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The possibility of using the Digital Image Correlation (DIC) technique, along with Lamb wave analysis, was investigated in this study for damage detection and characterization of polymer carbon fiber (CFRP) composites with the help of numerical modeling. The finite element model (FEM) of the composite specimen with artificial damage was developed in ANSYS and validated by the results of full-field DIC strain measurements. A quantitative analysis of the damage detection capabilities of DIC structure surface strain measurements in the context of different defect sizes, depths, and orientation angles relative to the loading direction was conducted. For Lamb wave analysis, a 2D spatial-temporal spectrum analysis and FEM using ABAQUS software were conducted to investigate the interaction of Lamb waves with the different defects. It was demonstrated that the FEM updating procedure could be used to characterize damage shape and size from the composite structure surface strain field from DIC. DIC defect detection capabilities for different loadings are demonstrated for the CFRP composite. For the identification of any composite defect, its characterization, and possible further monitoring, a methodology based on initial Lamb wave analysis followed by DIC testing is proposed. [ABSTRACT FROM AUTHOR]

Lamb waves are mechanically guided waves that propagate through plates and shells, and their speed depends on the frequency. Nowadays, researchers use these waves to detect defects in structures. This is due to the properties of Lamb waves that can propagate in the whole structure, and are quickly affected if there is a defect. Using this method instead of traditional methods due to their complexity, cost and time are desired. As an innovation in this research, the effects of lamb wave propagations in symmetrically variable stiffness fiber-metal laminated plates are investigated and for this purpose, the finite element method is used. In this analysis, curvilinear fibers are used in the composite layers instead of the straight fibers, and the effect of the number of layers, plate dimensions, and boundary conditions on the propagation of the mentioned waves are investigated. Also, to check the validity, the first three frequencies of the structure are compared with several different references. The obtained results show that the Lamb waves are propagated along the length of the plate and are reflected by hitting the boundaries of the plate or defect in the structure and lose their uniform propagation state. Based on this investigation, this method can be used to detect the defect in variable stiffness fiber-metal laminated structures.

During production or service life, lightweight components made of fibre-reinforced polymers (FRP) may suffer from different types of defects such as cracks or delamination, which may be detected, amongst others, with the help of ultrasonic lamb waves (LW). The study intends to illustrate the potentials of numerical models to assist the design of LW-based non-destructive testing (NDT) processes. For that, the complete NDT chain of an LW driven inspection system was supported by a Finite Element Analysis (FEA) using stringer-stiffened C FRP panels as an exemplary component. The study was divided in two publications: The first part focused on the description of the numerical model, illustration of its functionalities and – most importantly – the determination of LW related effects that were needed to design the inspection system (dispersion characteristics, excitation frequency, propagating modes etc.). In this part, the FEA driven design approach was first validated with self measured experimental data as well as experiments provided in literature. In the second step, the numerical model was applied for the performance of parameter studies in order to identify component specific aspects that proved important for the NDT process. Finally, general advices and FEA related working steps were derived from all findings to assist practitioners in the implementation of the technique. [ABSTRACT FROM AUTHOR]

Transcranial focused ultrasound (FUS) is a noninvasive treatment for brain tumors and neuromodulation. Based on normal incidence, conventional FUS techniques use a focused or an array of ultrasonic transducers to overcome the attenuation and absorption of ultrasound in the skull; however, this remains the main limitation of using FUS. A dual-mode conversion technique based on Lamb waves is proposed to achieve high transmission efficiency. This concept was validated using the finite element analysis (FEA) and experiments based on changes in the incident angle. Aluminum, plexiglass, and a human skull were used as materials with different attenuations. The transmission loss was calculated for each material, and the results were compared with the reflectance function of the Lamb waves. Oblique incidence based on dual-mode conversion exhibited a better transmission efficiency than that of a normal incidence for all of the specimens. The total transmission losses for the materials were 13.7, 15.46, and 3.91 dB less than those associated with the normal incidence. A wedge transducer was designed and fabricated to implement the proposed method. The results demonstrated the potential applicability of the dual-mode conversion technique for the human skull. [ABSTRACT FROM AUTHOR]

