Application of orthogonality-relation for the separation of Lamb modes at a plate edge: numerical and experimental predictions.

In this study the orthogonality relation-based method for post-processing finite element (FE) predictions and experimental measurements is applied in order to separate Lamb modes at a plate edge at normal incidence. The scattered wave field from the free edge is assumed to be a superposition of all the eigenmodes of an infinite plate. The eigenmode amplitudes of the reflected wave field are determined by implementing the orthogonality-based method on the measured plate edge displacements. Overlapping wavepackets of Lamb modes at a plate edge are simulated by using the FE model and the experiment in the case of an incident S0 mode in a plate with a notch. In the experiment a 3D Scanning Laser Doppler Vibrometer (3D SLDV) (Johansmann and Sauer, 2005) is used to measure 3 dimensional vibrations and thus the edge two-dimensional displacement components simultaneously. It is demonstrated that it is possible to extract signals of various propagating and non-propagating modes in time-domain. The influences of the errors in practical measurements on the extraction procedure have also been studied.

Acoustic scattering from a finite plate: generation of guided Lamb waves S(0), A(0) and A.

In the domain of renewable energies, marine current turbines constitute one of the possibilities of producing electrical energy. Naked-eye inspection, or with the aid of video monitoring systems of these machines to ensure their perfect working order, can be difficult in a turbid environment. Acoustic methods are conceivable. The study focuses on the blades of these machines, by considering rectangular plates. The propagation of Lamb waves in a plate is studied by analyzing experimental time signals obtained from acoustic scattering. These signals are analyzed employing the ray theory. In vacuum, the flexural wave is the A(0) Lamb wave, whilst in water this wave splits in a bifurcation: the A wave with a phase velocity always smaller than the sound speed in water, and the A(0) wave with a phase velocity always higher than the sound speed in water. In the central bandpass of the transducers used in the experiments, mainly the A and S(0) waves exist. However, signals observed in the third harmonic bandpass of the transducers are also analyzed. In order to complement these results, resonance frequencies of the plate studied are calculated taking into account the boundary conditions and compared with the resonance frequencies of the experimental spectra.

Prediction of the acoustic form function by neural network techniques for immersed tubes.

A new approach is used to predict the acoustic form function (FF) for an infinite length cylindrical shell excited perpendicularly to its axis using the artificial neural network (ANN) techniques. The Wigner-Ville distribution is used like a comparison tool between the FF calculated by the analytical method and that predicted by the ANN techniques for a stainless steel tube. During the development of the network, several configurations are evaluated for various radius ratios ba (a: outer radius: b: inner radius of the tube). The optimal model is a network with one hidden layer. It is able to predict the FF with a mean relative error about 1.61% for the cases studied in this paper.

Behavior of first guided wave on finite cylindrical shells of various lengths: experimental investigation.

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 k1a < or = 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.

Analysis of the acoustic scattering at variable incidences from an extra thin cylindrical shell bounded by hemispherical endcaps

Through an experimental approach, in this paper we investigate the acoustic wave scattering processes involved in the acoustic backscattering at variable incidences from an air-filled submerged cylindrical shell with hemispherical endcaps. Given the 1% shell thickness and the explored low frequency domain, the wave types studied are the circumferential or helical S0 wave and the helical T0 wave only. Between the axial (in the direction of the main axis of the object) and the normal incidences (normal to the main axis), two distinct angular zones can be observed depending on hemispherical or cylindrical excitation. In these zones, after a pressure wave excitation, different series of echoes on the echo wave forms are identified by their arrival times and related wave types. From results in the time domain and those obtained in the frequency domain, each acoustic response from the target corresponding to the two zones of excitation is compared with the acoustic response of canonical objects (spherical shell for axial excitation and tube for normal excitation). This analysis of the acoustic response from the target at various incidences, highlights the influence of both the endcaps and the finite length for a cylindrical shell on scattering. The study is intended to make a contribution to the knowledge of the identification of such geometrically complex objects.

Acoustic scattering from complex elastic shells: visualization of S0, A0 and A waves

Acoustic scattering phenomena from complex immersed elastic shells are studied by high-speed Schlieren visualization for spark-generated spherical quasi-Dirac excitations. The situations considered are air-filled cylindrical shells under normal insonification and air-filled cylindrical shells soldered at one end with a hemispherical endcap under axial and non-axial incidences. The results are compared with those provided by the Methode d'Isolement et d'Identification des Resonances (MIIR) method and corresponding analytical theoretical solutions. The combination of these complementary approaches better highlights some of the behaviors of the symmetric S0- and antisymmetric A0-Lamb waves, as well was the Scholte-Stoneley A-wave, on the considered targets. In particular, the influence of the internal solder inhomogeneities on the creation and/or the conversion of these modes is demonstrated.

Acoustic scattering from fluid-loaded stiffened cylindrical shell: analysis using elasticity theory

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.

Ultrasonic characterization of the quality of an epoxy resin polymerization.

This work concerns monitoring the polymerization of an epoxy resin and its hardener. An ultrasonic pulse echo technique was used to monitor the attenuation, the phase velocity, and the acoustic impedance of the resin as a function of time. The first two parameters give information about the average state of the hardening of the resin itself. The third parameter, acoustic impedance, indicates the state of the hardening of the resin at the interface with the vessel. These parameters are determined from spectral properties of echoes extracted from the experimental echo waveforms. Experiments were made for different proportions of hardener and allowed a determination of the best mixture (10% of hardener) that corresponds to the manufacturer's recommended value. Analysis of the results shows a progressive hardening from the center of the resin toward the walls of the vessel.