Lamb wave

Detecting and locating damage in structural components and joints that have high feature densities and complex geometry is a difficult problem in the field of structural health monitoring (SHM). Active propagation of diagnostic waves is one approach that is used to detect damage. But small cracks and damage are difficult to detect because they have a small effect on the propagating waves as compared to the effects the complex geometry itself which causes dispersion and reflection of waves. Another limitation of active wave propagation is that pre-damage data is required for every sensor-actuator combination, and a large number of sensors might be needed to detect small cracks on large structures. Overall, the problem of detecting damage in complex geometries is not well investigated in the field of SHM. Nevertheless, the problem is important because damage often initiates at joints and locations where section properties change.

Understanding the velocity and attenuation characteristics of ultrasonic waves in cortical bone and bone mimics is important for studies of osteoporosis and fractures. Three complementary approaches have been used to help understand the ultrasound propagation in cortical bone and bone mimics immersed in water, which is used to simulate the surrounding tissue in vivo. The approaches used were Lamb wave propagation analysis, experimental measurement and two-dimensional (2D) finite difference modelling. First, the water loading effects on the free plate Lamb modes in acrylic and human cortical bone plates were examined. This theoretical study revealed that both the S 0 and S 1 mode velocity curves are significantly changed in acrylic: mode jumping occurs between the S 0 and S 1 dispersion curves. However, in human cortical bone plates, only the S 1 mode curve is significantly altered by water loading, with the S 0 mode exhibiting a small deviation from the unloaded curve. The Lamb wave theory predictions for velocity and attenuation were then tested experimentally on acrylic plates using an axial transmission technique. Finally, 2D finite difference numerical simulations of the experimental measurements were performed. The predictions from Lamb wave theory do not correspond to the measured and simulated first arrival signal (FAS) velocity and attenuation results for acrylic and human cortical bone plates obtained using the axial transmission technique, except in very thin plates.

In this study the characterization of an aluminum nitride (AlN) double ended tuning fork (DETF) fabricated on a layer of silicon dioxide (SiO 2 ) is presented. The positive temperature coefficients of SiO 2 are used to achieve zero TCF for radio frequency (RF) Lamb wave resonators. This paper shows the possibility to integrate temperature compensated Lamb wave resonators with DETF-based devices on a single chip. The DETF resonates in a quasi-in-plane mode shape with a Q-factor of 578 in air and 3028 in vacuum. Through a laser Doppler velocity (LDV) measurement we also show that, due to the biomorph nature of the structure and the angle of the side walls, the motion of each tine of the DETF is a combination of in-plane bending, out-of-plane bending and torsional motion around the beam main axis.

Closed cracks under compressive normal stresses are difficult to detect by the conventional ultrasonic techniques. When the crack surfaces stay in very close contact with each other then the bond between the two surfaces of the crack is called a 'kissing bond'. This is a very dangerous bond. Catastrophic failures can result if the system is subjected to crack opening normal stresses or shear stresses. When the crack surfaces are smooth then kissing bonds cannot transmit shear stress very well but can carry compressive normal stress, these bonds are called 'slip bonds'. Conventional P-wave scans (C-scan or A-scan) are based on the assumptions that P-waves are reflected by the defective interface. However, an interface subjected to a large compressive stress cannot reflect P-waves effectively, hence these bonds remain invisible to the conventional P-wave based C-scan or A-scan techniques. In this paper it is shown that the kissing bonds can be effectively detected by some leaky Lamb mode. Theoretical and experimental results are presented to show that using the Lamb waves is an effective way of detecting kissing bonds. 0 1998 Elsevier Science B.V.

In this paper we review the state of the art in an emerging new technology: embedded ultrasonic non-destructive evaluation (NDE). Embedded ultrasonic NDE permits active structural health monitoring, i.e. the on-demand interrogation of the structure to determine its current state of structural health. The enabling element of embedded ultrasonic NDE is the piezoelectric wafer active sensor (PWAS). We begin by reviewing the guided wave theory in plate, tube, and shell structures, with special attention to Lamb waves. The mechanisms of Lamb wave excitation and detection with embeddable PWAS transducers is presented. It is shown analytically and verified experimentally that Lamb wave mode tuning can be achieved by the judicious combination of PWAS dimensions, frequency value, and Lamb mode characteristics. Subsequently, we address in turn the use of pitch-catch, pulse-echo, and phased array ultrasonic methods for Lambwave damage detection. In each case, the conventional ultrasonic NDE results are contrasted with embedded NDE results. Detection of cracks, disbonds, delaminations, and diffuse damage in metallic and composite structures are exemplified. Other techniques, such as the time reversal method and the migration technique, are also presented. The paper ends with conclusions and suggestions for further work.

