Showing papers on "Lamb waves published in 2008"

Temperature effects in ultrasonic Lamb wave structural health monitoring systems.

Abstract: There is a need to better understand the effect of temperature changes on the response of ultrasonic guided-wave pitch-catch systems used for structural health monitoring. A model is proposed to account for all relevant temperature-dependent parameters of a pitch-catch system on an isotropic plate, including the actuator-plate and plate-sensor interactions through shear-lag behavior, the piezoelectric and dielectric permittivity properties of the transducers, and the Lamb wave dispersion properties of the substrate plate. The model is used to predict the S(0) and A(0) response spectra in aluminum plates for the temperature range of -40-+60 degrees C, which accounts for normal aircraft operations. The transducers examined are monolithic PZT-5A [PZT denotes Pb(Zr-Ti)O3] patches and flexible macrofiber composite type P1 patches. The study shows substantial changes in Lamb wave amplitude response caused solely by temperature excursions. It is also shown that, for the transducers considered, the response amplitude changes follow two opposite trends below and above ambient temperature (20 degrees C), respectively. These results can provide a basis for the compensation of temperature effects in guided-wave damage detection systems.

Quantitative assessment of through-thickness crack size based on Lamb wave scattering in aluminium plates

Abstract: The interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analysed in terms of finite element method and experimental study. For oblique-wave incidence, both numerical and experimental results showed that the wave scattering from a crack leads to complicated transmission, reflection and diffraction accompanied by possible wave-mode conversion. A dual-PZT actuation scheme was therefore applied to generate the fundamental symmetrical mode (S0) with enhanced energy to facilitate the identification of crack-scattered wave components. The relationship between crack length and the reflection/transmission coefficient obtained with the aid of the Hilbert transform was established, through which the crack length was quantitatively evaluated. The effects of wavelength of Lamb waves and wave diffraction on the properties of the reflection and transmission coefficients were analysed.

Local vibration of an elastic plate and zero-group velocity Lamb modes

Abstract: Elastic plates or cylinders can support guided modes with zero group velocity (ZGV) at a nonzero value of the wave number. Using laser-based ultrasonic techniques, we experimentally investigate some fascinating properties of these ZGV modes: resonance and ringing effects, backward wave propagation, interference between backward and forward modes. Then, the conditions required for the existence of ZGV Lamb modes in isotropic plates are discussed. It is shown that these modes appear in a range of Poisson's ratio about the value for which the cutoff frequency curves of modes belonging to the same family intercept, i.e., for a bulk wave velocity ratio equal to a rational number. An interpretation of this phenomenon in terms of a strong repulsion between a pair of modes having a different parity in the vicinity of the cutoff frequencies is given. Experiments performed with materials of various Poisson's ratio demonstrate that the resonance spectrum of an unloaded elastic plate, locally excited by a laser pulse, is dominated by the ZGV Lamb modes.

Lamb Wave-Mode Tuning of Piezoelectric Wafer Active Sensors for Structural Health

Abstract: An analytical and experimental investigation of the Lamb wave-mode tuning with piezo-electric wafer active sensors (PWASs) is presented. The analytical investigation assumesa PWAS transducer bonded to the upper surface of an isotropic flat plate. Shear lagtransfer of tractions and strains is assumed, and an analytical solution using the space-wise Fourier transform is reviewed, closed-form solutions are presented for the case ofideal bonding (i.e., load transfer mechanism localized at the PWAS boundary). Theanalytical solutions are used to derive Lamb wave-mode tuning curves, which indicatethat frequencies exist at which the A0 mode or the S0 mode can be either suppressed orenhanced. Extensive experimental tests that verify these tuning curves are reported. Theconcept of “effective PWAS dimension” is introduced to account for the discrepanciesbetween the ideal bonding hypothesis and the actual shear-lag load transfer mechanism.The paper further shows that the capability to excite only one desired Lamb wave modeis critical for practical structural health monitoring (SHM) applications such as PWASphased array technique (e.g., the embedded ultrasonics structural radar (EUSR)) and thetime reversal process (TRP). In PWAS phased array EUSR applications, the basic as-sumption of the presence of a single low-dispersion Lamb wave mode (S0) is invokedsince several Lamb wave modes traveling at different speeds would disturb the damageimaging results. Examples are given of correctly tuned EUSR images versus detunedcases, which illustrate the paramount importance of Lamb wave-mode tuning for thesuccess of the EUSR method. In the TRP study, an input wave packet is reconstructed ata transmission PWAS when the signal recorded at the receiving PWAS is reversed in thetime domain and transmitted back to the original PWAS. Ideally, TRP could be used fordamage detection without a prior baseline. However, the application of TRP to Lambwaves SHM is impended by the dispersive and multimodal nature of the Lamb waves. Thepresence of more then one mode usually produces additional wave packets on both sidesof the original wave packet due to the coupling of the Lamb wave modes. The PWASLamb wave tuning technique described in this paper is used to resolve the side packetsproblem. Several tuning cases are illustrated. It is found that the