Components made of fibre-reinforced polymers (FRP) may suffer from various types of damages, such as cracks or delamination. In order to monitor the structures' material condition, non destructive testing (NDT) methods have emerged – among them, the use of ultrasonic waves. Inter alia, research focused on Lamb waves (LW), which offer promising conditions for NDT processes. However, prior implementation in a specific production environment, practitioners are faced with many open questions such as excitation frequencies to be used or which mode to select for defect identification. Therefore, the study aimed at demonstrating how a finite element analysis (FEA) can assist the experimental design of an LW driven NDT process. For that, a numerical model was developed, capable of computing LW propagations in an exemplary component: Stringer-stiffened C-FRP panels. The study was divided into two parts: In this part, all information regarding the model building is presented and principal functionality demonstrated. In the second part, the FEA is validated and then used to perform parameter studies, highlighting important conditions for practical application. The results of the present paper showed that stiffeners provoke varying interactions with LW modes – effects that increase wave field complexity but do not limit defect identification within or behind the stringer. [ABSTRACT FROM AUTHOR]

It is well known that acoustic emission (AE) signals, generated by external impacts or damages such as crack initiation, mainly propagate in the form of Lamb waves in plate-like structures. In this work, MEMS-based resonant capacitive micro-machined ultrasonic transducers (CMUTs), which are designed for sensing out-of-plane displacements, have been verified by finite element method (FEM) modeling and theoretical analysis for their feasibility of detecting low-order Lamb waves (A0 and S0). First, combining the propagation theory of Lamb waves and the "spring-mass-damper" model of CMUTs, the out-of-plane sensing mechanism has been explained, together with the analytical expression of sensitivity. Then, simulations based on FEM have been carried out to show that the designed CMUTs are sensitive to out-of-plane displacements, while extremely insensitive to in-plane displacements. Meanwhile, a transient analysis has found the potential abilities of CMUTs for sensing A0 and S0 lamb waves. Besides, the sensing characteristics of CMUTs have also been investigated, including the influence of squeezed-film damping, the amplitude of the input signal, the cell number, and cell space. Finally, the ball drop impact is simulated to show the potential of identifying the location of the AE source by CMUTs. Our studies reveal the out-of-plane sensing behaviors of CMUTs for Lamb waves and may have the potential in promoting the miniaturization and integration of AE sensors. [ABSTRACT FROM AUTHOR]

Propagation of Lamb waves in both laminated and functionally graded (FG) composites is analyzed. Anomalous dispersion at high frequencies is observed revealing substantial discrepancy in the high frequencies asymptotes for quasi-flexural and quasi-symmetric fundamental modes. The applied methodology is based on a variant of sextic complex formalism. [ABSTRACT FROM AUTHOR]

A new type of stepwise radial metamaterial (SRM) with ultralow-frequency and broadband characteristics is proposed in this study. In contrast to the traditional radial metamaterial (TRM), the proposed structure is periodically arranged in a stepwise shape along the radial direction. The propagation characteristics of Lamb waves in the SRM were investigated using the finite element method. For the numerical analysis, the degeneracy between the bands of the SRM was separated, resulting in the opening of the bandgaps in the ultralow-frequency range. The total bandwidth was 75 times that of the TRM, and the wave attenuation ability was increased by more than 70%. The introduction of a stepwise array in the SRM opened up the local resonance and Bragg scattering bandgaps, and as a result, the SRM exhibited ultralow-frequency broadband characteristics. Furthermore, the influences of the structural parameters of the SRM on the bandgap characteristics were discussed. With the increase in the stepped angle, the coupling relationship between the Lamb wave mode and the local resonance was enhanced, which caused the band structure to shift to a lower frequency. In addition, the hole rotation and shape played important roles in the bandgap tuning. Finally, the experimental sample was processed based on the model, and the vibration propagation characteristics were tested to prove its ultralow-frequency broadband characteristics. The proposed shielding approach could provide a better alternative in the field of ultralow-frequency noise reduction and vibration reduction. [ABSTRACT FROM AUTHOR]

