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Abstract In this study the nonlocal bond-based peridynamic (PD) theory was applied to simulate Lamb wave transmission in 2D bimaterial plates. Plates of unlike materials were jointed end-to-end to make a bimaterial plate. The surface of the plate was then excited tangentially by single-frequency and multi-frequency force signals. The influence of changing material properties on travelling of the symmetric and antisymmetric Lamb waves were studied in detail. The phase and group velocities of travelling wave in bimaterial plates were calculated and it was found that dissimilar material properties of two joint plates can significantly alter the amplitude and arrival time of the wave. Moreover, Fast Fourier Transform (FFT) of the time history of symmetric Lamb wave velocity was calculated and verified. Accuracy and consistency of PD wave model was confirmed entirely using Spectral Finite Element Method (SFEM). The comparison between the results of two applied methods shows a good agreement. [ABSTRACT FROM AUTHOR]

Abstract Scanning laser Doppler vibrometry (SLDV) is a promising method for visualizing defects in plate-like structures. The Lamb wave-based SLDV method using continuous excitations has been recently developed in nondestructive evaluation (NDE). In addition, various signal processing techniques have been developed to enhance damage detection capability for the method. To visualize defects, the A 0 mode is commonly used because of its sensitivity to the thickness variations, i.e. the wave speed changes according to the thickness of a plate. The A 0 mode is easily generated by lead zirconate titanate (PZT) transducers. However, for thick plates, the A 0 mode is found to be less sensitive to thickness variations compared to thin structures. To address this issue, a mode-tuned inter-digital transducer (IDT) is adopted to efficiently generate and utilize the symmetric modes for damage detection and visualization of thick plates. Based on the wavelength of the S 0 mode using the dispersion curve of the plate, the IDT is designed and fabricated for efficient symmetric mode generation. To evaluate the capability of the mode-tuned IDT-based SLDV, a 6-mm-thick plate (40 mm × 40 mm) with defects of 0.3 mm, 0.6 mm, 0.9 mm, and 1.2 mm depths is used. The S 0 mode is selected for its wavenumber sensitivity and the degree of separation between modes. It is demonstrated that the detectability of shallow defects in a thick plate is improved compared to that when using the A 0 mode. [ABSTRACT FROM AUTHOR]

This paper aims to use the Lamb wave local wavenumber approach to characterize flat bottom defects (including circular flat bottom holes and a rectangular groove) in an isotropic thin plate. An air-coupled transducer (ACT) with a special incidence angle is used to actuate the fundamental anti-symmetric mode (A0). A laser Doppler vibrometer (LDV) is employed to measure the out-of-plane velocity over a target area. These signals are processed by the wavenumber domain filtering technique in order to remove any modes other than the A0 mode. The filtered signals are transformed back into the time-space domain. The space-frequency-wavenumber spectrum is then obtained by using three-dimensional fast Fourier transform (3D FFT) and a short space transform, which can retain the spatial information and reduce the magnitude of side lobes in the wavenumber domain. The average wavenumber is calculated, as a real signal usually contains a certain bandwidth instead of the singular frequency component. Both simulation results and experimental results demonstrate that the average wavenumber can be used not only to identify shape, location, and size of the damage, but also quantify the depth of the damage. In addition, the direction of an inclined rectangular groove is obtained by calculating the image moments under grayscale. This hybrid and non-contact system based on the local wavenumber approach can be provided with a high resolution. [ABSTRACT FROM AUTHOR]

Ultrasonic Lamb wave is a promising tool for structural health monitoring and nondestructive evaluation of plate-like structures. Using an array with several piezoelectric discs for damage imaging (i.e. visual detection and localization) is of interest. Commonly used delay-and-sum method is limited for overlapped signals when several damages are closely distributed in the structure. To overcome this limitation, modal-based sparse reconstruction imaging method is applied for adjacent damages in this study. Firstly, Lamb wave dispersion curve is obtained by solving the Rayleigh-Lamb equations. Subsequently, propagation modal of the damage-reflected signal is constructed based on the solved dispersion curve. Finally, the modal is used for damage imaging via sparse reconstruction and basis pursuit de-noising. Experimental data measured in an aluminum plate is considered, and the result demonstrates that the sparse reconstruction imaging method is effective to detect and localize closely distributed damages in the presence of signal overlapping. [ABSTRACT FROM AUTHOR]