Intrinsic temperature compensation of aluminum nitride Lamb wave resonators for multiple-frequency references

Abstract: In this paper we present the temperature compensation of aluminum nitride (AlN) Lamb wave resonators for a future application to XOs and TCXOs for a frequency ranging from 100 MHz to 1000 MHz. The temperature coefficient of frequency (TCF) for the lowest symmetric Lamb wave mode S0 for AlN plates with h/lambda<0.3 is found to be around -30 ppm/K. A zero TCF resonator is obtained by adding a compensating silicon dioxide layer. The low dispersion of the phase velocity for the S0-mode propagating in thin AlN plates reduces not only the fabrication tolerances towards thickness variations of the AlN layer, but also enables resonators operating over a wide frequency range, i.e. from 100 MHz to 1000 MHz, based on two absolute film thicknesses for AlN and SiO2 achieving near zero TCF over the entire frequency range. The acoustic properties and different layer configurations of zero TCF Lamb wave devices are discussed in detail.

Time-Domain Spectral Element Method for Built-In Piezoelectric-Actuator-Induced Lamb Wave Propagation Analysis

Abstract: An investigation was performed to develop a computational model based on a spectral element method to simulate piezoelectric-actuator-induced acousto-ultrasonic wave propagation in a metallic structure. The model solves the coupled electromechanical field equations simultaneously in both three-dimensional and two-dimensional plane strain conditions, and so it can accept any arbitrary waveform of electrical voltage as input to any piezoelectric transducer and produce piezoelectric sensor output in voltage as a result of the excitation generated by the transducer. Basically, the model inputs electrical voltage to actuators and outputs electrical signals of sensors. To visualize the transient dynamic wave motions in the structure generated by the transducer, the code is integrated with commercial pre/postprocessing software to provide graphical outputs of the dynamic deformations of the structure. The code was verified by comparison with experimental results. Performance of the model was examined in terms of solution convergence compared with the finite element method.

Ultrasonic guided-waves characterization with Warped Frequency Transforms

Abstract: Guided wave (GW) dispersion curves can be extracted from a time-transient measurement by means of time-frequency representations (TFRs). Unfortunately, any TFR is subject to the time-frequency uncertainty principle. This, in general, limits the capability of TFRs to characterize closely spaced guided modes over a wide frequency range. To overcome this limitation, we implemented a new warped frequency transform that presents enhanced mode extraction capabilities because of a more flexible tiling of the time-frequency domain. The tiling is designed to match the dispersive spectro-temporal structure of a GW by selecting an appropriate map of the time-frequency plane. The proposed transformation is fast, invertible, and covariant to group delay shifts. An application to Lamb waves propagating in an aluminum plate is presented. Time-transient GWs propagation events obtained both numerically and experimentally are considered. The results show that the proposed warped frequency transform limits the interference patterns which appear with other TFRs and produces a sparse representation of the Lamb wave pattern that can be suitable for identification and characterization purposes.

Are there waves in elastic wave turbulence

Abstract: An thin elastic steel plate is excited with a vibrator and its local velocity displays a turbulentlike Fourier spectrum. This system is believed to develop elastic wave turbulence. We analyze here the motion of the plate with a two-point measurement in order to check, in our real system, a few hypotheses required for the Zakharov theory of weak turbulence to apply. We show that the motion of the plate is indeed a superposition of bending waves following the theoretical dispersion relation of the linear wave equation. The nonlinearities seem to efficiently break the coherence of the waves so that no modal structure is observed. Several hypotheses of the weak turbulence theory seem to be verified, but nevertheless the theoretical predictions for the wave spectrum are not verified experimentally.

