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We investigate guided (Lamb) waves in a human cadaver skull through experiments and computational simulations. Ultrasonic wedge transducers and scanning laser Doppler vibrometry are used respectively to excite and measure Lamb waves propagating in the cranial bone of a degassed skull. Measurements are performed over a section of the parietal bone and temporal bone spanning the squamous suture. The experimental data are analyzed for the identification of wave modes and the characterization of dispersion properties. In the parietal bone, for instance, the A0 wave mode is excited between 200 and 600 kHz, and higher-order Lamb waves are excited from 1 to 1.8 MHz. From the experimental dispersion curves and average thickness extracted from the skull computed tomography scan, we estimate average isotropic material properties that capture the essential dispersion characteristics using a semi-analytical finite-element model. We also explore the leaky and non-leaky wave behavior of the degassed skull with water loading in the cranial cavity. Successful excitation of leaky Lamb waves is confirmed (for higher-order wave modes with phase velocity faster than the speed of sound in water) from 500 kHz to 1.5 MHz, which may find applications in imaging and therapeutics at the brain periphery or skull-brain interface (e.g., for metastases). The non-leaky A0 Lamb wave mode propagates between 200 and 600 kHz, with or without fluid loading, for potential use in skull-related diagnostics and imaging (e.g., for sutures). [ABSTRACT FROM AUTHOR]

• Semi-analytical approach presenting derivation of the scattering matrix related to the region with adhesively bonded joint. • Effect of the energy balance convergence of local sub-waveguide sections on the global scattering matrix. • Effect of the number of considered complex wavenumbers entering the computational process on the predicted results accuracy. Problems related to scattering phenomena of the antisymmetric fundamental Lamb wave mode with an adhesively bonded lap joint containing a disbond in the adhesive layer is being solved within the framework of this paper. Overall, two methods are used in order to identify energy scattering coefficients of reflected as well as transmitted Lamb waves. These methods include a hybrid approach, which incorporates the finite element method (FEM) with general orthogonality relation, and semi-analytical approach, combining the modal expansion technique and orthogonal property of Lamb waves. With the use of the stress and displacement continuity condition on selected interfaces, it is possible to explicitly derive the global scattering matrix, containing the information related to the scattering process of present Lamb wave modes in a given geometry. Both approaches are then adopted on three configurations of the adhesively bonded lap joint in terms of the length of the disbond, which is then positioned within the overlapped region. The obtained results, among others, clarify the relationship between geometric parameters of the region containing the overlapping with the horizontal disbond and the role of non-propagating modes in the computational process of energy scattering coefficients of propagating modes. [ABSTRACT FROM AUTHOR]

In the present study, the asymptotic stress and displacement solutions related to mixed mode I/II loading are calculated in the vicinity of the V-notches with end holes (VO-notches). After deriving the asymptotic stress and displacement fields, in order to calculate the coefficients of the series, a numerical technique called the over-deterministic method is utilized. According to the over-deterministic method, the unknown coefficients are calculated by fitting the asymptotic displacement field to the nodal displacement values. To show the accuracy of the derived asymptotic stress series and the effectiveness of higher order terms, the Brazilian disk specimen is modeled using finite element analysis. Different examples under pure mode I, pure mode II and mixed mode I/II loading for three different notch opening angles and four notch tip radii are considered and the coefficients of stress series are computed for each example. Finally, the obtained coefficients are used to compare the stress distribution of the truncated stress series with its relevant finite element values. The results show that considering the singular terms alone will generate large amounts of errors while very good accuracy could be achieved by considering the higher order terms. [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]

The wall pressure generated by near-field underwater explosion bubble involves significant nonlinear problems. This has been the focus and difficulty of the research on the anti-explosion design of battleships. To quantify the load characteristics of bubble pulsation and water jet pressure, underwater explosion tests were conducted beneath a clamped square plate subjected to 2.5g TNT at different distances (R = 10, 15, 20, 22.5, and 25 cm). Results showed that as the stand-off distance increased to the maximum radius of the bubble, there was a peak value of the impulse for bubble pulsation and water jet. A finite element model was then established and benchmarked. The benchmarked model was used to further explore the load characteristics of the bubble pulsation and water jet, considering the effect of the stand-off distance, plate dimension, horizontal distribution, and explosive equivalent. It was found that: a) as the plate dimension increased, the pressure of the shock wave to the plate was basically unchanged, while the period and impulse of the bubble pulsation and water jet decreased; b) the impact of the water jet pressure was concentrated within 6.72 times the radius of the explosive. Finally, an empirical formula was proposed to predict the impulse of the bubble pulsation and water jet. This research is able to provide guidance to investigate the blast resistance of ships. • The wall pressure-time history curves of bubble jet were obtained. • The bubble jet load was accurately predicted by the CEL method. • The effect of the stand-off distance, plate dimension, horizontal distribution, and explosive equivalent on bubble jet load. • An empirical formula was proposed to predict the specific impulse of the bubble jet load. [ABSTRACT FROM AUTHOR]

