Leaky Lamb Waves Research Articles

Noncontact monitoring of immersed plates by means of laser-induced ultrasounds

This article presents the results of an experimental and numerical study where guided ultrasonic waves were used for the structural health monitoring/nondestructive evaluation of an immersed aluminum plate. Leaky Lamb waves were generated by means of a pulsed laser and detected by an array of immersion transducers. The signals were then processed using continuous wavelet transform to extract few damage-sensitive features that were fed to an unsupervised learning algorithm based on outlier analysis. The experimental setup was simulated numerically using a commercial finite element software to predict the time of arrival of the propagating modes. In order to assess the capability of the monitoring system to detect damage, four defects were devised on the plate prior to the immersion in water. We found that the noncontact probing system and the signal processing enable the detection of cracks and holes.

Detection of coatings within liquid-filled tubes and containers by mode conversion of leaky Lamb waves

Abstract. In this paper, a new acoustic sensor principle for coating detection within liquid-filled tubes and containers based on mode conversion of leaky Lamb waves is introduced. Leaky Lamb waves are excited and detected by single-phase transducers, which are attached on the outer side of a tube or container. By transmission time and amplitude measurements, coating formation within the liquid-filled tube and container is detected non-invasively. This new sensor principle is subdivided into the separate considerations of Lamb wave excitation, mode conversion and inverse mode conversion. The Lamb wave excitation by a single-phase transducer is visualized by scanning laser Doppler vibrometer imaging. The mode conversion process of leaky Lamb waves is measured by membrane hydrophone measurements and Schlieren visualization; afterwards, the measured emission angles are compared with the theoretical one. The inverse mode conversion process of pressure waves back to leaky Lamb waves is visualized by Schlieren images. By merging the results of Lamb wave excitation, mode conversion and inverse mode conversion, the new sensor concept is explained. Theoretical considerations and measurement results of adhesive tape coating inside a liquid-filled plastic tube and a liquid-filled stainless steel container verify the new acoustic sensor principle. Finally the measuring sensitivity and the technical realization are discussed.

J041044 半解析的有限要素法を用いた漏洩ラム波の分散曲線の導出([J041-04]超音波計測・解析法の新展開(4))

J041044 半解析的有限要素法を用いた漏洩ラム波の分散曲線の導出([J041-04]超音波計測・解析法の新展開(4))

Determination of leaky Lamb wave modal attenuation coefficient on a solid plate

Leaky Lamb wave is practically important in an immersion technique of nondestructive evaluation for a solid plate. It is necessary to determine its modal attenuation coefficient in order to evaluate plate defects in industry and to diagnose osteoporosis in medical applications. In this study we present a method to determine the modal attenuation coefficient of leaky Lamb wave using two-dimensional Fourier filtering. A complex leaky Lamb wave signal from a solid plate can be decomposed into several modal signals on frequency-space domain through two-dimensional Fourier filtering. It makes possible to experimentally determine the modal attenuation coefficient along an aluminum plate in water. It is well explained by theoretical estimation with the dispersion relation of leaky Lamb wave.

Effects of beryllium coating layer on performance of the ultrasonic waveguide sensor

Under-sodium viewing is one of the critical technical issues and requirements for the in-service inspection of the sodium-cooled fast reactor (SFR) that is currently under development. The waveguide sensor that uses leaky A0 mode Lamb waves has shown its potential for high-resolution viewing/scanning of the reactor core and in-vessel structures. However, a few problems arise under a liquid sodium environment due to high sound speed in liquid sodium and dispersion in the long waveguide plate, which simultaneously deteriorate the reconstructed C-scan images. This paper proposes coating the surface of the waveguide sensor plate with a thin layer of material that has a very high ultrasonic wave velocity. It is shown that this coating layer can largely reduce the size (width) and radiation angle of the acoustic beam from the waveguide sensor. This paper precisely analyzes the effects of coating parameters on the beam quality. The proposed idea is validated through ultrasonic experiments in which the radiation beam profiles and group velocities in waveguide sensors with different surface treatments are measured and compared.

Interaction between gas flow and a Lamb waves based microsensor

The interaction between the gas flow and Lamb waves is investigated in this paper. Depending on the fact that the phase velocity is higher or lower than the gas sound velocity, we will get evanescent waves (EW) or leaky Lamb waves (LLW) in the gas along the solid–gas interface. In the LLW case, experiments showed that gas flow had not evident effects on Lamb waves’ propagations. In the EW case, the interaction between the gas flow and the Lamb waves was observed clearly when the Lamb wave phase velocity is close to the gas sound velocity. This interaction is related with the gas flow velocity profile within the boundary layer. The experimental results show that this sensor is very promising for many experiments involving gas flows such in wind tunnels, micro channels characterization, and can lead to multi-parameters measurements.