Damage Identification of Metallic Structures Using A0 Mode of Lamb Waves

Abstract: A Lamb wave-based technique was developed for detecting damages in metallic structures, such as cracks and holes in metallic beams and plates. For metallic structures with transverse cracks and hol...

Monitoring of Distributed Pipeline Systems by Wireless Sensor Networks

Abstract: This paper describes a sensor network platform for pipeline system monitoring. Pipeline systems are widely used for distribution and transportation of petroleum, natural gas, water, and sewage. Leaks and ruptures due to an aging and fast decaying pipeline system infrastructure cost millions of dollars a year; they also make clear the necessity for continuous, automatic monitoring systems that can provide early detection and early warning of defects, such as corrosion and leaks, before they reach the magnitude of a major disaster. In this paper, we discuss how sensor networks can detect, localize, and quantify bursts, leaks and other anomalies in pipeline systems. Lamb waves are guided ultrasonic waves that can propagate for considerable distances in plates. Research has shown that it is possible to detect flaws over a large area with active sensing devices such as Lead Zirconate Titanate (PZT) for simultaneous actuation and sensing. PZT sensors can be mounted on the curve surface of the pipelines for generating and measuring guided waves that propagate along the pipes. A network of PZT actuators/sensors provide a real-time continuous and automatic monitoring of the health of the pipeline systems. Complications that encountered in Lamb wave propagation include the existence of multiple propagation modes and the dispersive nature of the modes, which makes the sensing, communication, and control a difficult task. In this paper, we will address the detection and localization problems for the proposed acoustic sensor networks using signal processing techniques. We will discuss strategies to deal with the Lamb wave dispersive propagation to achieve reasonable performance for real-time monitoring.

Study of the fundamental Lamb modes interaction with asymmetrical discontinuities

Abstract: The interaction of the fundamental Lamb modes with asymmetrical discontinuities in isotropic plates is studied numerically and experimentally. Three kinds of discontinuities that enable mode conversions are considered: steps down, steps up and asymmetrical notches. The anti-symmetrical and symmetrical Lamb modes contributions are separated by means of the simple arithmetic operations of addition and subtraction. The power reflection and transmission coefficients are then computed with the well-known average power flow equation described in a previous work. Furthermore, the asymmetrical notch is reconstructed from the superposition of a step down and a step up. It is shown that it allows directly the determination of all power coefficients of the reflected, the transmitted and the multiple converted Lamb modes contrarily to the finite element model. Finally, an experimental device is realized to test aluminum plates with different notch depths. Good agreement is found between experimental and numerical results.

Particle trajectories in linear deep-water waves

Abstract: Using phase plane analysis we show that within the framework of linear water wave theory the particle paths in a deep-water wave are not closed: there is a forward drift over a period, which decreases with greater depth.

Design and modelling of ultrasonic tomography for two-component high-acoustic impedance mixture

Abstract: This paper describes the design and modelling of ultrasonic tomography for two-component high-acoustic impedance mixture such as liquid/gas and oil/gas flow which commonly found in chemical columns and industrial pipelines. The information obtained through this research has proven to be useful for further development of ultrasonic tomography. This includes acquiring and processing ultrasonic signals from the transducers to obtain the information of the spatial distributions of liquid and gas in an experimental column. Analysis on the transducers' signals has been carried out to distinguish between the observation time and the Lamb waves. The information obtained from the observation time is useful for further development of the image reconstruction.

Study of the fundamental Lamb modes interaction with symmetrical notches

Abstract: The aim of this work is to predict the propagation of the fundamental Lamb modes in an isotropic structure containing discontinuities in a simple and a fast way. The key point is to decompose the symmetrical notch into two elementary abrupt changes in the plate section. The power reflection and transmission coefficients are computed, using two techniques, with the help of the finite element and the well-known average power flow equation. In the first technique, the through-thickness displacements and stresses are used while in the second technique only the normal or tangential displacement at a given location on the plate surface is used. An equality relation between the direct and the inverse abrupt changes of the plate section is given, which allows computing the power reflection and transmission coefficients for a symmetrical notch from those obtained from one elementary damage. Finally, aluminum plates with different notches depths are tested. Experimental and numerical results are in good agreement.

