Showing papers on "Lamb waves published in 2014"

Surface Wave Methods for Near-Surface Site Characterization

Abstract: Overview of surface wave methods Seismic waves Test methodology Historical perspective Challenges of surface wave methods Typical applications Advantages and limitations Linear wave propagation in verticallyinhomogeneous continua Basic notions of wave propagation Rayleigh waves in homogeneous elastic half-spaces Existence of Love waves Surface waves in vertically inhomogeneouselastic continua Surface waves in vertically inhomogeneous, inelastic continua Measurement of surface waves Seismic data acquisition The wave field as a signal in time and space Acquisition of digital seismic signals Acquisition of surface waves Equipment Dispersion analysis Phase and group velocity Steady-state method Spectral analysis of surface waves Multi-offset phase analysis Spatial autocorrelation Transform-based methods Group velocity analysis Errors and uncertainties in dispersion analyses Attenuation analysis Attenuation of surface waves Univariate regression of amplitude versus offset data Transfer function technique and complex wavenumbers Multichannel multimode complex wavenumber estimation Other simplified approaches Uncertainty in the attenuation measurement Inversion Conceptual issues Forward modeling Surface wave inversion by empirical methods Surface wave inversion by analytical methods Uncertainty Case histories Comparison among processing techniques with active-source methods Comparison among inversion strategies Examples for determining Vs and Ds profiles Dealing with higher modes Surface wave inversion of seismic reflection data Advanced surface wave methods Love waves Offshore and nearshore surface wave testing Joint inversion with other geophysical data Passive seismic interferometry Multicomponent surface wave analysis, polarization studies, and horizontal-to-vertical spectral ratio References Index

Assessment of delay-and-sum algorithms for damage detection in aluminium and composite plates

Abstract: Piezoelectric sensors are increasingly being used in active structural health monitoring, due to their durability, light weight and low power consumption. In the present work damage detection and characterization methodologies based on Lamb waves have been evaluated for aircraft panels. The applicability of various proposed delay-and-sum algorithms on isotropic and composite stiffened panels have been investigated, both numerically and experimentally. A numerical model for ultrasonic wave propagation in composite laminates is proposed and compared to signals recorded from experiments. A modified delay-and-sum algorithm is then proposed for detecting impact damage in composite plates with and without a stiffener which is shown to capture and localize damage with only four transducers.

Noncontact depth-resolved micro-scale optical coherence elastography of the cornea.

Abstract: High-resolution elastographic assessment of the cornea can greatly assist clinical diagnosis and treatment of various ocular diseases Here, we report on the first noncontact depth-resolved micro-scale optical coherence elastography of the cornea achieved using shear wave imaging optical coherence tomography (SWI-OCT) combined with the spectral analysis of the corneal Lamb wave propagation This imaging method relies on a focused air-puff device to load the cornea with highly-localized low-pressure short-duration air stream and applies phase-resolved OCT detection to capture the low-amplitude deformation with nano-scale sensitivity The SWI-OCT system is used here to image the corneal Lamb wave propagation with the frame rate the same as the OCT A-line acquisition speed Based on the spectral analysis of the corneal temporal deformation profiles, the phase velocity of the Lamb wave is obtained at different depths for the major frequency components, which shows the depthwise distribution of the corneal stiffness related to its structural features Our pilot experiments on ex vivo rabbit eyes demonstrate the feasibility of this method in depth-resolved micro-scale elastography of the cornea The assessment of the Lamb wave dispersion is also presented, suggesting the potential for the quantitative measurement of corneal viscoelasticity

Experimental demonstration of ordered and disordered multiresonant metamaterials for lamb waves.

Abstract: We demonstrate the experimental realization of a multiresonant metamaterial for Lamb waves, i.e., elastic waves propagating in plates. The metamaterial effect comes from the resonances of long aluminum rods that are attached to an aluminum plate. Using time-dependent measurements, we experimentally prove that this metamaterial exhibits wide band gaps as well as sub- and suprawavelength modes for both a periodic and a random arrangement of the resonators. The dispersion curve inside the metamaterial is predicted through hybridizations between flexural and compressional resonances in the rods and slow and fast Lamb modes in the plate. We finally underline how the various degrees of freedom of such system paves the way to the design of metamaterials for the control of Lamb waves in unprecedented ways.

