance spectra are particularly studied. In the given frequency range, the presence of six families of circumferential waves labeled lw (1≤lw≤6) was noted. The resonances of these families, except lw=1, are wide compared to those of aluminum or steel cylinders. Nevertheless, the resonances of the first family are easily detected in the Lucite cylinder, contrary to aluminum or steel cylinders. The scattering of a plane‐wave pulse can be represented by the Fourier integral in order to determine a theoretical resonance spectrum without choosing the background. A good agreement is obtained for the studied frequency range between experimental and theoretical results. ...

The aim of this work is to study theoretically and experimentally, the acoustic radiation of a cylindrical elastic shell excited by a point source with no internal loading and immersed in a water tank. We are primarily focused on the theoretical study of a cylindrical elastic shell receiving a point force in the form of a Dirac pulse and the acoustic pressure radiated along the tube into a point at far-field observation. The model used is based on the theory of elasticity. It is therefore interesting, in a second step, to conduct experiments to validate these theoretical calculations and to realize complementary measures corresponding to the case of a focalized observer. The used shell is a cylindrical stainless steel tube characterized by its ratio equal to 0.94. Two experiments are carried out. The first experiment is based on a bistatic method, and the transmission and reception are performed by two focalized transducers, characterized by a center frequency of 1 MHz. For the second experiment, the two ...

An experimental study of sound scattering by a thin‐walled cylindrical shell stiffened by an internal lengthwise rib is presented. The results show that, in the explored frequency band, the positions of the resonance frequencies of the wave S0 are not affected much by the presence of a stiffener. Measurements made during the transient state after the quasiharmonic excitation of the stiffened shell have shown that, at the S0 wave resonance frequencies, backscattering is at its maximum when the rib is located at the vibration node of the shell. On the other hand, when in the total backscattered pressure, forced vibration is also taken into account, the maxima are detected when the rib is located at the vibration antinodes. In the stiffened shell other additional resonances appear which are not excited in the case of an unstiffened shell. These resonances are attributed to the propagation of the flexural type waves which are not detected in an unstiffened shell. In a stiffened shell these resonances appear a...

Acoustic resonance scattering from a water‐immersed, solid tungsten carbide target in the form of a finite‐length circular cylinder terminated by two hemispherical endcaps has been studied experimentally, and analyzed theoretically. The resonance frequencies have been determined from the condition of phase matching of the generated surface waves encircling the object; for the surface waves propagating over its cylindrical portion in the axial direction, the dispersion has been properly taken into account, as well as that of surface waves on the spherical surfaces. Surface displacements corresponding to the standing waves at each resonance were determined, and were used to calculate the angular patterns of the scattered acoustic field, in excellent agreement with the observed angular distributions. The pattern of lobes is found to provide information on the mode N of the standing surface waves, although in a less straightforward fashion than this had been the case in previous studies for the simple example...

Acoustic scattering from an isotropic elastic hollow cylindrical shell of infinite length excited by an obliquely incident plane acoustic wave is investigated. The form functions of an aluminum cylindrical shell immersed in water have been calculated by the direct summation of the Rayleigh series. Computations are made at angles (with the normal to the cylinder axis) between α=0° and α=35°. The results of the theoretical calculation are in good agreement with the results of experiments. The experimental results have shown in a frequency range of k1a=0 –20 that the resonances are related to three wave families: the circumferential wave (l=2) detected for angles smaller than the ‘‘angle of longitudinal wave in thin rods’’ (αl), the guided wave (p=1) detected for angles smaller than the second critical angle (αT), and the Scholte–Stoneley wave (l=0). The evolution of the resonance frequencies is followed for different angles and one can note experimentally, that at an angle superior to the Rayleigh critical ...

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...

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 meridian have been observed simultaneously.

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...