• A non-contact ultrasonic method of gas flow metering in a duct is described. • Wave propagation was visualized by COMSOL Multiphysics®. • Multipath signals with different time shift were successively received by a receiver in this method. • Receiver could collect the signal in a wide region (from 50 mm to 250 mm in the x direction in this work). • Simulations and experiments show good agreement with the analytical model (RMSE are 0.0708 and 0.0767 respectively). This work investigated the signal reception from multipath including direct transmission and reflection paths in a non-contact gas flow measurement system based on air-coupled leaky Lamb waves. Analytical model was established to provide the multiple reflective signal propagation time differences between upstream and downstream paths at different flow velocities. A multiphysics model based on finite element methods was established to facilitated the visualisation of the leaky Lamb waves reflected between the plates and the Lamb waves excited by multiple reflective paths waves. The propagation time differences were evaluated by performing simulations and experiments, which are in good agreement with the analytical values (RMSE are 0.0708 and 0.0767 respectively). The receiver can collect multipath signals at a large distance separated from the transmitter. Time resolution is enhanced by using received signals from reflection paths, as the transit time increased with the ultrasound path between the transducers. [ABSTRACT FROM AUTHOR]

In this article, the baseline-free stress monitoring methodology has been proposed. Initially, it is theoretically and experimentally proven that the impact of the external stress on the phase velocity of the Lamb waves varies with the Lamb wave propagation direction in isotropic materials. The Lamb wave propagation time difference in the specific directions can be subsequently used to calculate the existing stress. The proposed methodology can be therefore eliminated from the usage of the referential ideal echo data under zero residual stress condition, which is however the origin of the stress measurements for the existing methods. Then, a semi-analytical finite element model is established to calibrate the stress coefficient of the homogeneous prestress waveguide. The piezoelectric wafer active sensors array is finally applied to excite and receive the Lamb wave signal in the specially designed directions. Experimental verification indicates that the maximum absolute error of the stress measurement is 4.7 MPa within the external stress range from 20 to 100 MPa. It is supposed that the proposed methodology is a promising alternative for structural quality assessment and safety assurance. [ABSTRACT FROM AUTHOR]

Bolts are commonly applied to lap joints in various industrial fields. Harsh environment, low quality of operation, and other unfavorable conditions may result in bolt looseness. Due to the structural complexity, conventional Lamb wave-based methods just extract statistical characteristics for looseness detection, while neglecting the information contained in the propagation process of Lamb waves. To take full advantage of the information on bolt states carried by array signals during the propagation, this study proposes a detection procedure based on propagation model and imaging methods of Lamb waves. Firstly, the contact status on the macro and micro scales is investigated with theoretical analysis and finite element simulation, and the theoretical model of Lamb wave propagation across bolted joints is established deriving from the analysis of contact. Subsequently, three imaging methods, including delay-and-sum, minimum variance, and sparse reconstruction, are adapted for the identification of bolt state. Since Lamb waves are mainly transmitted into another plate through the contact area near the bolts based on the proposed propagation model, pixel values are inclined to be concentrated around the bolts. On this basis, imaging intensity near each individual bolt can reveal its fastening state given that the transmitted energy around its location is associated with its torque. Finally, experiments are carried out in both single-bolt and double-bolt joints. The proposed propagation model is verified in the single-bolt joint and loose bolts can be identified in the imaging results of the double-bolt joint, which demonstrates the effectiveness of the proposed method for bolt state identification. [ABSTRACT FROM AUTHOR]

In this paper, a new structure of two-dimensional phononic crystals consisting of one or more rows of parallel rectangular rods placed periodically in a homogenous slab, in which the rods are not connected directly but linked through neck structures with the slab, is proposed, and the Lamb wave propagation in this structure is investigated with numerical analysis. The dispersion relations and the power transmission spectra are studied using the finite-element method. In contrast to the phononic crystals where the rods and the slab are completely in contact, the proposed structure with necks are proved to display band gaps at much lower frequencies. The displacement fields of the eigenmodes of the band edges are computed and analyzed to clarify the mechanism for the generation of the low-frequency band gaps. It is found that the low-frequency band gaps are attributed to the interaction between the local resonance of the rod inclusion connected with the neck and the Lamb modes of the four plates which are formed by the introduction of the neck. Furthermore, the influences of the geometry parameters of the neck on the band gaps are discussed. Numerical results show that band gaps are significantly dependent upon the width and the position of the necks while insensitive to the neck length. These properties of Lamb waves can potentially be applied to optimize band gaps, generate filters, and design acoustic devices. [ABSTRACT FROM AUTHOR]