Interface debonding may happen in steel structures repaired or strengthened by Carbon Fiber Reinforced Polymers (CFRP), which will reduce ultimate loads and fatigue life of the structures. In this paper, a non-destructive method to test the interface debonding using Lamb waves is proposed. The electromechanical finite element detection model of a steel plate strengthened by CFRP is constructed, and the results have good consistency with experimental results. The effects of debonding position and debonding degree on the received signals are discussed in this paper, the signal waves are analyzed by the wavelet time-frequency conversion method and wavelet packet analysis method, and the relationship between the signal waves and the debonding parameters are obtained, which could support the damage detection of the repaired or strengthened structures by CFRP in the future. • The signal of Lamb wave in steel plates strengthened by CFRP with interface debonding were analyzed. • The experimental model and the FE model of interface debonding analysis in CFRP strengthening steel plate were constructed. • The effects of debonding position and degree on the characteristics of received signals were quantitatively analyzed. [ABSTRACT FROM AUTHOR]

• Anisotropic wave skew effects can cause an apparent offset in guided wave velocities. • Difference from theory of UD CFRP plate experiments and FE simulations. • Reduced accuracy for damage detection and localization in composite structures. • Skew angle correction for point source excitation, implemented experimentally. Guided ultrasonic wave propagation in anisotropic structures results in directional dependency of velocity and wave skewing effects that can impact the accuracy of damage detection. Phase and group velocities of the A 0 guided wave mode, propagating in a unidirectional carbon fiber reinforced laminate, were investigated experimentally and through finite element analysis. A correction for the significant offset in phase and group velocities due to wave skewing effects is illustrated for both point and short line sources, achieving good agreement with theoretical calculations assuming planar wave fronts. The influence of the line excitation length on velocity measurements is discussed. [ABSTRACT FROM AUTHOR]

A method is investigated to calculate the vertical force on a horizontal plate above still water level under the action of waves in this paper. The wave uplift force is divided into buoyancy force and impact force. Based on the discrete finite element analysis method, the calculation formula of buoyancy force is given and the calculation formula of impact force under the action of regular waves is derived through impulse theory. Then, a formula is proposed to solve the additional mass of liquid impact on the plate. Taking model tests in literature as an example, two different approaches based on the Finite Element Method are presented to verify the calculation method. The comparative analysis shows that the results of the two methods are both consistent with the test results shown in the literature, computation theories of wave uplift force and calculation formula of added mass of impact liquid are verified. On the basis of verifying the calculation theories, the distributions of buoyancy and impact force on the horizontal plate, the time history curves of buoyancy and impact force of the differential element and the relationships between wave uplift forces and wave periods, wave height and plate clearances are discussed. • A method to calculate the vertical force on a horizontal plate above still water level under the action of waves is studied. • Based on the discretization, the calculation formula of buoyancy force is given after correcting the gravity acceleration. • A new formula is proposed to solve the additional mass of liquid impacting the plate. • Two different approaches based on the Finite Element Method are presented to verify calculation method. • The distributions of buoyancy and impact force on the horizontal plate are discussed. [ABSTRACT FROM AUTHOR]