Formation of two-way Lamb waves and force potential wells using single conventional ultrasonic transducer and sheep horn shaped metal piece

A new method is introduced that uses single conventional ultrasonic transducer and a sheep horn shaped metal piece to induce two-way Lamb waves, and furthermore collects leaky Lamb waves to form two force potential wells. The immersion acoustical measurements show that the two-way Lamb waves with adjustable amplitudes can be induced when a 1MHz transducer radiates ultrasonic waves on to the semicircle of a metal “sheep horn”. Furthermore, the results obtained by hydrophone measurement and fluorescence microscope observation verify that the force potential well can be formed in the circle of metal “sheep horn” by the leaky Lamb wave of each way and possibly used to collect the submicro particles. This designed method is intended to provide assistance in limited space for microscope observing by collecting particles within low force potential well. All experiment results also indicate that radiation force and the gradient of force potential well produced by this leaky Lamb wave device is very small due to several reasons including low generation efficiency at semicircle tip and propagation attenuation of Lamb wave in metal piece, near field properties of leaky Lamb wave in liquid, and so on.

Air-coupled sensing of leaky rayleigh waves and ZGV modes in concrete slabs

Rayleigh waves and zero-group velocity (ZGV) Lamb modes (the impact-echo test) are commonly used for non-destructive evaluation (NDE) of concrete structures to extract information about material property and interior delaminations. Traditional methods in civil engineering employ point impactors to generate the waves while accelerometers measure the resulting out-of-plane motion. Though well-established, this methodology does not lend itself to efficient scanning of large areas, which is critical for monitoring the safety of infrastructure. The ability to detect these two wave types through in-air sensing of Leaky Rayleigh or ZGV Lamb waves can greatly accelerate NDE for large structures. Unfortunately, air-coupled sensing suffers from significantly decreased signal amplitude. This paper presents theoretical analysis and experimental validation efforts to amplify in-air signals resulting from Leaky Rayleigh waves and ZGV Lamb modes using a parabolic focusing mirror. A time-domain impulse response to the Ki...

The Potential of Ultrasonic Non-Destructive Measurement of Residual Stresses by Modal Frequency Spacing using Leaky Lamb Waves

This paper investigates the potential of ultrasonic non-destructive measurements of residual stresses using the modal frequency spacing method based on the interference spectrum of leaky Lamb waves as an alternative to the commonly used flight-time approach in ultrasonic methods. Extensive experiments were carried out to verify the viability and robustness of the technique using an instrumented leaky Lamb wave setup with uniaxial stressed samples and welded steel samples. To improve the signal-to-noise ratio, multiple sets of raw signals of specularly reflected and leaky Lamb waves were acquired and then averaged in the time domain. The acquired data in the time domain were then transformed into the frequency domain to form the interference spectrum of leaky Lamb waves with a good repeatability. The acoustoelastic coefficient of carbon steel is then derived from the measured relationship of wave velocity and applied stress. Finally, a welded steel plate was examined and residual stress was evaluated. The current work demonstrates the feasibility and the potential of the proposed method in measuring residual stresses in welded plates and thin-walled structures.

Assessment of viscous and elastic properties of sub-wavelength layered soft tissues using shear wave spectroscopy: Theoretical framework and in vitro experimental validation

In elastography, quantitative imaging of soft tissue elastic properties is provided by local shear wave speed estimation. Shear wave imaging in a homogeneous medium thicker than the shear wavelength is eased by a simple relationship between shear wave speed and local shear modulus. In thin layered organs, the shear wave is guided and thus undergoes dispersive effects. This case is encountered in medical applications such as elastography of skin layers, corneas, or arterial walls. In this work, we proposed and validated shear wave spectroscopy as a method for elastic modulus quantification in such layered tissues. Shear wave dispersion curves in thin layers were obtained by finite-difference simulations and numerical solving of the boundary conditions. In addition, an analytical approximation of the dispersion equation was derived from the leaky Lamb wave theory. In vitro dispersion curves obtained from phantoms were consistent with numerical studies (deviation <1.4%). The least-mean-squares fitting of the dispersion curves enables a quantitative and accurate (error < 5% of the transverse speed) assessment of the elasticity. Dispersion curves were also found to be poorly influenced by shear viscosity. This phenomenon allows independent recovery of the shear modulus and the viscosity, using, respectively, the dispersion curve and the attenuation estimation along the propagation axis.