This work investigates the interaction of shear and Lamb waves with different kinds of defects in plates, in view of applications to defect characterization purposes. Using a finite element model, the reflection and transmission coefficients of shear and Lamb waves are determined, as a function of size parameters of the defect related to its extension and depth. Notches with elliptical and rectangular profile are examined, together with internal voids, covering both symmetric and asymmetric cases. Low and high frequency times height (2hf) regimes are considered in order to clarify how mode conversion can provide information on the shape of the defect. In this regard, also the role of symmetric and asymmetric waves is elucidated. [ABSTRACT FROM AUTHOR]

Lamb wave dispersion curves are useful for optimizing the inspection scanning distance that can be covered with good sensitivity in many current applications. However, one of the main problems concerning this calculation lies in selecting a numerical method that is computationally accurate and efficient. In this paper, Lamb waves dispersion curves are generated by the Scaled Boundary Finite Element Method, and by the Rayleigh–Lamb equation. For the semi-analytical case, waveguide cross-section discretization was performed using isoparametric elements and high-order spectral elements. The semi-analytical formulations lead to an eigenvalue problem that can be solved efficiently by calculating the couples of wavenumbers and frequencies that guarantee the wave mode propagation, the basis for generating the dispersion curves. These are compared with those obtained from the analytical solution for the symmetric and antisymmetric modes; in both cases, homogeneous plates of constant thickness are considered. The numerical results show good agreement when using a low number of isoparametric elements, or a single spectral element with shape functions of the order of six for computing the dispersion curves and wave structure. The calculation is given with low computational effort, and the relative variation with respect to the analytical reference values is less than 2%. [ABSTRACT FROM AUTHOR]

A semi-analytical finite element method algorithm was established to plot the dispersion curves of isotropic aluminum and orthotropic plates. The curves obtained are compared with those plotted by the DISPERSE software and with previous experimental work. The results showed that the accuracy of the method depends on the number of elements for meshing. To ensure good precision and speed of the method, the number of elements per plate thickness must be optimized. [ABSTRACT FROM AUTHOR]

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

• A method for liquid level measurement based on leaky lamb waves was presented to improve the sensitivity and accuracy of liquid level detection. • Liquid level detection with lamb waves was simulated using the finite element method. • The field experiment of liquid level detection was performed, and the comparison tests of the detection of lamb waves and longitudinal waves were conducted. The accurate detection of liquid-level plays an important role in industrial process. This paper presents a method for liquid-level measurement based on Lamb waves. The dispersion equation of Lamb waves under unilateral liquid load is deduced to obtain the dispersion curves. The structure and attenuation curves are analyzed under a certain frequency and thickness, and the modes with more energy loss are selected. The ratio of the signal energy of the liquid-level at the upper and lower positions is defined as the discrimination to indicate the change of the liquid-level. The models of the liquid storage tank are built and the Lamb waves of different modes are simulated using the finite element method. The experimental results reveal that it is easy to detect the difference in ultrasonic signals when the liquid-level changes. Compared with the longitudinal wave detection method, the proposed method improves the discrimination of liquid-level detection. The experimental results are consistent with the theoretical calculations. [ABSTRACT FROM AUTHOR]

The reflection and transmission characteristics of Lamb waves at an adhesive single lap joint of plates are examined theoretically by the hybrid finite element method. The adhesive joint is modeled by a linear spring-type interface, which is characterized by normal and tangential stiffnesses. For the incidence of the lowest-order antisymmetric (A0) Lamb mode in a low frequency range, it is shown that the reflection and transmission coefficients of the A0 mode take local maxima and minima at multiple frequencies. This behavior results from the interference of waves originating from the lowest-order antisymmetric guided wave mode in the overlap region. The peak frequencies of the transmission coefficient increase monotonically with increasing tangential stiffness, but are almost invariant with the normal stiffness of the adhesive joint. Furthermore, time-domain numerical simulation using the finite element method is carried out to discuss the theoretical results. As a result, for the A0 mode incidence, the reflection and transmission waveforms of the A0 mode from the lap joint are found to show long-oscillation tails. The spectral analysis for the obtained waveforms shows that these tails are necessary to identify the frequencies at which the reflection and transmission coefficients take local maxima and minima. [ABSTRACT FROM AUTHOR]