To recognize a target submerged in water, a spectroscopy based on the backscattering spectrum or on the resonance spectrum can be used. These spectra are experimentally obtained from the quasiharmonic or short pulse method of isolation and identification of resonances (MIIR). They show sharp amplitude variations that are related to the scatterer’s resonances. These resonances are related to the surface waves that propagate around the target. The backscattering spectrum and the resonance spectrum are power spectra. They are obtained from the computation of the modulus of the Fourier transform of the signal scattered by the target. In this paper, the computation of the phase tangent of the Fourier transform is presented. The spectra obtained are called the Im/Re spectra. The experimental results are compared with the computation of the ratio of the imaginary part over the real part of the complex pressure observed at a large distance from the target center. It is shown that the Im/Re spectra are highly useful for distinguishing resonances in scatterers with large absorptivity, where conventional backscattering or resonance spectra prove insensitive.

An experimental study is presented of the diffraction of Lamb waves at the end section of a plate imbedded in water. The amplitude of the diffracted wave is plotted versus angle of diffraction for different Lamb modes. The S modes have maxima of diffraction in their direction of propagation, as opposed to A modes that are associated with minima of diffraction in their direction of propagation. For all modes, the diffraction pattern is symmetric about the direction of propagation of the incident (Lamb) wave, and the peaks associated with maxima of the diffracted amplitude widen as the angle of observation increases. Those results are in good agreement with those obtained by a simple calculation of interferences between two sources, the relative phases of which are determined by the type (A or S) of diffracting Lamb wave.

The normal excitation of a tube immersed in water by the acoustic plane wave, circumferential waves are generated inside the shell. These circumferential waves, standing form stationary waves on the circumference of the tube for some frequencies. These stationary waves, constituting resonances of the tube which are perfectly visible on the backscattered spectrum. Moreover, the studies carried out on the diffusion of a plane acoustic wave by target were based primarily on the use of the monodimensional methods (Temporal domain and/or frequencial domain). To exceed the disadvantages of these methods, in this work, we used the time‐frequency representations such as the Short‐Term Fourier Transform (STFT), Wigner‐Ville Distribution (WVD) and Wavelet Transform method. These representations are applied to a theoretical signal backscattered by a tube of aluminium, copper and steel with radii ratio b/a = 0.95 (a is the external radius, and b the internal radius). From the time‐frequency images obtained we have visualized the dispersion of circumferential waves (S0, A1, S1,œ) and identified these different waves. This analysis permits to compare between these time‐frequency representations. And also we have compared between the cut‐off frequencies of circumferential waves obtained from these representations and those computed by the proper modes theory of the vibration.

In this work, we present two ultrasonic methods allowing to control the quality of adhesion of two aluminium sheets with the same thickness. These methods are based on the analysis of the ultrasonic signals retrodiffused by the Al/glue/Al structure. The first method consists in controlling the behaviour of the mode of the sheets which is splitted. Two parameters controlling the transfers were allowed to characterize any type of adhesion: good, bad and intermediate. The second method is based on control of the width of the mode of the adhesive. The representation of this width by Argan diagram allows to control the quality of adhesion from the measurement of the diagram diameter.

The problem of the acoustic diffusion by a tube is a theoretical problem which had known since few years a considerable interest. A good comprehension of the acoustic diffusion makes it possible the study of the opposite problem. Many theoretical and practical works were made by a tube in the acoustic diffusion field. These works showed in particular that the resonance phenomenons are directly related to the physical and geometrical properties of the target. The interest of the study presented in this paper is the identification of the circumferential wave. Several frequential analysis techniques were applied to characterize the target. These analysis techniques currently used present the major disadvantage to not be able to highlight the temporal structure of the field diffused by the target. Reciprocally the acoustic temporal analysis of the target ca not always separate the successive arrivals of the various type waves in the signal and to have information on their frequential contents. The joint time‐frequency representations provide this need. In this paper, the time‐frequency of Wigner‐Ville is applied to analyse the acoustic experimental signal backscattered by a steel tube with radii radio b/a=0.97 (a: external radius, b: internal radius) immersed in water. Through the Wigner‐Ville image we have identified the scholte A and the circumferential S0 waves.