Influence of low pressure on laser inducing leaky Lamb wave and Scholte wave at air–solid interface

A setup with Q-switched Nd:YAG laser inducing acoustic wave at air–solid interface and air-coupled optical deflection sensor was developed to research the influences of low air pressure on laser inducing leaky Lamb and Scholte waves at air–solid interface. The solid plate is settled in an airtight vessel abounded with standard air and its pressure can be adjusted by a vacuum pump. By experiments, the waveforms of laser induced interface acoustic waves, leaky Lamb and Scholte waves, under air pressures from 0.02 to 0.08 MPa with 0.01 MPa interval were first measured comprehensively. From the waveforms, we find that with increasing air pressure, the leaky energy of Lamb waves increase linearly and the amplitude of Scholte waves increase exponentially, but the velocity of Scholte waves decreases obviously.

Resonance frequencies of piezoelectric plates surrounded by solid and fluid half-spaces

The design of ultrasound transducers, resonators and other piezoelectric devices usually requires the calculation of the resonance frequencies of piezoelectric plates. Recent studies have shown that the resonance frequencies for plates in vacuum correspond to frequencies where the waveguide group velocity vanishes (zero-group-velocity points). However, those studies are limited to vacuum boundary conditions. The objective of the present study is to analyze the resonance frequencies of layered piezoelectric plates in contact with solid and fluid half-spaces and their relation to the dispersion behavior of the elastic guided wave propagation. Theoretical analysis using partial-wave approach of leaky Lamb waves is performed to study wave propagation in, and resonance behavior of, multilayered plates in contact with solid and fluid half-spaces. A novel observation resulted from this analysis is that, for plates in contact with solid and fluid half-spaces, the resonance frequencies occur at points where the magnitude of the wavenumber reaches a minimum. This frequency is named as a ‘ transition frequency’. Such observations are important because they allow an easy identification of resonance frequencies with high amplitude response directly from the dispersion curves. This study will be helpful for the design of piezoelectric components used for resonators and sensors.

Laser-induced thermoelastic Leaky Lamb waves at the fluid–solid interface

A model based on the theory of fluid–structure interaction is developed to simulate the laser thermoelastic generation and propagation of Leaky Lamb waves at the water–aluminum interface. Each component of displacement, stress, and temperature are derived in transform domain by the photothermoelastic transfer matrix method. The time domain solutions are obtained by numerically inverting the transforms while the dispersion curves and attenuation curves for the leaky waves are also calculated. Then the propagation characteristics of different modes are analyzed. The model establishes a quantitative relation between the laser parameters, the material parameters, the corresponding waveforms, and the dispersion curves, which provides a useful tool for the Leaky Lamb waves applied to nondestructive evaluation.

Development of ultrasonic waveguide sensor for under-sodium inspection in a sodium-cooled fast reactor

The reactor core and in-vessel structures of a sodium-cooled fast reactor (SFR) cannot be examined visually due to the opaque sodium. The examination of the in-vessel structures is possible using ultrasonics to penetrate the sodium. A plate-type ultrasonic waveguide sensor using a leaky Lamb wave (A 0 mode) has been developed for under-sodium visual inspection of the reactor core and in-vessel structures. In the plate waveguide sensor, the A 0 leaky Lamb wave is utilized for the single mode generation and the effective radiation capability in a fluid. The liquid wedge is applied for the generation of the A 0 mode in the low frequency range. The long pulse tone-burst excitation should be applied to minimize the dispersion effect in 10 m long distance propagation of the A 0 Lamb wave. And a novel technique which is capable of steering a radiation beam of a waveguide sensor without a mechanical movement of the waveguide sensor has been suggested. The control of the beam angle can be achieved by a frequency tuning method of the excitation pulse in the dispersive low frequency range of the A 0 Lamb wave. A 10 m long ultrasonic waveguide sensor module which consists of a plate waveguide, a liquid wedge, an ultrasonic sensor, and an acoustical shielding protection tube has been designed and manufactured. The possibility of applying the ultrasonic waveguide sensor module to an under-sodium visual inspection has been investigated. The experimental tests such as the long distance propagation test of A 0 Lamb wave, the beam profile measurements, and C-scanning experiments in water have been carried out for the performance of the ultrasonic waveguide sensor. The feasibility of the ultrasonic waveguide sensor technique has been successfully demonstrated.