Leaky Lamb waves have the potential to be used to perform non-destructive testing on a set of several parallel and immersed plates. Short-time Fourier transform and two-dimensional Fourier transform have both been successfully used to measure the propagation properties: phase and group velocity, and leaky attenuation. Experimental measurements were validated by comparison between theory, experimentation and finite-element simulations (using comsol multiphysics® software) in the case of one immersed plate in water. These signal processing techniques proved to be efficient in the case of multi-modal propagation. They were applied to two immersed plates to identify the leaky Lamb mode generated in the second plate. Dispersion curves of the system composed by two immersed and parallel plates are computed. When plates have the same thickness, leaky Lamb modes propagate from the first to the second plate without any mode change, with the apparent attenuation being weaker in the second plate. Considering that the second plate is continuously supplied in energy by the first one, an energy-based model is proposed herein to estimate the apparent attenuation in the second plate. Despite our extremely simplifying assumption, this model proved to be in good agreement with both finite-element modelling and experimentation. [ABSTRACT FROM AUTHOR]

High-density polyethylene (HDPE) pipes are becoming increasingly popular, being used in various fields, such as construction, marine, petroleum, water transfer, process water, methane gas collection, oil and gas gathering, gas distribution systems, mining, acid and wet gas lines, offshore oil and gas and in nuclear power plants. Higher-order guided Lamb wave (UGW) modes can be used to detect various defects in complex structures. We will apply this methodology to one of the types of plastic—the structure of high-density polyethylene (HDPE). However, the excitation of UGW modes faces numerous challenges, especially when there is a need to identify which mode is excited. It is essential to note that, in the higher frequency range, multiple different higher-order modes can usually be excited. This can make it difficult to determine which modes have actually been excited. The objective of this research was to successfully excite and receive various higher-order UGW modes in high-density polyethylene structures using both ultrasonic single-element transducers and a phased array. Theoretical calculations were performed using a variety of methods: semi-analytical finite element (SAFE) method, 2D spatial–temporal spectrum analysis and finite element modeling (FEM). The results obtained from both measurements and simulations clearly demonstrate the possibility of efficiently exciting and receiving different Lamb wave modes possessing different phase velocities. [ABSTRACT FROM AUTHOR]

Lamb waves were utilized to quantify micro-crack damage in aluminum plates, and the scattering and mode conversion of Lamb waves passing through cracks were analyzed. The dynamic time warping (DTW) method was used to match and compare each Lamb wave time series that represented different damage degrees. The matching difference between the damaged plate and undamaged plate was taken as a marker to measure the damage degree of the workpiece. At the same time, due to the pathological alignment of traditional DTW methods, the shape context (SC) profile recognition method was introduced to optimize the algorithm for calculating the distance between sampling points in the DTW method and solve the pathological alignment problem. Finally, the SC-DTW method based on Lamb waves was verified by the finite element simulation model and bending test of aluminum plates. The results showed that the method was feasible for quantifying the damage degree of aluminum plates and had a great advantage in the analysis and processing of time series in low-sampling frequency and high-noise scenarios. [ABSTRACT FROM AUTHOR]

Early damage in metal materials is primarily characterized by microdefects, such as microcracks and micropores. In this study, we developed a novel approach for quantitatively assessing the microdefect damage degree in dual-phase bainite–ferrite (B + F) X80 pipeline steel using ultrasound nonlinear parameters. A finite element analysis model with micron-scale three-dimensional ellipsoidal defects was established based on the Gurson–Tvergaard–Needleman material damage theory. The propagation process of Lamb waves in the model was simulated to investigate the interaction mechanism between microdefects and Lamb waves and explore the quantitative relationship between ultrasound nonlinearity and microdefect damage degree, which can be characterized by the size and number of microdefects. Furthermore, to validate the simulation findings, an ultrasound testing experiment was designed to evaluate the microdefect damage in (B + F) X80 pipeline steel. The results demonstrate that when Lamb waves propagate through microdefects, their amplitude is attenuated, resulting in changes in ultrasound nonlinear parameters. Within a specific range, the nonlinear coefficient of Lamb waves increases with an increase in microdefect length and number of microdefects but decreases with an increase in microdefect width. This finding holds significant value for evaluating damage degree and predicting the service life of X80 pipeline steel. [ABSTRACT FROM AUTHOR]