This paper describes the theory, design, and realization of a new type of Ultrasonic Lamb Wave Transducer. The excitation mechanims of this device is unlike any other as it relies on the Capacitive Micromachined Ultrasonic Transducer (CMUT). Built using fundamental integrated circuit techniques, this device has an insertion loss of 20 dB at an operating frequency of 2.1 MHz. The dominant propagating mode in the device is that of the lowest order antisymmetric flexural wave (A 0 ). The substrate upon which the device rests is 18 µm thick and is almost entirely made up of crystalline silicon. When configured as a delay line oscillator, the transducer functions well as a sensor to changes in environmental conditions.

It is a well-established idea in optics as well as in ultrasonics that a bounded Gaussian beam, when reflected from an interface, can be displaced in the forward or in the backward direction, depending on the propagation direction of leaky waves that are generated by the incident beam. Such a displacement is often accompanied by the so-called Schoch effect characterized by a null strip in between a specular and a nonspecular reflected beam, and a trailing field that is much further displaced. The current letter shows experimentally and numerically that a simultaneous forward and backward displacement is possible accompanied by two null strips and being only the result of forward propagating Lamb waves.

Statistics show that fatigue crack development comes ®rst and foremost as a damage source in aerospace metallic structures. Currently, widespread methods are available to inspect these structures, but they are quite time-consuming, costly and require the structural system to be idle. Next, attempts to develop damage detection integrated systems are paramount for the safety and cost of such structures. This paper describes an investigation into the feasibility of using an integrated system based on Lamb waves in order to assess the integrity of riveted aluminium joints during cyclical loading. In this experimental analysis, Lamb waves are excited and received outside the joint area using piezoelectric transducers coupled onto the plates. The detected damage is cracks in joint resulting from fatigue loading. The collected signals on the piezoelectric transducers are analysed using Hilbert transform and time±frequency analysis. It is shown that the ®nal interpretation of Lamb wave analysis may provide a means of sizing the defects and following the crack development. In addition to that, an acoustic emission system is used jointly with the Lamb wave analysis in order to discuss results and damage development. Finally, it is demonstrated that both methods can work together and the results obtained are in good agreement. q

In this article, the complete characterization of the interface delamination in a composite T-joint using ultrasonic guided Lamb waves is discussed. Correlation between the numerically simulated and experimentally obtained B-scans was established. In the B-scan images, different Zones were identified and could be utilized for differentiating the healthy and delaminated T-joints. The length and the width of delamination were computed from traversing the transducers in two orthogonal directions (i.e. called the B-scans, and the D-scans). For the first time, backward propagating mode (called ''Turning Lamb Modes'') that are generated at corner of the T-joint was observed and verfied. Using a single linear line scan of the ''Turning Lamb Modes'', both the width and length of interface delamination could be computed. In the present study, the experiments were carried out employing air-coupled ultrasonic transducers.

A capacitive micromachined ultrasonic transducer (CMUT) array for minimally invasive medical diagnosis has been developed. Unlike traditional ultrasonic transducers, which generally use a bulky piece of substrate, this transducer array was integrated on a 40-µm-thick micromachined silicon substrate into a probe shape with a typical shank width of 50-80 µm and a shank length of 4-8 mm. For 1-D arrays, 24-96 CMUT devices were integrated on one such silicon probe and formed an accurately configured phase array. In addition to miniaturization, reduction of the substrate thickness also decreases the intertransducer crosstalk due to substrate Lamb waves. Due to its miniature size, this array can be placed or implanted close to the target tissue/organ and can perform high-resolution high-precision diagnosis and stimulation using high-frequency ultrasounds. The issue of conflict between resolution and penetration depth of ultrasonic diagnosis can therefore be resolved. A two-layer polysilicon surface micromachining process was used to fabricate this device. Suspended polysilicon membranes of diameters ranging from 20 to 90 µms and thicknesses from 1.0 to 2.5 µm were used to generate and detect ultrasounds of frequencies ranging from 1 to 10 MHz. B-mode imaging using this transducer array has been demonstrated.