An important challenge for current naval research is the modernization of battleships. Their target detection system must be increasingly efficient and they must be increasingly undetectable. Due to turbulent flows and bubbles, ship wakes are a detectable acoustic signature and ship bow waves disturb sonar detection. In this work, we study sound propagation through bubble clouds in water. We have developed an experimental set‐up which permits us to acquire, in synchronization, acoustical signals and optical images. The phenomenon of bubble monopole resonance in very low frequency, related to bubble size, provokes effects of strong sound damping and sound speed dispersion. These experimental results, related to theoretical results, permit to estimated sizes and concentrations of bubbles. The acquired bubble images permit to know the real bubble sizes and concentrations, in order to correlate with the experimental acoustical results. Air bubbles are generated with a high pressure water jet introduced into t...

In some submarine structures, the acoustic radiation of vibrations at the neighbourhood of sonar equipment limits their use. So the aim of this study is to understand the process of acoustic radiation of a submerged plate subjected to a vibration. For the low frequency domain, only two types of wave can propagate: the first antisymmetric Lamb wave (A0) and the first symmetric Lamb wave (S0). When the plate is immersed in water the A0 wave is modified and the new wave is named A wave. In this work, experimental and numerical analysis of vibration modes in a plate are carried out. The studied rectangular plate of thickness 10 mm is made of steel. Its length and width are respectively 1.0 m and 0.5 m. The plate is partially immersed in water (90%). Flexural vibrations are generated by a shaker normally connected to the emerging plate part. The applied signal is one sinusoidal period at a frequency which is under the critical frequency. Relations between the admittance of the plate and the radiated pressure in water are highlighted.

The non‐destructive evaluation of pipes using guided waves is extensively used for the detection and the size estimation of defects. The important number of investigations concerning with the interaction between a guided wave and a defect show the growing need to improve our knowledge of this phenomenon in order to detect and to characterize as well as possible the defect. This work is devoted to the evaluation of the width of an axisymmetric resin deposit coupled to the wall of a metallic tube by means of longitudinal modes S0, S1. To that end, we interest to damping of these modes provoked by the deposit. The study is carried out both from the numerical point of view and from the experimental point of view in considering a deposit of variable width. The finite element method is employed to model the propagation and the diffraction of Lamb‐type waves. The experiment is based on the generation of axisymmetric waves on the section of a tube by a piezoelectric transducer. Measurements of temporal echoes ref...

Numerous papers show that it is possible to characterize an air‐filled cylindrical shell immersed in water from a time‐frequency representation. The time scattered signal is constituted by echoes related to the reradiation of waves circumnavigating around the cylindrical target. For particular frequencies, these echoes make resonances which characterize this target. In first part, the scattered signal is calculated, the cylindrical shell is in water or in a medium with characteristics similar to the mixture sand/water used for the experimental measurements. In second part, the cylindrical shell is partially or completely buried in a thin sand/water mixture. It is insonified in normal or oblique incidence relatively to the interface water/mixture. The impulse time scattered signal is plotted and it is treated with a time‐frequency representation. The comparison between the theoretical and experimental time‐frequency representations shows us that it is possible to detect and classify the object buried in thin sand/water mixture.

Commander and Prosperetti works [J. Acoust. Soc. Am. 85, 732–746 (1989)] show that an acoustic wave, propagating in a bubbly water, is very strongly damped and its phase speed is disturbed for bubble resonance frequency neighboring. In relation to our experimental results, numerical results obtained with these analytical works are presented. Experiments are realized in a large water‐filled tank. A frequency modulated burst, generated with broadband transducers of central frequency 100, 200, and 500 kHz, propagates through the bubble cloud obtained by a high pressure water jet. Our measurements show simultaneously, after the stop of jet, a strong damping of acoustic signal and a small change of its phase speed. After some minutes, this signal gets back progressively its initial amplitude and phase speed values. One can notice that the acoustic signal amplitude and phase speed are not identical in all emission frequency domain. To explain this phenomenon, we assume the existence of frequency band gaps related to the multiple scattering from the bubble cloud. [Work supported by Bassin d’Essais des Carenes (France).]