Due to its multi-mode and dispersion characteristics, Lamb waves cause interference to signal processing, which profoundly limits their application in nondestructive testing. To resolve this issue, firstly, based on the traditional EMAT, a horizontal polarization periodic permanent magnet electromagnetic acoustic transducer (HP-PPM-EMAT) was proposed. A 2-D finite element model was then developed to compare magnetic flux density, Lorentz force, and signal strength between the traditional EMAT and the HP-PPM-EMAT. The simulation results show that the HP-PPM-EMAT enhances the A0 mode Lamb wave (A0 wave) and suppresses the S0 mode Lamb wave (S0 wave). Finally, the influence of structural parameters of the HP-PPM-EMAT on the total displacement amplitude ratio of A0 and S0 was investigated using orthogonal test theory, and the width of magnet units was improved based on the orthogonal test. The results show that the total displacement amplitude ratio of A0 to S0 of the improved HP-PPM-EMAT can be improved by a factor of 7.74 compared with that of the traditional Lamb wave EMAT, which can produce higher-purity A0 mode Lamb waves. [ABSTRACT FROM AUTHOR]

Most cracks do not grow straight due to the inhomogeneity of material properties. Furthermore, quantifying structural damage using Lamb waves is difficult due to uncertain crack shape. Here, we present a newly developed finite element model to simulate the propagation of Lamb waves in a cracked plate to quantify the influence of crack size and orientation on the characteristics of Lamb wave propagation. This model includes a piezoelectric actuator and sensor to generate and receive the tone-burst signals of Lamb wave. The damping coefficient of the finite element model was calibrated with the experimental data of plates without a crack. The Lamb wave propagation simulations were performed with various crack sizes, crack orientations, and distances between the actuator and sensor. Our model was utilized to investigate the influence of crack orientation on damage quantification based on the signal phase change and normalized amplitude. We successfully evaluated the effect of the incident angle of the crack, crack size, and distance between the sensors on the change of the plate wave signal characteristics. The signal characteristics obtained from our novel finite element model can be used to develop a quantification model to estimate the crack size, direction, and distance from the sensor. [ABSTRACT FROM AUTHOR]

Abstract Lamination defect is one of the common defects in the manufacturing process of seamless pipes. In this paper, the quantitative detection of a lamination defect in thin-walled metallic pipe using circumferential Lamb waves is studied. To interpret the received time-domain signals and extract useful information about the lamination defect, wavenumber analysis is performed on these signals. A three-dimensional finite-element model is established using Matlab and ABAQUS commercial software. Owing to the processing technique, an aluminum ring structure with a three-quarters circumference is considered to represent the metallic pipe. The lamination defect constructed in the model is a "zero-volume" crack, which stretches from θ = 180° to 270° and locates in the mid-plane of the wall. A five-cycle 0.41-MHz sinusoidal tone-burst signal modulated by a Hanning window is carefully chosen to generate the appropriate excitation wave, in CL 0 mode. According to the received signals, the conclusion that the incident CL 0 mode interacts with the lamination defect for numbers of times can be obtained. The space-amplitude curve of incident waves is also depicted to reveal the amplitude distribution of incident waves. A fully non-contact experimental platform that adopts an electromagnetic acoustic transducer as a transmitter and a laser ultrasonic inspection system as a receiver is set up to verify the finite-element model. Three different wavenumber analysis methods are performed on the wavefield signals to explain the detectability of the lamination defect separately through both numerical and experimental studies. It can be concluded that, from the variation of wavenumber, the continuity of structure can be deduced. Not only can the location be calculated with an error of less than 10%, but the profile of the lamination defect can also be imaged. It is also found that very good consistency exists between numerical and experimental results. Highlights • Lamination defect in thin-walled metallic pipe is detected using C-Lamb waves. • Wave-propagation characteristics are depicted between C-Lamb waves and lamination. • Wavenumber analysis methods are used to quantitatively identify lamination defect. • A fully non-contact EMAT-laser platform is set up to perform wavenumber analysis. [ABSTRACT FROM AUTHOR]

Delamination is one of the most common types of damage in carbon fiber reinforced polymer composites. This paper analyzes the interaction of fundamental symmetric (S0) and anti-symmetric (A0) mode Lamb waves with the start and the end of a delamination using finite element simulations. Propagations of Lamb waves in a 4-layer [0/90]s laminate were compared with propagations obtained in a 1-layer [0] sub-laminate and in a 3-layer [90/90/0] sub-laminate, which are the two sub-laminates separated by the delamination. Results show that the Lamb waves travel at different velocities in the delaminated region and in the healthy region. Comparing the signals obtained in the delaminated region with the excitation, it was found that mode conversions happen at the edges of a delamination for both S0 and A0 waves. The mode conversion of S0 wave at the end of a delamination generates large A0 waves and causes most significant changes to the detection signal. Based on this feature, a reference-free localization method is proposed in this paper. By measuring the difference between the arrival times of two modes, the distance between pick-up sensor and the end of delamination can be estimated. This method was verified by signals obtained in finite element simulation. [ABSTRACT FROM AUTHOR]

