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A multi-stage damage detection method based on Lamb waves is proposed for plate-like structures, which is not only for locating the damage position accurately, but also for estimating the damage size precisely. Considering a layout of PZT wafers in a square grid arrangement, the multi-stage damage detection method is based on the same PZT layout to achieve the damage detection in multiple stages with improving accuracy. Firstly, an aluminum plate with a hole is identified numerically by finite element method based on the multi-stage method. After the group velocity of Lamb wave in an aluminum plate is obtained numerically by the time of flight (ToF) and cross-correlation method, the position and size of holed-damage can be assessed by the first stage detection. The second stage is carried through one more data obtained by the same layout of PZT wafers, and the size of hole can be estimated more precisely. The third stage only depends on the need of improving detection results of the second stage. An experimental verification is carried through the multi-stage detection method applied in an aluminum plate with a hole. Therefore, the proposed method based on PZT wafers can conduct a multi-stage damage detection in the same PZT layout, and it can reach to a more accurate assessment quantitatively without requiring more PZT wafers. The method is simple and straightforward, and very cost effective in damage detection of plate-like structures. [ABSTRACT FROM AUTHOR]

Lamb waves have proven to be a valuable tool for structural health monitoring (SHM) of plate-like structures susceptible to degradation and failure. It is well-known that their multi-modal propagation characteristic could be both a challenge and an opportunity. Piezoelectric transducers are widely used in SHM applications because of their low cost, small profile and high electromechanical coupling. Properly designing a piezoelectric transducer to excite a particular mode is of great importance to successful SHM practice. Mode tuning capability of piezoelectric transducers has been studied both theoretically and experimentally in the literature for exciting A0 and S0 modes. However, the higher order Lamb waves are not fully studied for their tuning capability. Also, the transducer is usually modeled separately from the waveguide and their coupling is through the in-plane surface traction. This assumption may induce inaccuracy if the dynamics of the actuator are not negligible. Additionally, the driving circuit is not usually included in the current actuator-waveguide models such that the power of excited wave could not be evaluated. In this work, a fully coupled finite element model created for general Lamb wave excitation using piezoelectric transducers is developed. The model comprises three components, electrical driving circuit, piezoelectric element and linear elastic waveguide. The preferential excitation of higher order Lamb wave modes using a single piezoelectric element has been studied and demonstrated experimentally on aluminum plates. [ABSTRACT FROM AUTHOR]

The interaction between Lamb waves and a sessile water droplet (i.e., a droplet resting on a horizontal surface) is quite complex and limited research has addressed this issue. However, the similar interaction of Rayleigh waves with a water droplet has been considered, both experimentally and via models. Motivated by these results, this current work applies 3D finite element modeling to study far-field scattering of the fundamental antisymmetric Lamb wave mode from a single, axisymmetric water droplet on an aluminum plate. The profile of the droplet, which is affected by gravity, surface tension, and its volume, determines the scattering behavior of incident Lamb waves. For small droplets, the effects of gravity can be ignored and the droplet is well-described by a spherical cap. The effects of the diameter and contact angle of this cap on far-field scattering are investigated by analyzing guided wave signals in the plate with and without the droplet. Scattered wave amplitudes expressed as a function of the scattered angle are compared for a number of different droplet geometries. [ABSTRACT FROM AUTHOR]

A common practice in guided wave structural health monitoring is collecting measurements from a transducer array using the pitch-catch method. Among different array configurations, the spatially distributed array provides a cost-effective solution for rapid interrogation of large, plate-like structures. Several guided wave imaging techniques have been proposed and successfully demonstrated for damage detection and localization. However, the performance of these imaging methods can be compromised by a mismatch between a particular transducer array geometry and the scattering characteristics of a defect of interest. This study proposes a method, which is based upon estimating scattering matrices, to quantify the ability of a specific array geometry to interrogate a scatterer. Several array geometries are evaluated using this method, and a Monte Carlo simulation is then performed to vary the transducer locations to find the array geometry that is best matched to a specific directional scatterer. The efficacy of the proposed method is evaluated experimentally by introducing notches of different orientations and locations on an aluminum plate specimen that is instrumented with a spatially distributed array. [ABSTRACT FROM AUTHOR]

Experiments are conducted on aluminum plates containing fastener holes as well as aluminum plates containing a bolted joint. Cyclic mechanical loading is interrupted to perform structural health monitoring with the S0 fundamental Lamb wave mode. An array of surface bonded piezoelectric disk transducers having a pair of transducers at each of five bolt holes is employed. A correlation between the transmission coefficient and crack size is suggested by the data. [ABSTRACT FROM AUTHOR]