The transmission behavior of Lamb waves and the possible occurrence of resonance at an adhesive butt joint of plates are studied experimentally. To this purpose, two 2.5-mm thick aluminum alloy plates are bonded at their edges using cyanoacrylate-based adhesive. Bonded plate specimens with different joint conditions are prepared by changing the bonding procedure. The measurements are performed for the transmission characteristics of the lowest-order symmetric (S0) and antisymmetric (A0) Lamb modes for the frequency range of 0.4–0.6 MHz below the cut-off frequency of the higher-order modes. The experimental results show that the transmission coefficients of the S0 and A0 modes exhibit different frequency-dependent characteristics depending on the joint condition. Furthermore, for the incidence of the S0 mode at the center frequency of 1 MHz, the transmitted S0 mode in weakly bonded specimens shows a long oscillation tail due to the resonance effect. The experimental results are discussed in the light of the theoretical results based on the spring-type interface model. The interfacial stiffnesses identified from the transmission coefficients are shown to be correlated with the bonding condition of the joint and give reasonable estimates of the resonance frequencies of weakly bonded specimens. [ABSTRACT FROM AUTHOR]

Effective application of the Lamb waves for structural health monitoring and damage identification intensively relies on the accurate damage-related feature extraction in the received signals. Most of existing signal processing methods extract the damage-related features from the time–frequency joint spectrum which requires a quite amount of effort. In this paper, the soft-thresholding process, based on different signal decomposition methods, is introduced to damage identification so that the damage-related signal features can be manifested more distinctively. By applying two popular signal decomposition methods (i.e., the discrete wavelet transform (DWT) and the empirical mode decomposition (EMD)), the signal of interest can be represented by a series of components with different frequencies. Since most noises exist in the high frequency range, it is feasible to alleviate noise by restricting the energy of high-frequency components. Finally, a denoised signal is synthesized using the corresponding reconstruction method. As an application, the soft-thresholding process is performed to detect a small crack on an isotropic aluminum plate under the white Gaussian noise contamination. The results, from both the numerical finite element simulation and experimental test, indicate that the soft-thresholding process is capable of effectively reducing the effect of noise, convincingly improving the sensitivity of damage identification, and discriminating relatively small damage. [ABSTRACT FROM AUTHOR]

Ultrasonic Lamb wave testing is one important nondestructive testing technology for the plate-like structural health monitoring. To improve the detection accuracy, a fractional derivative method is presented to reduce the noise of Lamb wave signals. First, the amplitude spectrum fractional derivative of the Lamb signal with noises at different orders is obtained. Next, the characteristic parameters of the amplitude spectrum are calculated according to the proposed relationship between the peak amplitude and the derivative order as well as that between the peak frequency and the derivative order. Finally, the Lamb wave signal is rebuilt by combining the amplitude and phase spectra. Both the simulated and experimental data are used to evaluate the developed method qualitatively and quantitatively. The results indicate that this method can effectively reduce the noise for Lamb wave signals. [ABSTRACT FROM AUTHOR]

In this paper, we proposed a practical method for classifying damages in pipes and plates using ultrasonic guided waves. The A-scan Pulse-Echo lamb wave ultrasonic tests used in this study. Tests accomplished on isotropic 1050 Aluminum with 0.4 mm thickness. Damages studied here were corrosion and crack which is common in pipe lines and steel structures like vehicles body or aerospace structures. This investigation is done in three steps. First step, experimental testing (making standard sample, lamb wave tests), second step, signal processing (window function, normalizing, wavelet function), third step, using the proper algorithm for classification. In first step, 206 ultrasonic lamb wave tests are measured on standard damaged samples (on pipe and plate) and the signals digitalized. After that, these signal processed and classified by classification algorithm. In this the classification algorithm is the support vector machine (SVM). In machine learning, support vector machines are supervised learning models with associated learning algorithms that analyze data and recognize patterns, used for classification and regression analysis. The results show that the corrosion damage can be distinguished from crack damages with 99% accuracy by proposed algorithm. [ABSTRACT FROM AUTHOR]

This paper theoretically investigates the propagation of Lamb waves in a two-dimensional (2D) phononic crystal (PC) structure composed of multiple square stubs symmetric or anti-symmetric deposited on a thin homogeneous plate. The dispersion relations, the power transmission spectra and the displacement fields of the eigenmodes are studied by using the finite-element method (FEM). We investigate the effect of the distance between adjacent square stubs on the band gaps (BGs). The results show that the distance between adjacent square stubs can remarkably change the location and the width of the BGs. Meanwhile, we conducted a numerically study on the different results between the anti-symmetric and symmetric distribution of the scatterers. The band structure of the anti-symmetric structure PC is significantly different from the symmetric structure. In general, the distribution of the square stubs has an influence on the band structures. We further identify the physical mechanism of the distinction in the formation of the BG between the anti-symmetric and the symmetric model through the displacement fields of the eigenmodes. In addition, we find that the variation of the PC BGs in anti-symmetric and symmetric structure are sensitive to the distance between adjacent square stubs, which could be indispensable to practical applications such as BG tuning. [ABSTRACT FROM AUTHOR]