In this paper, we continue our work on the propagation of Lamb waves in a poroelastic infinite strip containing a linear crack. In the previous work, the crack was placed horizontal to the upper and lower surfaces and moreover, there was zero pressure on the upper and lower surfaces, a physically impossible situation. In the present paper, the crack is situated perpendicular to the side and situated at the origin. A step stress is applied to the crack surface resulting in transient behavior. By using the Fourier transform and Laplace transform methods, we arrive at dual integral equations. From this formulation, we subsequently obtain, by reduction, a Fredholm integral equation of the second kind. This equation is solved numerically, and the dynamic stress intensity factor is shown graphically. [ABSTRACT FROM AUTHOR]

Recently, a perturbation approach was adopted for the non-collinear mixing of plate waves in an isotropic elastic plate with quadratic nonlinearity by Ishii, Biwa, and Adachi [J. Sound Vib. 419, 390–404 (2018)] and a resonance condition was derived theoretically, namely, that a significant scattered plate wave is generated when its frequency and wavevector coincide with the sum/difference of those of its primary waves. However, that analysis assumed monochromatic plane waves that interacted everywhere in the plate. To apply the non-collinear mixing of plate waves to nondestructive evaluation of plate-like structures, the influence of the spatial and temporal finiteness of the primary waves on the generation of the scattered wave must be elucidated. To that end, the present study conducts three-dimensional dynamic finite-element analyses on the mixing of lowest-order antisymmetric Rayleigh–Lamb waves that have finite beam widths and time durations. The generation of scattered lowest-order symmetric Rayleigh–Lamb and shear horizontal waves with sum frequencies is discussed for various beam widths, intersection angles, and primary frequencies. The resonance condition derived originally for monochromatic plane waves in the aforementioned study is found to be valid even when the wave interaction is allowed only within limited space and time. [ABSTRACT FROM AUTHOR]

In this paper two techniques: the zero-crossing and the spectrum decomposition are implemented together to obtain the guided waves phase velocity and their dispersion properties. This proposed hybrid technique has been investigated in two different mediums: an homogenous using signals obtain by 2D finite element modeling and experimental investigation and a non-homogenous environment using 3D finite element model. The results are compared with those obtained by the semi-analytical finite element method (SAFE). It was demonstrated that in the case of correctly selected filters the dispersion curves are reconstructed in approximately 90% for the Lamb wave A 0 mode, for the non – dispersive S 0 mode up to 81% of the incident signal frequency bandwidth in a homogeneous medium. In the case of non-homogenous CFRP plate the phase velocity dispersion curve of the A 0 mode can be reconstructed even up to 96% incident frequency bandwidth. [ABSTRACT FROM AUTHOR]

Lamb waves are extensively used in non-destructive tests (NDT) to detect corrosion and another defects in thin plates. Several NDT methods require to compute the phase velocity of the Lamb wave, which depends on the frequency. In this work we show the potential of the combination of the phase velocity method (PVM) with the finite element method (FEM) for computing the phase velocity dispersion curve of an ultrasonic pulse traveling in a transversally annular isotropic thin plate. The FEM-PVM is based on the numerical solution of the wave propagation equations for several selected frequencies. To solve these equations, a second order difference scheme is used to discretize the temporal variable, while spatial variables are discretized with FEM. The open software FreeFem++ is used with quadratic triangular elements to compute the displacements. The phase velocity for a given frequency is obtained from the computed displacements at few points on the top of the plate. Repeating the procedure for several frequencies we compute a sample of points that are fitted to obtain an approximation of the phase velocity dispersion curve. A deeper understanding on the behavior of the phase velocity dispersion curve with respect to the plate curvature is obtained as result of an extensive experimentation with annular isotropic plates of two materials. [ABSTRACT FROM AUTHOR]