The backscattering acoustic spectra of a cylinder show resonances due to the circumferential waves. Other resonance types are seen on the spectra, they are due to the thickness e of the shell. When this thickness is thin, the circumferential waves can be compared with the Lamb waves. The phase velocity of these waves depends on the frequency. The Lamb waves (Ai,Si, i≳1) have, in normal incidence, a cutoff frequency. In this case, only the longitudinal wave generates some Lamb modes. In oblique incidence, the transversal wave also generates Lamb modes. As the thickness is proportional to an integer of half‐wavelengths, it is easy to measure the thickness of a flat plate: The thickness resonances are clearly isolated. On the contrary, for a cylindrical shell, the thickness resonances are mixed with the circumferential resonances. It is difficult to determine the exact thickness resonance frequencies. To solve this problem, focused beam is used. The focal spot is smaller than the cylinder diameter. This meth...

Acoustic scattering from a finite air-filled elastic cylindrical shell, immersed in water, is investigated. The shell is made of stainless steel and has a thickness to outer radius ratio of 17%. The considered dimensionless frequency range extends over 7k1a22 (k1: wave number in water, a: outer radius). Bistatic measurements are carried out to identify vibration modes related to the phase matching of the first guided wave, T0, propagating on the shell. Both transducers, the emitter and the receiver, are positioned at the same angular distance with regard to the normal axis of the shell. The emitter transducer is fixed at a given position. In order to identify circumferential modes of vibration, the receiver transducer is made to rotate in the azimuthal plane, normal to the shell axis. Results obtained are plotted in functions of dimensionless frequency and azimuthal angle. Vibration modes along the shell's length are identified by moving the receiver transducer parallel to the shell axis. In this case, results are plotted in functions of dimensionless frequency and axial wave number. The experimental investigation is corroborated by theoretical results obtained from approximate calculations for thick finite cylindrical shells [Scot F. Morse et al., J. Acoust. Soc. Am. 103, 785-794 (1998)]. The evolution of the mode position with respect to the incidence angle is discussed so as to clarify peak patterns in backscattered resonance spectra.

Acoustic scattering from an isotropic elastic hollow cylindrical shell of infinite length excited by an obliquely incident plane acoustic wave is investigated. The waves generated at an incident angle α on a determined area of a cylindrical shell immersed in water and filled with air, propagate in a direction parallel to the cylinder axis and re‐emit their energy during the propagation at an angle α. The various velocities of propagation of helical waves allows one to record separately the resonance spectrum of each type of waves. Then, the radiation, in the far field, can be observed for each wave, at the end of a shell of semi‐infinite length, with a quasiharmonic method. In this case, no phenomenon of stationary waves can occur in the cylinder length. The axial displacement is analyzed and the use of the Rayleigh integral allows one to determine a model of the pressure radiated by the circular section of the tube. The results of the theoretical calculation are in agreement with the results of experiments.

It is well known that several experimental methods using quasiharmonic insonification allow a direct verification of the resonance scattering theory. They all provide, for elastic cylindrical targets immersed in water, the resonance frequencies and the mode number of each one. So far, pulsed techniques only allowed resonance isolation. Presented here is a new method: the short pulse method of isolation and identification that completes these works and allows a total comparison with theoretical and quasiharmonic experimental results. This method consists in the digitizing of the time signals that characterize the scattering of an elastic target insonified by a short pulse for different angular positions of the receiving transducer. For each so obtained signal, a resonance spectrum is obtained after a spectral amplitude analysis. From these data, a computer processing allows the plotting of identification patterns, and, by the same way, the knowledge of the mode of vibration at a given resonance frequency. ...