In this paper, the temperature compensation of aluminum nitride (AlN) Lamb wave resonators using edge-type reflectors is experimentally studied and demonstrated. By adding one compensating silicon dioxide (SiO 2 ) layer with an appropriate thickness, the Lamb wave resonator can achieve a zero linear temperature coefficient of frequency (TCF). With the composite membranes including 1 μm AlN and 0.83 μm SiO 2 , a Lamb wave resonator operating at 711 MHz exhibits a secondorder TCF of -21.5 ppb/°C 2 . The temperature-dependent frequency variation is less than 250 parts per million (ppm) over a wide temperature range from -55 °C to 125 °C. This temperature compensated AlN Lamb wave resonator is promising for future applications to thermally stable oscillators, filters, and sensors.

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.

In this paper the propagation of plane and circular crested viscothermoelastic waves in a homogeneous isotropic, Kelvin-Voigt type viscoelastic thermally conducting, plate sandwiched between inviscid liquid layers is investigated in the context of classical and non-classical theories of thermoelasticity. The secular equations for the symmetric and skew-symmetric modes of plane and circular crested waves are derived in closed form and isolated mathematical conditions. It is noticed that the motion for both the plane and cylindrical waves in plates is governed by Rayleigh-Lamb-type secular equations. The secular equations for thin plate and short wave length waves are also obtained and discussed. The results in the absence of fluid loading, coupled and uncoupled theories of thermoelasticity have been obtained as particular cases from the derived secular equations. The dispersion curves, attenuation profiles and specific loss in case of symmetric and skew-symmetric wave modes are also presented graphically for a polymethyl methacrylate material plate under fluid loadings. The effect of dissipation due to viscosity is noticed to be quite significant and clearly visible from various curves in the graphs.

general map factors that make it easy to configure in oblate spherical, cylindrical, or Cartesian coordinates. The code in-We describe a new general circulation model (GCM) designed cludes optional Message Passing Interface (MPI) library calls for planetary atmospheric studies called the EPIC model. This and hence runs on any Unix-based parallel computer or netis a finite-difference model based on the isentropic-coordinate work cluster. An optional graphical user interface to commerscheme of Hsu and Arakawa (1990. Mon. Wea. Rev. 118, 1933cial visualization software facilitates control of the model and 1959). We report on previously undocumented modifications, analysis of output. Memory is allocated dynamically such that additions, and key practical issues that experience running the the user does not recompile to change horizontal or vertical model has revealed to be important. The model integrates the resolution or range. Applications to date include comet impact hydrostatic primitive equations, which are valid for large-scale forecasts and hindcasts for Jupiter, meridional circulation studatmospheric dynamics and include gravity waves (buoyancy ies of Uranus and Neptune, and the accompanying paper on waves), planetary waves (Rossby waves), and horizontally propthree-dimensional simulations of Neptune's Great Dark Spot agating sound waves (Lamb waves), but not vertically propagat-(1998. Icarus 132, 239-265). © 1998 Academic Press ing sound waves because of the hydrostatic approximation. The vertical coordinate is entropy in the form of potential temperature, which coincides with material surfaces for adia-1. INTRODUCTION batic motion. This means that there is no vertical velocity except where there is heating, which improves accuracy and helps the This paper describes the general circulation model model maintain conservation properties over long integrations. (GCM) used in the companion paper by LeBeau and Dow-An isentropic vertical coordinate is natural for the atmospheres ling (1998) to study the dynamics of Neptune's Great Dark of Jupiter, Saturn, Uranus, and Neptune, which are believed Spot (GDS). An early version of this GCM was used to to have essentially adiabatic interiors that match up with the make predictions of the response of Jupiter's atmosphere bottom of the model and is also excellent for middle-atmosphere to the Shoemaker-Levy 9 comet impacts and successfully studies on any planet. Radiative processes are parameterized by predicted that there would be no long-lasting vortices

The use of Lamb waves is attractive for structural health monitoring of plate and shell structures since their propagation in thin-walled structures is disturbed at damage locations especially at high frequencies. Lamb waves can easily be generated by thin piezoelectric plates which are attached to the surface of the structure. For the observation of the oscillating actuator as well as the propagating Lamb waves laser vibrometry is a powerful tool. Examples of vibrations of free and bonded piezoelectric actuators are given with special regard to the influences of contacts and other parameters, affecting the effectiveness of the wave generation. The determination of important features of Lamb wave propagation in carbon fibre reinforced plastics includes the measurement of dispersion curves and the estimation of attenuation and anisotropy. The interactions of Lamb waves with defects represented by reflections, transmissions and mode conversions are visualised and are easily to interpret.

Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate m ethods for in-situ damage detection of composite materials. Experimental results are presented for the application of Lamb wave techniques to quasi-isotropic graphite/epoxy thin coupons and sandwich beams containing representative damage modes, including delamination, transverse ply cracks and through-holes. Optimization experiments provided a procedure capable of easily and accurately determining the presence of damage by monitoring the transmitted waves with piezoceramic sensors (PZT). Lamb wave techniques have been proven to provide more information about damage type, severity and location than previously tested methods, and may prove suitable for structural health monitoring applications since they travel long distances and can be applied with conformable piezoelectric actuators and sensors that require little power.

Air-coupled ultrasonic systems are usually limited by poor signal-to-noise ratios. The use of pulse compression techniques based on Golay codes can help to improve the ultrasound signal-to-noise ratio. This type of codes allows virtually side lobe free operation and causes the cancellation of self-induced noise. This paper proposes the use of Golay sequences to code Lamb waves excited in thin materials with a NDT system based on air-coupled piezoelectric arrays. Furthermore, a system for the generation of Lamb waves modulated by 16-bits Golay codes is also proposed. Successful results are presented in copper plates. An improvement in signal-to-noise ratio up to 21 dB is achieved when compared with conventional pulse transmission.

The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in-situ nondestructive evaluation (NDE) is explored. Laboratory tests are used to prove that PWAS can satisfactorily perform Lamb wave transmission and reception. Subsequently, crack detection in an aircraft panel with the pulseecho method is illustrated. For large area scanning, a PWAS phased array is used to create the embedded ultrasonics structural radar (EUSR). Opportunities for implementation into Air Force vehicles and the research needs are also discussed.

This paper describes work performed in the development of a set of specification for the construction of an integrated electronic system for piezoelectric wafer active sensor (PWAS). The paper starts with a comprehensive review of the PWAS material properties, dimensions, and electrical characteristics. PWAS of various shapes and sizes are considered. Two boundary conditions were examined: free PWAS and PWAS attached to actual structures. For both, the PWAS immittance and the allowable dc and ac voltages were considered. The predicted values were compared with measurements performed over a wide frequency range (10 kHz to 2 MHz). Next, the electronic-equipment specifications were considered. The PWAS can be used in a number of different ways to actively detect damage in structures. Our aim was to develop electronic-equipment specifications that would extract the optimum performance from the PWAS, i.e., maximize the coupling with the structure and obtain large-amplitude Lamb wave transmission and reception. Analytical predictions were compared with measurements made using current laboratory equipment. The comparative analysis revealed that the current electronic equipment does not fully exploit the PWAS capabilities. Hence, the PWAS equipment specifications were divided into two categories: "existing" and "desired". The former category designates integrated electronic equipment that would offer the same PWAS performance as the existing lab equipment, but be of a lower volume/weight/cost. The latter category refers to advanced electronic equipment that will exploit the full potential of PWAS transducers while being of lower volume/weight/cost than the lab equipment. Both categories are presented and discussed in the paper.

Guided acousto-ultrasonic waves have great potential for structural health monitoring applications. Lamb waves are the most widely used guided waves for damage detection in metallic and composite structures. These waves, once introduced to a structure, can propagate long distances. Nevertheless the method still requires a significant number of transducers for monitoring large structures. Recent years have shown various applications of non-contact generation and sensing of Lamb waves. This paper proposes a new scanning technique based on laser vibrometry. The method is applied for damage detection in simple aluminium plates and validated using numerical simulations. The study shows the potential of the method for simple and rapid detection, location and monitoring of damage in structures.