• 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 scope of this study is the monitoring of the adhesion integrity of composite Single Lap Joints (SLJs) through the assessment of the M U L 2 CODE, software developed by the M U L 2 Research Group - Department of Mechanical and Aerospace Engineering of Politecnico di Torino. The M U L 2 CODE is implemented through the Carrera Unified Formulation (CUF) for 2D structures based on Hierarchical Legendre Expansion (HLE) polynomials. An efficient method for the Structural Health Monitoring (SHM) of bonded composite joints is experimentally verified and simulated by commercial FE software and by the CUF approach, in order to reduce the computational cost of analyses: using transient excitations via toneburst signals, the structural health of damaged SLJ is analytically, experimentally and numerically evaluated. The interaction mechanism between the waves traveling through the investigated specimens is numerically simulated with a simple Finite Elements (FE) model and it is solved via M U L 2 CODE and commercial software Ansys Workbench (with 2D and 3D models), respectively. Experimental campaigns data are compared with analytical, CUF and Ansys results demonstrating the consistence of the M U L 2 formulation that is computationally simpler, but very efficient for the joint analysis. The presented and discussed CUF application is able to quantify with a high accuracy the debonding extension in the damaged SLJ, simply tuning the excitation frequency of the SHM technique. • Elastic waves interference is evaluated by using analytical approach, experimental data, CUF and Ansys simulations. • The method is tested on a composite single lap joint, simple experimental set-up representative of an actual scenario. • A good agreement between experiments and simulations confirmed the validity of the proposed method. • The CUF approach provides finer results with lower elapsed computational time respect to the Ansys FE model. [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]

Abstract The paper deals with numerical and experimental investigations aimed to develop a Finite Element (FE) model for predicting wave propagation in a blended composite winglet. Material anisotropy, inhomogeneity, multi-layered configuration and complex geometries tend to increase the complexity of the wave propagation phenomena and consequently the development of established FE models. Moreover, even if 2D finite elements seem to be not appropriate for modelling guided waves propagation, especially for complex anisotropic structural components, they are more attractive than 3D ones, because of the computational cost saving. For this reason, part of the presented research activity is addressed to investigate the efficiency of shell finite elements in modelling guided waves propagation in a such complex structure as a winglet. The development of an efficient model depends also on the numerical characterization of the medium within which guided waves propagate through. As a consequence, preliminary simple experimental bending and modal tests have been carried out to support the material properties modelling. Subsequently, guided wave propagation FE analyses were performed and the results compared with provided experimental data. A good agreement between numerical and experimental results of the different analyses has been achieved in terms of both signal time of flight and amplitudes. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a theoretical, numerical and experimental analysis of Lamb wave propagation and joint-debond identification in an advanced composite structure with core-junction. In the process, the wave modes in the recorded sensor-signals are effectively identified based on the theoretically obtained time-history response and dispersion curves for the sample structure. In order to study the core-junction and joint-debond effects, the finite element based numerical simulations were carried out in ABAQUS and the results verified with laboratory experiments. It was observed that the presence of core-junction in the structure reduces the propagating Lamb wave mode amplitudes, whereas the presence of localized joint-debond significantly increases the modal amplitudes. The study was further extended for the analysis of variable core-junction thickness and joint-debond size effects on the amplitude difference between the baseline and affected Lamb wave signals. Finally, a baseline-free debond detection strategy is proposed for the detection of hidden joint-debond locations in the target structure. Highlights • Lamb wave based analysis of core-core joint and joint-debond effects in an advanced composite structure are presented. • Presence of core-junction in the sandwich composite structures substantially reduces the wave mode amplitudes. • The increase in adhesive bond thickness further decreases the A0 mode amplitudes. • Presence of joint-debond at the core-junction significantly amplifies the propagating Lamb wave modes. • The increase in joint-debond size leads to a further increase in A0 mode amplitude. • A SHM strategy is proposed to effectively locate such joint-debond locations in the advanced composite structure. [ABSTRACT FROM AUTHOR]

Abstract Elastic guided wave based evaluation is useful to detect delamination in composites. This paper reports a detailed account of guided wave interaction with delamination in laminated composites modeled using time domain spectral finite element (TSFE) method. Wave scattering due to different delamination positions is studied. Wave field near the delamination in different layer-wise positions is investigated, which is useful to further characterize and correlate the far field wave packet carrying the parametric signature of delamination. Off-axis delamination in composite laminate creates an asymmetry in the elastodynamic stress transfer. It leads to wave mode conversions. Sensitivity of the delamination to the wavelengths is studied. The outcome of this study indicates potential possibilities to use frequency-wavelength information to discriminate various sizes of delamination. Energy of the scattered waves and dissipation/conversion of the wave energy due to the defects depend on the resonance characteristics of the sub-laminates. Wave scattering effect due to length-wise multiple delaminations and edge delamination is also analyzed. The resonance patterns in the signals are analyzed with reference to the defect quantification problem. [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]