The acoustic scattering from an air‐filled finite cylindrical shell with hemispherical endcaps is considered. In axial incidence, experimental studies have shown formation of compressional wave S0 resonances. These resonances are predicted and identified by applying the phase‐matching condition over the meridian circumference. However, frequency irregularities observed experimentally on spectra have suggested that the assemblage quality during the manufacturing of the object may be not negligible. Indeed, internal nonuniformities of the shell at the junctions of cylindrical and hemispherical shells are present and modify certain wave propagation phenomena. The aim of this study is to discover these experimental conditions of the studied object through a numerical model with internal annular inhomogeneities localized at the junctions of the substructures and for two different b/a ratios (internal to external radius ratio) equal to 0.97 and 0.99. In the studied frequency range (50–300 kHz), in addition to the S0 wave, another peripheral wave A can be generated for the ratio b/a=0.97. The numerical approach is based on the discretization of the shell and determination of its eigenmodes (FEM). Acoustical pressure field is found by the boundary integral equation procedure (BEM). The comparison of numerical and experimental results is presented in frequency and time domains.

Let a thin‐walled finite cylindrical shell with the hemispherical end caps be immersed in a fluid medium. The shell contains structural inhomogeneities like radial solders and internal ribs and is excited by an axisymmetric pressure load. The paper deals with the numerical and experimental investigation of the sound‐pressure field scattered by the shell and its internals. The numerical solution of the problem is based on the finite element method (FEM) with the use of the thin shell theory elements. The Green’s function of the structure is obtained by the FEM and is further used in the boundary element solution scheme using both physical and modal coordinates. Flexural and membrane type eigenmodes of the fluid‐loaded structure can be determined and identified. A time‐domain solution to the problem obtained by the inverse Fourier transform gives echo‐signals of the sound scattered by the structure. Individual pulses in the echo‐signal help to localize structural inhomogeneities and to appreciate their importance in the scattered sound field. The membrane type vibrations are shown to be substantially modified by the internal solders and substructures. Experimental investigation is performed for a steel target immersed in a water tank and the numerical results correlate well with the experiment.

Acoustic scattering from a Lucite cylinder of infinite length immersed in water and excited by a normal incident plane acoustic wave is investigated at low frequencies. Numerical calculations and experiments are performed for a cylinder with a radius a=0.01 m. The experimental results are obtained with the method MIIR. The resonances of the first circumferential wave are easily isolated in the frequency range 100–300 kHz. A good agreement is obtained between experimental and theoretical results. First, in order to analyze the physical nature of vibration modes associated with this wave, the evolution of the resonance location and that of their corresponding widths are investigated theoretically when the fluid density is perturbated. Contrary to aluminum or steel cylinders, the computed calculus in vacuum are not in good agreement with the experimental results related to the first wave. The detection of this wave depends mainly on the outer fluid. Second, the dispersion curves of the phase velocity for thi...

A pulse ultrasonic technique has been developed to study the quality of the liquid/solid transformation of an epoxy resin made up of two components (resin and hardener). The epoxy resin is in a tank with parallel faces. A reflection technique allows the measurement of the phase velocity, the attenuation, and the acoustical impedance. The first and second parameters give an average state of the resin hardening in the thickness. The temporal evolution of the last parameter allows an evaluation of the state of the resin at the interface resin/tank wall. These experiments have been achieved for different proportions of the hardener. The velocity and the attenuation increase at first and then stabilize after a time which is as short as the hardener concentration is high. The experiment shows that 10% of the hardener is the optimum value and that 20% of it produces a vitreous transition. The experimental results also show that the acoustical impedance stabilizes after a time longer than the one observed for the velocity and the attenuation. The velocity and the attenuation are significant parameters of the optimum value of the hardener concentration. The acoustical impedance is the significant parameter of the bond quality.

The method of isolation and identification of resonances (MIIR) has allowed plotting of resonance spectra that characterize the shape, the structure, and the material of an object. On the spectra, the resonance peaks are related to surface waves propagating around the object. The backscattering, from thin‐walled cylindrical shell stiffened by internal lengthwise rib and insonified normally to its axis, is studied. In the explored frequency band, the frequencies of S0 wave resonances are not much affected by the presence of stiffener. Measurements made during transient state after quasiharmonic excitation of the stiffened shell have shown that, at the S0 wave resonances, backscattering is maximum when the rib is located at the vibration nodes. In the total backscattered pressure, considering also forced vibration, the maxima are detected when the rib is in the antinodes. In the stiffened shell, other additional resonances appear that are not detected in the case of the unstiffened shell. These resonances a...