In this paper we present the results of a systematic theoretical and experimental investigation of the fundamental aspects of using piezoelectric wafer active sensors (PWASs) to achieve embedded ultrasonics in thin-gage beam and plate structures. This investigation opens the path for systematic application of PWASs for in situ health monitoring. After a comprehensive review of the literature, we present the principles of embedded PWASs and their interaction with the host structure. We give a brief review of the Lamb wave principles with emphasis on the understanding the particle motion wave speed/group velocity dispersion. Finite element modeling and experiments on thin-gage beam and plate specimens are presented and analyzed. The axial (S 0 ) and flexural (A 0 ) wave propagation patterns are simulated and experimentally measured. The group-velocity dispersion curves are validated. The use of the pulse-echo ultrasonic technique with embedded PWASs is illustrated using both finite element simulation and experiments. The importance of using highfrequency waves optimally tuned to the sensor-structure interaction is demonstrated. In conclusion, we discuss the extension of these results to in situ structural health monitoring using embedded ultrasonics.

Active sensor wave propagation technique is a relatively new method for in-situ nondestructive evaluation (NDE). Elastic waves propagating in material carry the information of defects. These information can be extracted by analyzing the signals picked up by active sensors. Due to the physical property of wave propagation, large area can be interrogated by a few transducers. This simplifies the process

Due to their small size and flexibility fiber optics can be embedded into composite materials with little negative effect on strength and reliability of the host material. Fiber optic sensors such as Fiber Bragg Gratings (FBG) or Etched Fiber Sensors (EFS) can be used to detect a number of relevant parameters such as flow, degree of cure, quality and structural health throughout the life of a composite component. With a detection algorithm these embedded sensors can be used to detect damage in real time while the component remains in service. This paper presents the research being conducted on the use of fiber optic sensors for process and Structural Health Monitoring (SHM) of Resin Transfer Molded (RTM) composite structures. Fiber optic sensors are used at all life stages of an RTM composite panel. A laboratory scale RTM apparatus was developed with the capability of visually monitoring the resin filling process. A technique for embedding fiber optic sensors with this apparatus has also been developed. Both FBGs and EFSs have been embedded in composite panels using the apparatus. EFSs to monitor the fabrication process, specifically resin flow have been embedded and shown to be capable of detecting the presence of resin at various locations as it is injected into the mold. Simultaneously these sensors were multiplexed on the same fiber with FBGs, which have the ability to measure strain. Since multiple sensors can be multiplexed on a single fiber the number of ingress/egress locations required per sensor can be significantly reduced. To characterize the FBGs for strain detection tensile test specimens with embedded FBG sensors have been produced. These specimens have been instrumented with a resistive strain gauge for benchmarking. Both specimens and embedded sensors were characterized through tensile testing. Furthermore FBGs have been embedded into composite panels in a manner that is conducive to detection of Lamb waves generated with a centrally located PZT. To sense Lamb waves a high speed, high precision sensing technique is required to acquire data from embedded FBGs due to the high velocities and small strain amplitudes of these guided waves. A technique based on a filter consisting of a tunable FBG was developed. Since this filter is not dependant on moving parts, tests executed with this filter concluded with the detection of Lamb waves, removing the influence of temperature and operational strains. A damage detection algorithm was developed to detect and localize cracks and delaminations.

Environmental factors such as temperature and humidity influence the efficacyo fd efect imaging procedures based on the identification of changes between an intact state and the current state of as ample/compo-nent/structure in the presence of ad efect. In this paper,w ef ocus on the Reconstruction Algorithm for Proba-bilistic Inspection of Damage (RAPID)a nd propose an onlinear Lamb wave version of RAPID to visualize a delamination in acomposite structure without having to knowanything about the intact state, i.e. abaseline free RAPID. Once the optimal frequencyselection of Lamb wavesinapitch-catch configuration mode is performed, lowa nd high excitation amplitude signal responses within as parse array at that frequencya re evaluated along each transducer-receiverpath by analyzing aset of damage sensitive parameters: the correlation coefficient, the energy of the scaling subtracted signal, and the third harmonic ratio. Processing of this information leads to acor-responding probabilistic damage map of the area within the sparse array.The obtained results from av alidation experiment demonstrate the capability of this nonlinear variant of RAPID for the identification of adelamination in acomposite structure.