A high electromechanical coupling coefficient SH0 Lamb wave lithium niobate micromechanical resonator and a method for fabrication

Abstract: We present a high coupling coefficient, k eff 2 , micromechanical resonator based on the propagation of SH0 Lamb waves in thin, suspended plates of single crystal X-cut lithium niobate (LiNbO 3 ). The thin plates are fabricated using ion implantation of He to create a damaged layer of LiNbO 3 below the wafer surface. This damaged layer is selectively wet etched in a hydrofluoric (HF) acid based chemistry to form thin, suspended plates of LiNbO 3 without the wafer bonding, layer fracturing and chemical mechanical polishing in previously reported LiNbO 3 microfabrication approaches. The highest coupling coefficient is found for resonators with acoustic propagation rotated 170° from the y -axis, where a fundamental mode SH0 Lamb wave resonator with a plate width of 20 μm and a corresponding resonant frequency of 101 MHz achieves a k eff 2 of 12.4%, a quality factor of 1300 and a resonator figure of merit ( M ) of 185. The k eff 2 and M are among the highest reported for micromechanical resonators.

Micromachined One-Port Aluminum Nitride Lamb Wave Resonators Utilizing the Lowest-Order Symmetric Mode

Abstract: The characteristics of one-port aluminum nitride (AlN) Lamb wave resonators utilizing the lowest-order symmetric mode with electrically open, grounded, and floating bottom electrode configurations are theoretically and experimentally investigated in this paper. Finite element analysis is performed to take an insight into the static capacitance characteristics of the AlN Lamb wave resonators with various bottom surface conditions. Without sacrificing the transduction efficiency, the floating bottom electrode is capable of reducing the static capacitance in the AlN thin plate and then promotes an efficient improvement in the effective coupling coefficient (k2eff). In addition, in comparison with the grounded bottom electrode, the employment of the floating bottom electrode offers simple fabrication processes for the micromachined Lamb wave resonators. Experimentally, the AlN Lamb wave resonators without the bottom electrode exhibit an average loaded quality factor (Q) as high as 2676 at the series resonance frequency, but a low average k2eff of 0.19%. On the contrary, the Lamb wave resonators with the electrically floating bottom electrode show the largest average k2eff up to 1.13% among the three topologies but a low average loaded Q of 800 at the series resonance frequency. In contrast to the floating bottom electrode, the Lamb wave resonators with the electrically grounded bottom electrode show a smaller average k2eff of 0.78% and a similar average loaded Q of 758 at the series resonance frequency.

Predictive modeling of nonlinear wave propagation for structural health monitoring with piezoelectric wafer active sensors

Abstract: This article presents predictive modeling of nonlinear guided wave propagation for structural health monitoring using both finite element method and analytical approach. In our study, the nonlinearity of the guided waves is generated by interaction with a nonlinear breathing crack. Two nonlinear finite element method techniques are used to simulate the breathing crack: (a) element activation/deactivation method and (b) contact analysis. Both techniques are available in ANSYS software package. The solutions obtained by these two finite element method techniques compare quite well. A parametric analytical predictive model is built to simulate guided waves interacting with linear/nonlinear structural damage. This model is coded into MATLAB, and the WaveFormRevealer graphical user interface is developed to obtain fast predictive waveform solutions for arbitrary combinations of sensor, structural properties, and damage. The predictive model is found capable of describing the nonlinear wave propagation phenomen...

Lamb wave frequency–wavenumber analysis and decomposition

Abstract: Lamb waves have shown great potentials in damage detection of thin-walled structures due to their long propagation capability and sensitivity to a variety of damage types. However, their practical ...