In this paper, an algorithm for identifying rectangular notch parameters as damage in a plate using Lamb waves is presented. In this algorithm, a combination of pulse-echo and pitch-catch methods is used. The method is divided into two steps: notch localization and notch geometry detection. The bases for this algorithm are mode conversion and scattering phenomena because of interaction of Lamb wave modes with defects. The method is applied to some numerical examples, and the results show that it can successfully identify all of rectangular notch parameters, i.e., its location, depth, and width. [ABSTRACT FROM AUTHOR]

Structures with composite materials in daily use are subjected to impact. Determining if the material is damaged is an important aspect of structural health monitoring of composites. In this paper, a method to determine structural damage as a result of impact and localize the position of the damage based on lamb wave mode is presented. First, the paper analyses the characteristics of lamb wave mode generated by force loading on the plate. These results are validated based on the analysis of experimental results. From the results, it can be concluded that S 0 mode lamb waves were generated by damage of the plate structure and it can be used as the characteristic signal of damage. Then, empirical mode decomposition (EMD) is utilized to decompose to the Intrinsic Mode Functions (IMF) of S 0 mode inside the emission signals of complex acoustics to realize if damage occurs and localize the position of impact damage on the composite material plate. [ABSTRACT FROM AUTHOR]

In this paper, a kind of sandwich phononic crystal (PC) plate with silicon rubber scatterers embedded in polymethyl methacrylate (PMMA) matrix is proposed to demonstrate its low-frequency Lamb wave band gap (BG) characteristics. The dispersion relationship and the displacement vector fields of the basic slab modes and the locally resonant modes are investigated to show the BG formation mechanism. The anti-symmetric Lamb wave BG is further studied due to its important function in reducing vibration. The analysis on the BG characteristics of the PC through changing their geometrical parameters is performed. By optimizing the structure, a sandwich PC plate with a thickness of only 3 mm and a lower boundary (as low as 23.9 Hz) of the first anti-symmetric BG is designed. Finally, sound insulation experiment on a sandwich PC plate with the thickness of only 2.5 mm is conducted, showing satisfactory noise reduction effect in the frequency range of the anti-symmetric Lamb BG. Therefore, this kind of sandwich PC plate has potential applications in controlling vibration and noise in low-frequency ranges. [ABSTRACT FROM AUTHOR]

Traditionally dispersion characteristics for any plate structure are evaluated using the Rayleigh-Lamb solution and an assumption that the material properties are known. In cases where the material properties may be difficult to obtain the opposite is proposed. Specifically, the dispersive characteristics of Lamb waves may be used to extract the mechanical properties of a given test coupon. This paper presents a particle swarm optimisation (PSO) methodology to achieve this. The results from a series of tests on an aluminium coupon were used to validate the approach. The PSO was found to successfully match dispersion characteristics identified in a 2D FFT. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Traditional structural health monitoring (SHM) techniques are vulnerable to factors such as temperature change and vibration noise that are not relevant to structural damages. To overcome these drawbacks, this paper develops a reference-flee crack detection method based on Lamb wave mode conversion. Neither baseline data nor damage threshold is required in this method. According to PZT polarization characteristics, feature signals which contained crack-lead Lamb wave converted modes are obtained from PZT components of two sets of side-by-side, and their amplitudes are obtained in the frequency domain to represent signal energy. Whether there being a crack is judged by comparing energy of each feature signal. Simulation and experiments show that the proposed method is not sensitive to optimal excitation frequency and sampling time, therefore it has strong robustness and applicability. [ABSTRACT FROM AUTHOR]

Abstract: Numerical simulations were carried out to quantify the reflection and transmission characteristics of the fundamental Lamb modes propagating in aluminium sub-plates, which are formed due to a semi-infinite horizontal crack. It was observed that, a Lamb mode propagating in a sub-plate when incident at the edge of a crack, undergoes reflection and transmits through the main plate, as well as the other sub-plate. The mode transmitted through the sub-plate has been termed the ‘Turning Lamb Mode’ (TLM). Furthermore, a mode converted mode also propagates along with the TLM. This mode has been termed the ‘Mode Converted Turning Lamb Mode’ (MCTLM). Reflection and transmission characteristics of the fundamental Lamb modes in aluminium sub-plates were studied at frequencies 150kHz, 175kHz, and 200kHz. Experiments conducted to validate the observations made in numerical simulations, confirmed that the transmission and reflection characteristics depend on the thickness ratio. From this study it is surmised that when a Lamb mode propagates through a plate containing horizontal crack, the TLM and the MCTLM start propagating from one sub-plate to the other at the rear edge of the crack and amplitude of these modes depends on the location of the crack across the plate thickness. [Copyright &y& Elsevier]