Wind turbine blade failure is the most prominent and common type of damage occurring in operating wind turbine systems. Conventional nondestructive testing systems are not available for in situ wind turbine blades. We propose a portable long distance ultrasonic propagation imaging (LUPI) system that uses a laser beam targeting and scanning system to excite, from a long distance, acoustic emission sensors installed in the blade. An examination of the beam collimation effect using geometric parameters of a commercial 2 MW wind turbine provided Lamb wave amplitude increases of 41.5 and 23.1 dB at a distance of 40 m for symmetrical and asymmetrical modes, respectively, in a 2 mm-thick stainless steel plate. With this improvement in signal-to-noise ratio, a feasibility study of damage detection was conducted with a 5 mm-thick composite leading edge specimen. To develop a reliable damage evaluation system, the excitation/sensing technology and the associated damage visualization algorithm are equally important. Hence, our results provide a new platform based on anomalous wave propagation imaging (AWPI) methods with adjacent wave subtraction, reference wave subtraction, reference image subtraction, and the variable time window amplitude mapping method. The advantages and disadvantages of AWPI algorithms are reported in terms of reference data requirements, signal-to-noise ratios, and damage evaluation accuracy. The compactness and portability of the proposed UPI system are also important for in-field applications at wind farms.► Portable long distance laser ultrasonic propagation imaging (LUPI) system. ► A new anomalous wave propagation imaging (AWPI) methods based platform was developed. ► It provided enhancement and accuracy in damage visibility, localization and size. ► The system feasibility was verified considering a 2 MW commercial wind turbine system. ► LUPI system shows applicability for in-field long distance wind turbine applications.

A detailed study is undertaken to analyze the non-steady interaction of plane progressive pressure pulses with an isotropic, homogeneous, fluid-filled and submerged spherical elastic shell of arbitrary wall thickness within the scope of linear acoustics. The formulation is based on the general three dimensional equations of linear elasticity and the wave equation for the internal and external acoustic domains. The Laplace transform with respect to the time coordinate is invoked, and the classical method of separation of variables is used to obtain the transformed solutions in the form of partial-wave expansions in terms of Legendre polynomials. The inversion of Laplace transforms have been carried out numerically using Durbin's approach based on Fourier series expansion. Special convergence enhancement techniques are invoked to completely eradicate spurious oscillations (Gibbs' phenomenon), and obtain uniformly convergent solutions. Detailed numerical results for the transient and vibratory responses of water-submerged steel shells of selected wall thickness parameters with various internal fluid loadings under an exponential wave excitation are presented. Many of the interesting dynamic features in the transient shell-shock interaction such as shock transparency, shell-radiated negative pressure waves, formation of triple points, and focusing of the reflected waves are examined using appropriate 2D images of the internal pressure field. Also, the temporal behavior of the specularly-reflected, the lowest symmetric S 0 -and antisymmetric A 0 -Lamb waves, as well as appearance of the Franz's creeping waves are discussed through proper visualization of the external scattered field. Likelihood of cavitation is addressed and regions proned to cavitation are identified. Moreover, the effects of internal fluid impedance in addition to shell wall thickness on the dynamic stress concentrations induced within the shell are analyzed. Limiting cases are considered and fair agreements with well-known solutions are established.

Large-area PVDF thin films have been prepared and characterized for quasi-static and high frequency dynamic strain sensing applications. These films are prepared using hot press method and the piezoelectric phase (␤-phase) has been achieved by thermo-mechanical treatment and poling under DC field. The fabricated films have been characterized for quasi-static strain sensing and the linear strain-voltage relationship obtained is promising. In order to evaluate the ultrasonic sensing properties, a PZT wafer has been used to launch Lamb waves in a metal beam on which the PVDF film sensor is bonded at a distance. The voltage signals obtained from the PVDF films have been compared with another PZT wafer sensor placed on the opposite surface of the beam as a reference signal. Due to higher stiffness and higher thickness of the PZT wafer sensors, certain resonance patterns significantly degrade the sensor sensitivity curves. Whereas, the present results show that the large-area PVDF sensors can be superior with the signal amplitude comparable to that of PZT sensors and with no resonance-induced effect, which is due to low mechanical impedance, smaller thickness and larger area of the PVDF film. Moreover, the developed PVDF sensors are able to capture both A 0 and S 0 modes of Lamb wave, whereas the PZT sensors captures only A 0 mode in the same scale of voltage output. This shows promises in using large-area PVDF films with various surface patterns on structures for distributed sensing and structural health monitoring under quasi-static, vibration and ultrasonic situations.