The effect of stress on Lamb wave propagation is relevant to both nondestructive evaluation and structural health monitoring because of changes in received signals due to both the associated strain and the acoustoelastic effect. A homogeneous plate that is initially isotropic becomes anisotropic under uniaxial stress, and dispersion of propagating waves becomes directionally dependent. The problem is similar to Lamb wave propagation in an anisotropic plate, except the fourth order tensor in the resulting wave equation does not have the same symmetry as that for the unstressed anisotropic plate, and the constitutive equation relating incremental stress to incremental strain is more complicated. Here we consider the theory of acoustoelastic Lamb wave propagation and show how dispersion curves shift anisotropically for an aluminum plate under uniaxial tension. Theoretical predictions of changes in phase velocity as a function of propagation direction are compared to experimental results for a single wave mode. [ABSTRACT FROM AUTHOR]

This research experimentally characterizes an efficient Lamb wave mode pair that generates the cumulative second harmonic in an undamaged aluminum plate. It has been shown that inherent material nonlinearity will generate a second harmonic component in an originally monochromatic ultrasonic Lamb wave signal if certain conditions are satisfied, and the normalized second harmonic amplitude should increase with propagation distance. Previous research has identified five Lamb wave mode types exhibiting second harmonic generation (SHG) and theoretically compares the rate of accumulation of the second harmonic over propagation distance for each mode. This research experimentally investigates a new Lamb mode pair that has a higher rate of accumulation than those studied in the existing literature, and develops improved experimental techniques that can be applied to other modes for SHG measurements. [ABSTRACT FROM AUTHOR]

A guided wave technique for measurement of remaining thickness distribution is presented for the purpose of maintenance of large structures like oil storage tanks and pipe networks. This study used a laser source to excite the guided wave at many locations on aluminum plates with defect that have high ratio of A0 mode transmission and small mode conversions. The A0 mode was used to interrogate the through-thickness over the rastered areas in the test plates. It was expected that many modes of Lamb wave were excited in intact and defect regions of the plates through the thermoelastic stresses from the laser source. The fundamental antisymmetric mode A0 was extracted to indicate the thickness distribution in aluminum test plates with the defect. The results exhibit greater amplitude distributions of A0 mode over thinner regions around defects and smaller amplitudes distributions over the intact regions. [ABSTRACT FROM AUTHOR]

Lamb waves in aluminum plate were analyzed in a time-frequency domain for the improvement of mode identification, the measurement of Lamb wave propagation distance and the measurement of thickness of the plate. The time-frequency analysis was performed by short time Fourier transforms, which revealed the patterns of dispersion that provided relative time-of-flight of each frequency component of received signals. Hence, the dispersive characteristics of group velocity were obtained from the patterns of dispersion that enabled the mode identification regardless of dispersion and multi mode propagation. By utilizing this enhanced mode identification capability, the measurement of Lamb wave propagation distance was demonstrated without knowing the absolute time-of-flight. In addition, the utilization of multi mode propagation was demonstrated for the thickness measurement. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This research applies wavelet and short time Fourier transform (STFT) techniques to enhance the interpretation of experimentally measured transient ultrasonic waveforms. Specifically, this work applies these advanced signal-processing methodologies to two applications: (1.) detection of cracks in plate specimens; and (2.) the interpretation of acoustic emission (AE) signals. The first case examines experimentally measured (with laser ultrasonics) guided waves in both uncracked and cracked plates. This study investigates the sensitivity, accuracy, and robustness of both the wavelet transform and STFT to determine crack size and location. The second case examines AE signals that emanate from a fatigue crack. This work shows that wavelets enable the quantitative characterization of AE signals, enhancing the ability to determine both source type and location. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Guided-wave based structural health monitoring methods employ piezoelectric wafer active sensors (PWAS) to excite and record guided waves. A mathematical model simulating the dynamics of a structure with two PWASs operating as an actuator and a sensor is presented. Experimental investigations on Lamb waves excitation and sensing on an aluminum plate are performed. Guided waves are measured by laser Doppler vibrometry and with PWAS; the differences between signals obtained using these two approaches are discussed. Propagation of symmetric and antisymmetric Lamb waves through the domain with sensing PWAS is simulated within the obtained mathematical model to investigate the efficiency of a PWAS to record various Lamb waves. [ABSTRACT FROM AUTHOR]