Dynamic analysis of plate structures based on the Mindlin plate theory has become one of the usual modeling methods for the structural health monitoring (SHM) of composite structures in recent years. Compared to the classical plate theory (CPT) based on Kirchhoff hypothesis, the Mindlin plate theory considers the influence of transverse shear deformation and moment of inertia on displacements. Thus it is more suitable for dynamic analysis of composite laminate with low transverse shear stiffness and large transverse shear deformation. Combining the adhesive layer coupling model of the piezoelectric actuator with the Mindlin plate theory, the dispersion curve of Lamb wave in any direction and mechanical parameters of any point in the composite are obtained, and thus after the substitution of boundary condition, the modeling of piezoelectric wafer excited Lamb wave propagating in composite laminate is realized. The validation experiment is performed on a carbon fiber composite laminate. It proves that the analytical modeling effectively reflects the propagation characteristics of Lamb wave in composite laminate and promotes the engineering application of SHM. [ABSTRACT FROM AUTHOR]

A new spectral plate element is presented for modeling selectively or simultaneously the symmetric and/or anti-symmetric modes of Lamb waves propagating in thin plate structures. The Legendre polynomials and the extended form of the displacement field are used in the formulation. The diagonal mass matrix is obtained by using a simple method with less computational effort. Detailed derivations are provided. Comparisons with existing results are performed to validate the formulations as well as the written programs. Numerical calculations have been carried out for thin aluminum plates with and without damages by using the proposed spectral plate element. Comparisons reveal that the proposed spectral plate element is more effective than the spectral plate element based on Chebyshev polynomials and the 3-D spectral finite elements with the same order of Legendre polynomials as the shape functions. [ABSTRACT FROM AUTHOR]

Abstract: A new type of hydrogen sensor using Lamb waves is proposed and fabricated. The sensor consists of an aluminum thin plate-based hydrogen sensing medium that can be easily replaced, a medium grip component, and a piezoelectric sensor. In addition to structural uniqueness, the proposed hydrogen sensor provides two wave parameters, peak amplitude and phase shift, to detect changes in hydrogen concentration. The proposed sensor can provide reliable sensitivity, decreased fabrication and operational costs, and significantly increased lifetime because its hydrogen sensing medium can be replaced when it shows low sensitivity. [Copyright &y& Elsevier]

Abstract: Using finite element method, we have made a theoretically study of the band structure of Lamb wave in a locally resonant phononic crystal strip waveguide with periodic soft rubber attached on the two sides of epoxy main plate. The numerical results show that the Lamb wave band gap based on local resonant mechanism can be opened up in the stub strip waveguides, and the width of the local resonant band gap is narrower than that based on the Bragg scattering mechanism. The results also show that the stub shape and width have influence on the frequency and width of the Lamb wave band gap. [Copyright &y& Elsevier]

Abstract: The thermal degradation in ferritic Cr–Ni alloy steel plates is measured using the nonlinear effect of Lamb wave propagation. Experiments were carried out to introduce controlled levels of thermal damage to determine the nonlinear response of Lamb waves. A “mountain-shape” change in the normalized acoustic nonlinearity of Lamb wave versus the level of thermal degradation in the specimens has been observed. The variation in the measured acoustic nonlinearity reveals, based on metallographic studies, that the normalized acoustic nonlinearity increases due to the second phase precipitates in the early stage and it decreases as a combined result of dislocation change and micro-void initiation in the material. The results show a potential application of the nonlinear Lamb waves for the quantitative assessment of thermal damage in metallic plates or pipes. [Copyright &y& Elsevier]