The effect of material viscoelasticity on the propagation of Lamb wave in a viscoelastic thin plate has been investigated quantitatively. Based on the plane strain theory, a numerical model of the laser-generated Lamb wave is developed in the frequency domain by using the finite element (FE) method. To verify the correctness of the FE model in the frequency domain, a classical FE model in the time domain is established for an elastic plate. The transient waveforms calculated in the frequency domain are in very good agreement with those obtained in the time domain. And then, the laser-generated Lamb waves in the elastic and viscoelastic thin plates are calculated, respectively. By comparing the waveforms of Lamb wave in the two plates, the influence of the material viscoelasticity on the propagation of Lamb waves is studied. Moreover, the effect of the material viscoelastic rate on Lamb waves is analyzed in detail. [ABSTRACT FROM AUTHOR]

Structural health monitoring (SHM) is becoming very significant for preventing catastrophic failures and uninterrupted operation. Due to continuous developments in SHM Lamb wave technique, finite element simulation is emerging as an initial step to be performed to visualize the potential solution to problem. The present study focus on the simulation of Lamb wave response using finite element method and its application to crack detection and identification in 3D aluminium plates and brass tubes using commercially available finite element package ABAQUS. Phase velocity and group velocity dispersion curve are initially plotted for aluminium and brass material. Subsequently simulation of distinct specimens with and without presence cracks is performed. Simulation results were validated and compared with actual results and were found to be in reasonably good agreement. A damage index parameter i.e. amplitude ratio is defined to notice the effect of crack dimensions variation. Interaction of Lamb waves with rectangular, semi-elliptical and semi-circular cracks (shallow to deep) is studied in detail. Also the application of signal processing techniques such as Spectrogram analysis, Fourier transform and Hilbert transform is also reported in this paper. [ABSTRACT FROM AUTHOR]

[Display omitted] • A meter-scale ultra-low frequency wide-bandgap seismic metamaterial for Lamb waves is proposed for the first time, and the bandgap can cover the 2 Hz seismic peak spectrum. • Band structures, vibration modes and transmission spectrum of the seismic metamaterial are calculated and analyzed. • The influences of geometrical parameters, material parameters value on the bandgap width and location are discussed. • The structure is optimized, and the effects of the different forms of matrix on the attenuation degree of the transmission spectrum is discussed. Seismic metamaterials (SMs) have attracted the attention of many researchers in the field of damping and elastic wave isolation because of their bandgap properties, i.e., attenuating elastic waves in the range of bandgaps. However, the dimension of these SMs often reaches the 10-meter scale, which impedes practical engineering application. To break through this bottleneck and achieve a smaller scale, a meter-scale novel SM composed of steel and rubber is proposed for attenuating ultra-low frequency seismic Lamb waves. Firstly, the finite element method is used to analyze the band structure of SMs and calculate the bandgaps. To explain the mechanism of bandgap, the vibration modes of the waves at the bandgap boundary frequency are further analyzed. Subsequently, the transmission spectrum of Lamb waves incidents on the finite SMs system is analyzed to prove the authenticity of the bandgaps. Finally, parameter analyses including the geometric variables, material properties, equivalent mass density, and structural matrix forms with identical equivalent mass density are investigated numerically. The results show that Lamb waves in the range of 0–20 Hz are significantly attenuated by SMs, and the aforementioned parameters are significant factors affecting the bandgap properties and transmission properties. The proposed SM has a smaller size while maintaining some wide bandgaps at 0–20 Hz for ultra-low frequencies. It is worth noting that the 2 Hz seismic peak spectrum causing the destruction of building structures is covered effectively. [ABSTRACT FROM AUTHOR]

We previously formulated a semi-analytical finite element technique for Lamb waves in a plate surrounded by fluids and investigated the dispersion curves and wave structures for leaky Lamb waves. Herein, this technique is extended to the calculation of transient responses both in a plate and in fluids for dynamic loading on the plate surface. To gain fundamental insights into guided wave inspection for a water-filled pipe or tank, guided waves generated upon transient loading of a flat plate water-loaded on one side were analyzed. The results show that a quasi-Scholte mode propagating at the plate-water interface is useful for the long-range inspection of a water-loaded plate because of its non-attenuation and minimal dispersion; moreover, this mode has superior generation efficiency in the low-frequency range, while it is localized near the plate-water interface at higher frequencies. [ABSTRACT FROM AUTHOR]