The observation of the guided waves p=1 and p=2 from an isotropic elastic hollow cylindrical shell of infinite length excited by an obliquely incident plane acoustic wave is investigated. The use of ‘‘classic’’ MIIR (method of isolation and identification of resonances) is limited because of the refraction effect of the surface wave: The state of stationarity generated at a resonance frequency occurs out of the insonified area of the cylindrical shell. This problem has been solved by a new method called MIIR ‘‘in propagation.’’ Then, an isolation and an identification are possible in a large range of incidence, contrary to the configuration of transducers according to Snell–Descartes’ law (‘‘classic’’ MIIR). The resonances of the guided wave p=1 can be detected in a broad range of incidence angle. Contrary to the guided wave p=1, the experimental results achieved with incidence angles varying between 0° and the second critical angle show a particular evolution of the resonance frequencies of the guided wa...

The elastic scattering by a thin shell cylinder bounded by hemispherical endcaps is analyzed using a coupled finite element–boundary element method (ATILA and EQI codes). The target is made of steel and filled with air. The ratio of the inner radius b to the outer radius a is 0.97. The ratio of the total length L to a is 4. Axial, normal, and oblique incidences are considered for ka≤10 (k is the acoustic wave number in the fluid). The displacement field of the scatterer, the radiated and scattered pressures in the near field and far field, and the directivity patterns are provided. From these results, the peaks in the backscattered pressure are correlated to resonances of the structure and flexural or extensional waves propagating in the solid. Computed results are compared to experimental data obtained with the same target immersed in water. These results are performed by the quasiharmonic method M.I.I.R. (method of isolation and identification of resonances). This method allows one to obtain backscattered pressure spectra and patterns. [Work supported by D.R.E.T. Paris.]

The acoustic scattering from infinite cylinders or spheres has shown the great influence of surface wave propagation. Two types of surface waves are distinguished: the Rayleigh or Whispering Gallery waves and the Scholte–Stoneley waves. In this study, experimental results obtained on the finite cylindrical shells bounded by two hemispherical endcaps are presented. These objects are made of stainless steel and filled with air. Their radius is 27 mm and their radius ratio b/a is 0.97 (b is the inner radius and a the outer radius). The direction of insonification is parallel to the main object axis. The reduced frequency range k1a of the study varies from 10 to 40. These experimental results are explained by writing the stationary condition on the large circumference of objects, in the meridian plane. This condition depends on the phase velocity in the length of the cylindrical shell and in the spherical shell. These two velocities are computed separately in relation to the frequency and used to apply the st...

Tank experiments on acoustic bistatic and backscattering from elastic cylinders with hemispherical endcaps have been performed at the University of Le Havre and at Catholic University. The resonances apparent in these experiments have been interpreted on the basis of the phase‐matching principle [H. Uberall et al., J. Acoust. Soc. Am. 61, 771 (1977)] for surface waves that encircle these objects along closed geodesics. Upon axial incidence, the resonances of meridionally propagating surface waves are visible. Upon axial incidence, the resonances of meridionally propagating surface waves are visible. Upon broadside incidence, circumferential wave resonances are visible as well as the meridional ones. The frequencies of both types of resonances are explained very well by the phase‐matching principle. [Work supported in part by the David Taylor Res. Ctr.]

Two different kinds of experimental methods allow the isolation of resonances of cylindrical elastic targets immersed in water: a quasiharmonic one, the method of isolation and identification of resonances (MIIR) [G. Maze et al., J. Acoust. Soc. Am. 73, 41 (1983)] and an impulsive one, the short‐pulse MIIR [P. Pareige et al., Phys. Lett. A 135, 143 (1989)]. In both cases the experimental results only involve the spectral amplitude of the signal, so that the information provided by the phase spectrum is never used. A new method is proposed which uses the evolution with the frequency N of the ratio Im(N)/Re(N) of the complex number provided by the FFT of the whole backscattered temporal signal (for details see O. Lenoir, preceding abstract). The Im/Re spectra are compared to the spectra obtained by the two previous methods. Several elastic and viscoelastic cylinders and tubes are investigated. That the Im/Re spectrum provides as much information as the backscattered and resonance spectra obtained by the two different MIIR can be noticed. The first two methods allow the identification of the mode number of each isolated resonance; in the same way the Im/Re ratio at a given frequency versus the azimuthal angle is studied. The signification of the symmetric shape of the so‐obtained angular diagrams is discussed.