In this paper, we propose a parametric study of aluminum nitride Lamb wave resonators. A Film Bulk Acoustic Resonator (FBAR) technology was used and integrated on a 200 mm silicon substrates process flow performed on a standard CMOS fabrication line. Experimental results show that frequency is defined mostly by the period of interdigitated electrodes, but also by a competing contribution of membrane width.

In this paper an investigation is carried out to predict the most efficient leaky Lamb mode and associated frequency to detect defects in a specific layer of a multilayered composite plate. To this end the stress and displacement fields inside a plate are theoretically calculated for different propagating leaky Lamb modes. It is observed that for every leaky Lamb mode the stress field inside the plate varies significantly with depth. A five-layer fiber reinforced composite plate with pre-existing defects, such as missing or broken fibers, and delamination is ultrasonically scanned with different leaky Lamb modes for which internal stress and displacement fields have been theoretically computed in absence of any defect. In presence of defects such as broken fibers and delamination the stresses on the defective surface are significantly reduced, Hence, if these defects are located in a region where the stress level is high for certain leaky Lamb modes then the presence of the defects will affect these modes significantly by releasing the high stress. These specific modes would then be most effective in producing the image of the defect. Theoretical predictions are then experimentally verified. 0 1997 Elsevier Science B.V.

A phononic crystal exhibiting a band gap in the near-gigahertz frequency range for surface acoustic waves was fabricated in a lithium niobate substrate. Reflection and transmission properties of the sample were characterized both electrically and optically, by means of embedded broadband interdigital transducers and optical heterodyne interferometry, respectively. Measurements performed for (XZ) propagating surface waves show the existence of a band gap between 660 and 900 MHz. Optical measurements confirm that the phononic crystal behaves as a perfect mirror for waves propagating at frequencies within the band gap. Outside the band gap, transmission can be observed for frequencies below, but also above the forbidden frequency range, hence showing that losses experienced by high frequency surface acoustic waves, i.e. for modes located beyond the sound line, can be partially overcome.

The dispersion curves of Lamb-wave modes propagating along a multilayer structure are important for the operation of thin-film bulk acoustic wave (BAW) devices. For instance, the behavior of the side resonances that may contaminate the electrical response of a thin-film BAW resonator depends on the dispersion relation of the layer stack. Because the dispersion behavior depends on the materials parameters (and thicknesses) of the layers in the structure, measurement of the dispersion curves provides a tool for determining the materials parameters of thin films. We have determined the dispersion curves for a multilayer structure through measuring the mechanical displacement profiles over the top electrode of a thin-film BAW resonator at several frequencies using a homodyne Michelson laser interferometer. The layer thicknesses are obtained using scanning electron microscope (SEM) measurements. In the numerical computation of the dispersion curves, the piezoelectricity and full anisotropy of the materials are taken into account. The materials parameters of the piezoelectric layer are determined through fitting the measured and computed dispersion curves.

Wind turbine blade failure is the most prominent and common type of damage occurring in operating wind turbine systems. Conventional nondestructive testing systems are not available for in situ wind turbine blades. We propose a portable long distance ultrasonic propagation imaging (LUPI) system that uses a laser beam targeting and scanning system to excite, from a long distance, acoustic emission sensors installed in the blade. An examination of the beam collimation effect using geometric parameters of a commercial 2 MW wind turbine provided Lamb wave amplitude increases of 41.5 and 23.1 dB at a distance of 40 m for symmetrical and asymmetrical modes, respectively, in a 2 mmthick stainless steel plate. With this improvement in signal-to-noise ratio, a feasibility study of damage detection was conducted with a 5 mm-thick composite leading edge specimen. To develop a reliable damage evaluation system, the excitation/sensing technology and the associated damage visualization algorithm are equally important. Hence, our results provide a new platform based on anomalous wave propagation imaging (AWPI) methods with adjacent wave subtraction, reference wave subtraction, reference image subtraction, and the variable time window amplitude mapping method. The advantages and disadvantages of AWPI algorithms are reported in terms of reference data requirements, signal-to-noise ratios, and damage evaluation accuracy. The compactness and portability of the proposed UPI system are also important for in-field applications at wind farms.