Stress Evaluation Using Ultrasonic Interference Spectrum of Leaky Lamb Waves

This paper presents a nondestructive stress evaluation technique using the ultrasonic interference spectrum of leaky Lamb waves. By using a specific pitch-catch ultrasonic setup, the symmetric and antisymmetric modes of Lamb waves in a finite plate are decoupled, leading to simple relationships between the modal frequency spacing of two adjacent modes in the interference spectrum and the acoustic wave velocities that are functions of stress. As a result, the stress in the plate can be determined by measuring the modal frequency spacing instead of the relative flight times to calculate the acoustic wave velocity. Extensive experiments were carried out to verify the viability and robustness of the new technique using a simple testing system. It has been demonstrated that the new technique is about 25 times more accurate than existing flight-time approaches using the same testing system. The experimental results agree well with the results obtained by other ultrasonic methods using expensive equipment.

Optical visualization of coupling modes of leaky Lamb waves with negative group velocity in the solid/fluid/solid trilayer

Optical visualization of coupling modes of leaky Lamb waves with negative group velocity in the solid/fluid/solid trilayer

AXISYMMETRIC VIBRATIONS IN MICROPOLAR THERMOELASTIC CUBIC CRYSTAL PLATE BORDERED WITH LAYERS OR HALF SPACES OF INVISCID LIQUID

The present study is concerned with the propagation of ax- isymmetric vibrations in a homogenous isotropic micropolar thermoelas- tic cubic crystal plate bordered with layers or half spaces of inviscid liq- uid subjected to stress free boundary conditions in context of Lord and Shulman (L-S) and Green and Lindsay (G-L) theories of thermoelastic- ity. The secular equations for symmetric and skew-symmetric leaky and nonleaky Lamb wave modes of propagation are derived. The amplitudes of displacement components, microrotation and temperature distribution are also computed numerically and presented graphically. Finally, in or- der to illustrate the analytical developments, numerical solution of secu- lar equations corresponding to stress free thermally insulated micropolar thermoelastic cubic crystal plate is carried out for magnesium crystal material bordered with water layers of finite and infinite thickness.

Surface wave testing of pavements.

Pavements are constructed using several layers of materials, and their durability depends on the quality of all of these strata. It is, therefore, valuable to be able to determine the properties of the layers nondestructively. A method is presented for evaluating the thickness and stiffness of multilayered pavement structures from guided waves measured at the surface. In this type of layered structure, the interaction of leaky Lamb waves in the embedded layers generates surface waves corresponding only to certain portions of the guided wave dispersion curves and branches measurable at the pavement surface. To resolve the different mode branches, the wavefield is measured at the surface by using a light hammer as the source and an accelerometer as receiver, generating a synthetic receiver array. The recorded data are transformed to a phase velocity spectrum, which is then inverted to give the layer properties using a global inversion algorithm. The theoretical background along with experimental results of the application to nondestructive testing of pavements will be presented. Ongoing research on noncontact air-coupled measurements is also demonstrated. This opens up the possibility for faster on-the-fly surface wave testing of pavement layers, since surface contact is no longer required.

Numerical simulation and experimental detection of leaky Lamb waves induced by pulse laser at air-solid interfaces

Based on the thermoelastic mechanism of laser ultrasonic, the problems of the thermal conduction and the coupling between the motion of solid and fluid are solved by using the finite element method. And then the transient waveforms of leaky Lamb waves induced by pulse laser action on the air-aluminum interface are obtained. Experimental signals of laser-induced leaky Lamb waves at the air-aluminum interface are detected by applying an our-developed detector, based on the light deflection principle. The dispersion and attenuation properties of leaky Lamb waves are analyzed through the phase spectral analysis.

Determination of Condition for Fastest Negative Group Velocities of Lamb‐Type Waves under each Density Ratio of Solid and Liquid Layers

Lamb‐type waves are coupling modes of leaky Lamb waves on a layer structure. The Lamb‐type waves have complicated propagation characteristics more than ordinary Lamb waves on a uniform elastic plate. In the characteristics, we examine the negative group velocities. Generally, the negative group velocities of Lamb waves are slower than positive group velocities under the same condition. If the negative group velocities are applied to fabricating some new application, it is desired that the speeds of negative group velocities are comparable to those of positive group velocities. Consequently, we aim to obtain the faster negative group velocities. Lamb‐type waves show more discriminative characteristics in negative group velocities than ordinary Lamb waves. In this research, we consider the Lamb‐type waves in a solid/liquid/solid structure. It is described the conditions for obtaining the fastest negative group velocities of Lamb‐type waves. The conditions, which are the acoustical impedance ratio, are expressed as the function of the density ratio of solid and liquid layers. These results are verified by numerical calculations.