Abstract: In this paper we propose a concept and report experimental results based on a circular array of Piezoelectric Wafer Active Sensors (PWASs) for rapid localization and parametric identification of corrosion type damage in metallic plates. Implementation of this circular array of PWASs combines the use of ultrasonic Lamb wave propagation technique and an algorithm based on symmetry breaking in the signal pattern to locate and monitor the growth of a corrosion pit on a metallic plate. Wavelet time-frequency maps of the sensor signals are employed to obtain an insight regarding the effect of corrosion growth on the Lamb wave transmission in time-frequency scale. We present here a method to eliminate the time scale, which helps in identifying easily the signature of damage in the measured signals. The proposed method becomes useful in determining the approximate location of the damage with respect to the location of three neighboring sensors in the circular array. A cumulative damage index is computed from the wavelet coefficients for varying damage sizes and the results appear promising. Damage index is plotted against the damage parameters for frequency sweep of the excitation signal (a windowed sine signal). Results of corrosion damage are compared with circular holes of various sizes to demonstrate the applicability of present method to different types of damage. [Copyright &y& Elsevier]

An inversion procedure is described to determine the elastic constants of an orthotropic plate by using experimental data on Lamb wave propagation and a curve fitting algorithm based on the Nelder-Mead simplex method. A sample made from glass-epoxy was tested and eight out of nine engineering constants were recovered from Lamb wave propagation measurements in the principal material directions. To improve convergence, the phase velocity sensitivities to different elastic constants are considered. The inversion results agree rather well with experimental data and static tension measurements for the longitudinal elastic modulus. [ABSTRACT FROM AUTHOR]

Abstract: An analysis is given to the band structure of the two-dimensional phononic crystal plate constituted of a square array of elastic anisotropic, circular Pb cylinders embedded in elastic isotropic epoxy. The numerical results show that the band gap can be tuned by rotating the anisotropic material orientation. It is found that the influence of anisotropy on band gap of Lamb wave is clearly different from that on the band gap of bulk waves. The thickness of the system under study is a sensitive parameter to affect the influence of anisotropic materials on the normalized gap width. [Copyright &y& Elsevier]

Abstract: Propagation of the anti-symmetric fundamental Lamb mode (A 0) in a composite laminate containing a semi-infinite delamination was studied through numerical simulations and experimental studies. Here, the wave was generated on a sub-laminate above the delaminated region, and the reflection and transmission of the Lamb modes at the edge of the discontinuity were quantitatively studied. The wave group (named as ‘turning mode’), which was made to propagate from one side of the delamination to the other side (through delaminations) was captured and its transmission factor was estimated. Reflection and transmission factors of wave groups were estimated for three different laminates – quasi-isotropic, unidirectional and cross-ply, containing delamination at various interfaces across thickness. Propagation of ‘turning modes’ were also captured experimentally, employing air-coupled transducers, in quasi-isotropic laminates containing delaminations at different interfaces. Attenuation compensation was incorporated in the experimental signals to improve the comparison of transmission and reflection factors between numerical simulations and experiments. [Copyright &y& Elsevier]

Abstract: This paper proposes an acoustic multi-touch tactile sensing method. The proposed method is based on an amplitude disturbed ultrasonic wave diffraction pattern. An A 0 Lamb wave transmitted in a thin finite copper plate is processed to provide tactile information, for one or two fingers. A touch event is localized by identifying the diffraction signals among a database of diffracted Lamb wave references. Statistic models are used to improve the localization reliability. An artificial silicone finger is used in the calibration procedure. This touch interface is evaluated as a 2-touch interface. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the A0 Lamb wave propagating in a thin finite copper plate is investigated to design a new multi-touch tactile instrument, which could localize one or several fingers. The position of index is localized by identifying the absorption signals of Lamb waves when it is in contact with the plate. Statistic models are used to improve the robustness of this interface. An artificial silicone finger is fabricated in the calibration procedure. The multi-touch (two fingers or more) applications using this plate is also presented. [Copyright &y& Elsevier]

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 holes, A0 mode of Lamb waves was employed due to its shorter wave length compared with S0 mode, which leads to high sensitivity to small damages. Two kinds of excitation techniques for generating comparatively pure A0 mode using piezoelectric lead zirconate titanate (PZT) actuators were realized experimentally. In one technique, two PZT actuators with applied out-of-phase voltages were attached on both sides of the structures. While in the other technique, a kind of grease lubricant was used between the bottom surface of one PZT actuator and the surface of the specimens. Both techniques were able to enhance the component of A0 mode and reduce the component of S0 mode effectively. Secondly, in terms of the arrival time of the A0 wave mode reflected from damages obtained using the wavelet analysis, the positions of damages were identified accurately. The above two techniques were then validated by identifying the transverse cracks and holes in aluminum beams and plates, respectively. Numerical simulations using the finite element method (FEM) for the wave propagation in these structures with damages were carried out. The obtained experimental and numerical results demonstrate that it is possible to identify damage position very accurately by using only sensor data of defective structures without referring benchmark signals (sensor data of intact structures). [ABSTRACT FROM AUTHOR]