Numerical analysis of Lamb waves using the finite and spectral cell methods

Abstract: SUMMARY An accurate and efficient simulation of wave propagation phenomena plays an important role in different engineering disciplines. In structural health monitoring, for example, ultrasonic guided waves are used to detect and localize damage and to assess the structural integrity of the component part under consideration. Because of the complexity of real structures, the numerical simulation of structural health monitoring systems is a computationally demanding task. Therefore, to facilitate the analysis of wave propagation phenomena, the authors propose to combine the finite cell method with the spectral element method. The ensuing novel method is referred to as the spectral cell method. Because it does not rely on body-fitted meshes, the resulting approach eliminates all discretization difficulties encountered in conventional finite element methods. Moreover, with the aid of mass lumping, it paves the way for the use of explicit time-integration algorithms. In the first part of the paper, we show that using a lumped mass matrix instead of the consistent one has no detrimental effect on the accuracy of the spectral element method. We introduce the spectral cell method in the second part, showing that, when applied to wave propagation analysis, the spectral cell method yields results comparable with other standard higher order finite element approaches.Copyright © 2014 John Wiley & Sons, Ltd.

Guided wave generation, sensing and damage detection using in-plane shear piezoelectric wafers

Abstract: This work presents guided wave generation, sensing, and damage detection in metallic plates using in-plane shear (d36 type) piezoelectric wafers as actuators and sensors. The conventional lead zirconate titanate (PZT) based on induced in-plane normal strain (d31 type) has been widely used to excite and receive guided waves in plates, pipes or thin-walled structures. The d36 type of piezoelectric wafer, however, induces in-plane (or called face) shear deformation in the plane normal to its polarization direction. This form of electromechanical coupling generates more significant shear horizontal (SH) waves in certain wave propagation directions, whose amplitudes are much greater than those of Lamb waves. In this paper, an analysis of SH waves generated using in-plane shear electromechanical coupling is firstly presented, followed by a multiphysics finite element analysis for comparison purposes. Voltage responses of both the conventional d31 and the new d36 sensors are obtained for comparison purposes. Results indicate that this type of wafer has the potential to provide a simple quantitative estimation of damage in structural health monitoring.

Acoustic black holes: recent developments in the theory and applications.

Abstract: Acoustic black holes are relatively new physical objects that have been introduced and investigated mainly during the last decade. They can absorb almost 100% of the incident wave energy, and this makes them very attractive for such traditional engineering applications as vibration damping in different engineering structures and sound absorption in gases and liquids. They also could be useful for some ultrasonic devices using Lamb wave propagation to provide anechoic termination for such waves. So far, acoustic black holes have been investigated mainly for flexural waves in thin plates, for which the required gradual changes in local wave velocity with distance can be easily achieved by changing the plates' local thickness. The present paper provides a brief review of the theory of acoustic black holes, including their comparison with optic black holes introduced about five years ago. Review is also given of the recent experimental work carried out at Loughborough University on damping structural vibrations using the acoustic black hole effect. This is followed by the discussion on potential applications of the acoustic black hole effect for sound absorption in air.

WaveFormRevealer: An analytical framework and predictive tool for the simulation of multi-modal guided wave propagation and interaction with damage:

Abstract: This article presents the WaveFormRevealer—an analytical framework and predictive tool for the simulation of guided Lamb wave propagation and interaction with damage. The theory of inserting damage effects into the analytical model is addressed, including wave transmission, reflection, mode conversion, and nonlinear higher harmonics. The analytical model is coded into MATLAB, and a graphical user interface (WaveFormRevealer graphical user interface) is developed to obtain real-time predictive waveforms for various combinations of sensors, structural properties, and damage. In this article, the main functions of WaveFormRevealer are introduced. Case studies of selective Lamb mode linear and nonlinear interaction with damage are presented. Experimental verifications are carried out. The article finishes with summary and conclusions followed by recommendations for further work.

Numerical Simulation of Nonlinear Lamb Waves Used in a Thin Plate for Detecting Buried Micro-Cracks

Abstract: Compared with conventional linear ultrasonic inspection methods, which are sensitive only to severe defects, nonlinear ultrasonic inspection methods are better for revealing micro-cracks in thin plates. However, most nonlinear ultrasonic inspection methods have only been experimentally investigated using bulk or Rayleigh waves. Numerical studies, especially numerical simulations of Lamb ultrasonic waves, have seldom been reported. In this paper, the interaction between nonlinear S0 mode Lamb waves and micro-cracks of various lengths and widths buried in a thin metallic plate was simulated using the finite element method (FEM). The numerical results indicate that after interacting with a micro-crack, a new wave-packet was generated in addition to the S0 mode wave-packet. The second harmonics of the S0 mode Lamb waves and the new wave-packet were caused by nonlinear acoustic effects at the micro-crack. An amplitude ratio indicator is thus proposed for the early detection of buried micro-cracks.