The problem of Lamb wave propagation in an axially multi-layered waveguide is treated by a multi-modal approach. A general formalism is proposed that avoids the numerical divergence due to evanescent modes and that is based on an impedance matrix. To describe the fields, we choose a 4-vector composed of the displacements and the horizontal stresses. Due to symmetry properties of the right-and left-going modes, this 4-vector can be split into two 2-vectors described by only two sets of modal components. Moreover, the modal 2-vectors have a biorthogonality relation that allows us to express the fields continuity at the interface between two media in a simple manner. Formally, this approach permits us to extend the multi-modal formalism from fluidic to elastic waveguides. In this context, the impedance matrix is defined as the linear operator that links the two sets of modal components. As in the fluidic case, the impedance matrix has the advantage of avoiding numerical divergence, and can be used to obtain the reflection and transmission matrices, as well as the wave fields. The technique is validated in the case of two semi-infinite elastic plates bounded along their lateral faces (succession of two media) and is also applied to a thick bonding (succession of three media) and to a periodic waveguide (succession of multiple media).

A parametric 2D finite-element inversion scheme is used for sizing cracked zone type strip defect caused by a linear and uniform impact on a composite material plate. The reflection and transmission coefficients produced by mode conversion phenomenon when a pure incident S 0 Lamb wave mode is sent towards the defect are used as input data for the inversion process. The inversion process consists in quantifying two unknown parameters, i.e., width and depth representing the assumed triangular geometry of cracked zone like defect. The inversion scheme consists of a finite element based model, which is used to simulate the Lamb wave scattering for various values of the aimed parameters, and specific post-processing based on Shkerdin's orthogonality relation is applied to predict the needed reflection and transmission coefficients. A laboratory experiment is performed for creating a linear and uniform impact on a carbon epoxy composite plate sample. Then another ultrasonic experiment is performed in laboratory for measuring reflection and transmission coefficients by sending a pure S 0 mode towards the defect. The measured coefficients are then used for the inversion purpose. The two principles used in inversion process give almost similar optimised values for these parameters, showing the usefulness of this technique.

This research examines the propagation of guided Lamb waves in fiber-reinforced polymer ͑FRP͒ bonded components, establishing the effectiveness of combining laser ultrasonic techniques with a time frequency representation ͑TFR͒ to experimentally measure the dispersion curves of a concrete component repaired with a FRP plate. A TFR is used to operate on experimentally measured, guided Lamb waves to resolve a FRP bonded component's individual modes and generate its dispersion curves. The objective of this research is to demonstrate that it is possible to develop the dispersion curves of FRP bonded components from a single, experimentally measured guided Lamb wave. The experimental results show that the stiffer the bond, the more deviation from the behavior of a free plate specimen, and the less modes that are present.

Embedded-ultrasonics structural radar (EUSR) is a new concept and methodology for in situ nondestructive evaluation (NDE) and structural health monitoring (SHM) of thin-wall structures. EUSR consists of: (a) an array of piezoelectric wafer active sensors (PWAS) embedded into the structure; and (b) electronic modules for signal transmission/reception, processing, and interpretation. EUSR utilizes guided elastic waves (Lamb waves) generated omnidirectionally into the thin-wall structure by surface-mounted permanently attached PWAS. The paper starts with the general concepts of the EUSR algorithm: transmission beamforming, reception beamforming, and time-of-fight (TOF) determination. Next, details of the Lamb wave generation with PWAS, verification of group-velocity dispersion curves, identification of optimal excitation frequency, and confirmation of wave front omnidirectionality are discussed. In the third part of the paper, the actual implementation of the EUSR method in a proof-of-concept demonstration is presented. The construction of the PWAS-phased array is described, and detection of cracks located broadside and offside of the PWAS array is illustrated. The method is shown to be easy to use through a visually interactive LabView TM interface. Very good detection accuracy is observed. The proof-of-concept experiments presented in this paper were illustrated on metallic structures; however, the EUSR concept may also work on composite and hybrid structures, although the range of detection may be reduced by the medium attenuation.

Exact and approximate theoretical analysis of the wavelengths of plate wave mode propagation in all planar directions for the dispersive antisymmetric Lamb wave mode are compared with measurements from a laser ultrasonic imaging approach that records the out of plane ultrasonic motion over a large area in a single frame without scanning. Good agreement is demonstrated, based on independent determination of the elastic constants, for two different types of paper.