Structural health monitoring by extraction of coherent guided waves from diffuse fields.

Abstract: Recent theoretical and experimental studies in a wide range of applications have demonstrated that Green's functions (impulse responses) can be extracted from cross-correlation of diffuse fields using only passive sensors. This letter demonstrates the passive-only reconstruction of coherent Lamb waves (dc-500 kHz) in an aluminum plate of thickness comparable to aircraft fuselage and wing panels. It is further shown that the passively reconstructed waves are sensitive to the presence of damage in the plate as it would be expected in a typical "active" guided wave test. This proof-of-principle study suggests the potential for a structural health monitoring method for aircraft panels based on passive ultrasound imaging reconstructed from diffuse fields.

Propagation of Rayleigh-type surface waves in a transversely isotropic piezoelectric layer on a piezomagnetic half-space

Abstract: Propagation of Rayleigh-type surface waves in a piezoelectric-piezomagnetic layered half-space is investigated. The materials are assumed to be transversely isotropic crystals. The dispersion relations have been numerically derived and computed by considering the coupling piezoelectric and piezomagnetic behaviors. The phase velocities are obtained for four kinds of electric-magnetic boundary conditions at the free surface. The variations of mechanical displacements, electric and magnetic potentials along the thickness direction of the layer are obtained. The effects of different electric-magnetic boundary conditions on the phase velocity and mode shapes of displacements, electric and magnetic potentials have been discussed. The results show that the lowest mode is Rayleigh mode and that the phase velocities of the higher modes tend to the shear wave velocity of the piezoelectric layer as the frequency increases. The electric boundary conditions dominate the phase velocity. The magnetic boundary conditions have a significant effect on the mode shapes of the displacements, electric and magnetic potentials of the first mode. It is also found that piezoelectric material properties have an important effect on wave propagation. The result is relevant to the analysis and design of various acoustic surface wave devices constructed from piezoelectric and piezomagnetic materials.

Comparative study of nonlinear acoustic and Lamb wave techniques for fatigue crack detection in metallic structures

Abstract: The paper deals with fatigue crack detection in metallic structures. A simple fatigue experiment is performed to initiate and propagate a crack in an aluminium plate. The plate is instrumented with two low-profile piezoceramic transducers and one low-profile piezoceramic stack actuator. Nonlinear acoustics and Lamb waves are used for crack detection. The former utilizes the high-frequency acoustical wave and low-frequency modal excitation for damage detection. Modulations sidebands around the acoustical spectral component are used to detect the crack. The latter produces Lamb wave responses that are de-correlated due to the presence of the crack. Both effects are due to growing fatigue crack. Two simple signal parameters, namely the intensity of modulation and the normalized cross-correlation coefficient are used as damage indicators. The study demonstrates similar sensitivity of both methods to small fatigue cracks.

Group delay measurements using modally selective Lamb wave transducers for detection and sizing of delaminations in composites

Abstract: A group delay measurement technique is proposed using modally selective Lamb wave transducers for the detection and sizing of delaminations in unidirectional and cross-ply composites. Unlike amplitude or energy based Lamb wave methods, this method is insensitive to transducer coupling. Specifically, modally selective array transducers are used to generate the lowest antisymmetric A0 Lamb mode in a zone with minimal dispersion. The change in the modal group velocity is used as a damage indicator while the accumulated time delay of the traveling ultrasonic wavepacket is used for size estimation of the delaminations. The results are repeatable and consistent, suggesting time delay as a reliable damage parameter for quantitative monitoring of delaminations and impact damage in composites.

Band gaps of lower-order Lamb wave in thin plate with one-dimensional phononic crystal layer: Effect of substrate

Abstract: We study the substrate effect on the band gaps of lower-order Lamb waves propagating in thin plate with one-dimensional phononic crystal layer coated on uniform substrate. The transmitted power spectra are calculated by using the finite element method (FEM). The results show that when the substrate is hard, the influences on band gap are significant and the band gaps disappear rapidly as the substrate becomes thicker. However, when the substrate is soft, the depth of band gaps becomes larger as the thickness of the substrate increases. A virtual plane wave expansion method is developed to calculate the dispersion curves of Lamb wave. The locations and widths of band gaps on the dispersion curves are in good agreement with the results from the transmitted power spectra by FEM.