Guided Wave Damage Characterization via Minimum Variance Imaging with a Distributed Array of Ultrasonic Sensors

Abstract: Guided wave imaging with a distributed array of inexpensive transducers offers a fast and cost-efficient means for damage detection and localization in plate-like structures such as aircraft and spacecraft skins. As such, this technology is a natural choice for inclusion in condition-based maintenance and integrated structural health management programs. One of the implementation challenges results from the complex interaction of propagating ultrasonic waves with both the interrogation structure and potential defects or damage. For example, a guided ultrasonic wave interacts with a surface or sub-surface defect differently depending on the angle of incidence, defect size and orientation, excitation frequency, and guided wave mode. However, this complex interaction also provides a mechanism for guided wave imaging algorithms to perform damage characterization in addition to damage detection and localization. Damage characterization provides a mechanism to help discriminate actual damage (e.g. fatigue cracks) from benign changes, and can be used with crack propagation models to estimate remaining life. This work proposes the use of minimum variance imaging to perform damage detection, localization, and characterization. Scattering assumptions used to perform damage characterization are obtained through both analytical and finite element models. Experimental data from an in situ distributed array are used to demonstrate feasibility of this approach using a through-hole and two through-thickness notches of different orientations to simulate damage in an aluminum plate.

Creep damage characterization using nonlinear ultrasonic guided wave method: A mesoscale model

Abstract: The early deformations in materials such as creep, plasticity, and fatigue damages have been proved to have a close relationship with the nonlinear effect of ultrasonic waves propagating in them. In the present paper, a theoretical mesoscale model of an ultrasonic non-destructive method has been proposed to evaluate creep deformed states based on nonlinear guided waves. The model developed here considers the nonlinear generation of Lamb waves response from precipitates variation in the dislocation network, which can be applicable to all precipitate stages including coherent and semi-coherent precipitates in the metallic alloy undergoing creep degradation. To verify the proposed model, experiments of titanium alloy Ti60 plates were carried out with different creep strains. An “increase-decrease” change of the acoustic nonlinearity of guided wave versus the creep life fraction has been observed. Based on microscopic images analyses, the mesoscale model was then applied to these creep damaged Ti60 specimens, which revealed a good accordance with the measured results of the nonlinear guided waves. It is shown that the change of the nonlinear Lamb wave depends on the variations of the α2 precipitation volume fraction, the dislocation density, the growth of the creep-voids, and the increasing mismatch of the phase velocities during the creep deformation process. The results indicate that the effect of the precipitate-dislocation interactions on the nonlinear guided wave is likely the dominant mechanism responsible for the change of nonlinear guided wave propagation in the crept materials.

Lamb waves in anisotropic plates (review)

Abstract: Propagation of Lamb waves in elastic anisotropic plates is studied in the framework of the six-dimensional Cauchy formalism. Closed-form secular equations for dispersion curves for Lamb waves propagating in a plate with arbitrary elastic anisotropy are obtained.

A new omni-directional EMAT for ultrasonic Lamb wave tomography imaging of metallic plate defects.

Abstract: This paper proposes a new omni-directional electromagnetic acoustic transducer (EMAT) for the ultrasonic Lamb wave (ULW) tomography imaging (TI) of defects in metallic plates. The proposed EMAT is composed of a permanent magnet and a coil with a contra-flexure structure. This new EMAT coil structure is used for omni-directional ULW transmission and reception and ULW TI for the first time. The theoretical background and the working principles of this EMAT are presented and analyzed. The experimental results of its use on a 3 mm thick aluminum plate indicate that the EMAT with a contra-flexure coil (CFC) can transmit and receive a pure single A0 mode ULW with a high signal-to-noise ratio (SNR). Thus, the extraction of the projection data used for ULW TI may be performed accurately. The circumferential consistency of the projection data is only slightly influenced by the distortion of the eddy current field that is induced by the new CFC with an irregular shape. When the new EMAT array is used for ULW TI using the cross-hole method and SIRT arithmetic, a desirable imaging quality can be achieved, and the estimated size of an artificial corrosion defect agreed well with its actual value. The relation between the reconstruction resolution and the number of the new EMATs used is analyzed. More TI experiments are carried out when the aluminum plate defect is in two different locations relative to the EMAT array, for the further investigation of the performances of the new EMATs.