A recent experimental investigation [Maze et al., J. Acoust. Soc. Am. 77, 1352 (1985)] has shown that the spectrum of the resonance frequencies of plates can be obtained using ultrasonic pulse techniques. The distortion of incident wave trains upon their reflection, when the carrier frequency is near a resonance frequency, is analyzed here; the physical origin of plate resonances can be elucidated, the observed ringing of the resonances being caused by multiple internal reflections. A series of backscattering experiments, employing wave trains with carrier frequencies on and off a resonance, has been interpreted in the mentioned fashion, with very good agreement between theory and experiment.

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.

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.

Experiments have been carried out with 30‐μs‐long sinusoidal wave trains incident on an aluminum plate submerged in water. The excitation of plate resonances by such pulses has been observed, in a fashion used previously for cylinders [G. Maze et al., J. Acoust. Soc. Am. 77, 1352 (1985)]. The pulse distortions (in the form of initial transients, a quasi steady‐state regime, and a final transient) have been interpreted by the interference of reflected and multiple internally refracted pulses, as previously done by us for a layered ocean bottom [A. Nagl et al., Inverse Problems 1, 99 (1985)]. The final transient, showing a step structure, represents the ringing of the resonance. Overlapping resonances are shown to lead to beat effects in the ringing. [Work supported by the Direction des Recherches, Etudes et Techniques, France, and by the Office of Naval Research and the Naval Research Laboratory, U.S.]

The ultrasonic scattering from circular cylinders is described in numerous papers. If the insonification is perpendicular to the cylinder axis, the scattering is strongly influenced by the propagation of circumferential waves. These circumferential waves propagate either in the elastic shell, where they are the Whispering Gallery waves, or at the interface water/elastic shell, where they are the Scholte‐Stoneley waves. These waves, for some frequencies called resonances, form standing waves around the cylinder. Previously, it has been shown that the resonance frequencies are functions of the cylinder diameter, the shell thickness, and the target composition. In this presentation, the influence of the curvature radius on the position and the amplitude of resonances is studied. The method of isolation and identification of resonances (MIIR) is used to plot resonance spectra of air‐filled prolate cylinders. The incident ultrasonic beam perpendicular to the cylinder axis insonifies the cross section at different positions. The resonance spectra of an air‐filled prolate cylinder are compared to those obtained with a circular cylinder that has the same thickness and the same circumference.

The acoustic signatures of elastic targets immersed in water or inclusions in materials are more easily obtained and interpreted since the discovery and implementation, in the 1980s, of the “Method of Isolation and Identification of Resonances” (MIIR) [G. Maze and J. Ripoche, Phys. Lett. A 84, 309–312 (1981); J. Acoust. Soc. Am. 73, 41–43 (1983)] and derived pulse methods. The first interest is the isolation of resonances in quasiline spectra, which are obtained for passive or active targets when the excitation is external or internal. The second interest is the experimental identification of each resonance by the determination of the n mode number, which allows a classification of resonances. The results corroborate the “Resonance Scattering Theory” [H. Uberall, L. R. Dragonette, and L. Flax, J. Acoust. Soc. Am. 61, 711–715 (1977)]. Quasiharmonic MIIR and short‐pulse MIIR can be applied to cylinders, air‐ or liquid‐filled shells, and multilayered structures (or other targets) immersed in water by means of an external or internal excitation. Experiments give resonance spectra of liquid cylindrical inclusions in elastic materials. [Work supported by D.R.E.T., D.G.A., Contract 86/045.]