Block-sparse reconstruction and imaging for lamb wave structural health monitoring

Abstract: A frequently investigated paradigm for monitoring the integrity of plate-like structures is a spatially-distributed array of piezoelectric transducers, with each array element capable of both transmitting and receiving ultrasonic guided waves. This configuration is relatively inexpensive and allows interrogation of defects from multiple directions over a relatively large area. Typically, full sets of pairwise transducer signals are acquired by exciting one transducer at a time in a round-robin fashion. Many algorithms that operate on such data use differential signals that are created by subtracting prerecorded baseline signals, leaving only signal differences introduced by scatterers. Analysis methods such as delay-and-sum imaging operate on these signals to detect and locate point-like defects, but such algorithms have limited performance and suffer when potential scatterers have high directionality or unknown phase-shifting behavior. Signal envelopes are commonly used to mitigate the effects of unknown phase shifts, but this further reduces performance. The blocksparse technique presented here uses a different principle to locate damage: each pixel is assumed to have a corresponding multidimensional linear scattering model, allowing any possible amplitude and phase shift for each transducer pair should a scatterer be present. By assuming that the differential signals are linear combinations of a sparse subset of these models, it is possible to split such signals into location-based components. Results are presented here for three experiments using aluminum and composite plates, each with a different type of scatterer. The scatterers in these images have smaller spot sizes than delay-and-sum imaging, and the images themselves have fewer artifacts. Although a propagation model is required, block-sparse imaging performs well even with a small number of transducers or without access to dispersion curves.

Negative effective mass density of acoustic metamaterial plate decorated with low frequency resonant pillars

Abstract: We investigate the elastic wave dispersion by a phononic metamaterial plate containing low frequency resonator stubs arranged periodically over the plate. We show that this system not only provides stop bands for wavelengths much larger than the periodicity but also displays negative behavior of its effective mass density under the homogenization assumption. A numerical method is used to calculate the plate's effective dynamic mass density as function of the frequency where the metamaterial is considered as homogeneous plate for these large wavelengths. Strong anisotropy of the effective mass density matrix is observed around the resonance frequencies where the gaps are opened. In these regions, we demonstrate that the effective matrix density components take negative values. For each of these components, the negative behavior is studied by taking into account the polarization of the involved resonant modes as well as their associated partial band gaps opened for each specific Lamb symmetry modes. We found that coupling between Lamb waves and resonant modes strongly affects the effective density of the whole plate especially in the coupling frequency regions of the gaps.

Possible second-order nonlinear interactions of plane waves in an elastic solid.

Abstract: There exist ten possible nonlinear elastic wave interactions for an isotropic solid described by three constants of the third order. All other possible interactions out of 54 combinations (triplets) of interacting and resulting waves are prohibited, because of restrictions of various kinds. The considered waves include longitudinal and two shear waves polarized in the interacting plane and orthogonal to it. The amplitudes of scattered waves have simple analytical forms, which can be used for experimental setup and design. The analytic results are verified by comparison with numerical solutions of initial equations. Amplitude coefficients for all ten interactions are computed as functions of frequency for polyvinyl chloride, together with interaction and scattering angles. The nonlinear equation of motion is put into a general vector form and can be used for any coordinate system.

Mixing of collinear plane wave pulses in elastic solids with quadratic nonlinearity

Abstract: This paper derives a set of necessary and sufficient conditions for generating resonant waves by two propagating time-harmonic plane waves. It is shown that in collinear mixing, a resonant wave can be generated either by a pair of longitudinal waves, in which case the resonant mixing wave is also a longitudinal wave, or by a pair of longitudinal and transverse waves, in which case the resonant wave is a transverse wave. In addition, the paper obtains closed-form analytical solutions to the resonant waves generated by two collinearly propagating sinusoidal pulses. The results show that amplitude of the resonant pulse is proportional to the mixing zone size, which is determined by the spatial lengths of the input pulses. Finally, numerical simulations based on the finite element method and experimental measurements using one-way mixing are conducted. It is shown that both numerical and experimental results agree well with the analytical solutions.