The characterization of the full set of elastic parameters for an orthotropic material is a complex non-linear inversion problem that requires sophisticated optimization algorithms and forward models with thousands of iterations. The intricacy of this type of inversion procedure limits the possibility of using these algorithms for large-scale automation and real-time structural health monitoring. At this point, the introduction of machine learning-based inversion strategies might become helpful to overcome the existing limitations of conventional inversion algorithms. In the present study, a multilayer perceptron algorithm is used to identify elastic stiffness parameters of orthotropic plates using guided wavefield data. A large and diverse training dataset is created by using a semi-analytical finite element model, and the effect of both the training dataset size and the signal-to-noise ratio on the inference outcome are examined. The performance of the multilayer perceptron-based inversion method is first validated on a numerical dataset, and the method is then further applied on experimental data obtained from a multilayered glass-fiber reinforced polyamide 6 composite plate. Finally, the multilayer perceptron-based inference results are compared with the outcome of a traditional inversion algorithm, showing a difference of less than 0.5%. • A multi-layer perception method is used to determine the elastic orthotropic parameters of materials. • SAFE is embedded as a forward model to create training dataset. • COMSOL simulations of ultrasonic wave propagation are conducted to generate virtual data and to verify the inversion. • 3D IR-SLDV is employed to measure full field guided wave data and to validate the inversion for wood and GFRP. [ABSTRACT FROM AUTHOR]

This paper presents a delamination damage detection technique based on the nonlinear response of Lamb waves in composite laminates. In the approach, a network of transducers is employed to sequentially scan the structure by actuating and receiving dual guided waves. Using combined frequency waves originated due to contact nonlinearity when the incident dual frequency wave interacts with the damage, a damage localization imaging algorithm is proposed to locate the damage and determine the damage location. The proposed damage localization imaging algorithm is based on analysing the cross-correlation of the signal envelope at combined frequency wave with the excitation pulse envelope at incident waves for each transducer pair, the damage localization image is constructed. A numerical study using an experimentally validated finite element model is used to verify the proposed damage detection technique. The results of the numerical study show that, despite the small amplitude of the nonlinear components, the proposed method can effectively predict the damage location, and it does not require baseline measurements, which makes it potential to be a baseline-free damage detection technique. [ABSTRACT FROM AUTHOR]

In this work, two numerical procedures based on Finite Elements Method (FEM) have been developed in order to simulate the Lamb wave propagation in Low Velocity Impact (LVI) damaged CFRP (Carbon Fibre Reinforced Polymer) laminate. The former (softening representation), usually adopted in literature, consists of modelling LVI damages by lowering the elastic material properties which allowed investigating the Lamb wave propagation at different stages of LVI damages evolution. The latter, proposed in this paper, conversely to the first one and the most of techniques presented in literature, consists of simulating Lamb wave propagation in a plate characterized by an initial stress-strain state and the related failures carried out by a previous impact simulation involving the same model. Such technique allows a better damage modelling and, consequently, overcoming the damage modelling approximations introduced by the former strategy; the lowering of the elastic material properties leads to a bad damage modelling which does not allow reproducing accurately what happens in the reality. Such procedure allowed investigating the Lamb wave propagation at different impact energy levels. The interaction between Lamb waves and damages has been investigated under three central frequencies of the actuation signal: 150 kHz, 200 kHz and 250 kHz which resulted in interesting observations to minimize the effect of the first lamina's fibres orientation on the wave propagation velocity. It is well known that different wave propagation velocities along fibres and matrix lead to different RMSD (Root Mean Square Deviation) damage index values, even if the sensors are mounted at the same distance from the damage location, resulting in wrong or less accurate information about the identification of both damage size and location during the post-processing phase. Moreover, the relationships between the RMSD damage index values, recorded at different instants of time of the impact history, and the impactor phases has been achieved. Finally a comparison between the results achieved by the two investigated strategies has been carried out and presented here. [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]

A semi-analytical technique for estimating the energy loss in a resonator mounted to an infinite plate substrate is proposed in this paper. In a plate, only Lamb waves have to be considered, leading to a simplified characterization of the energy carried away from a vibrating source on the plate surface. Instead of employing absorbing elements at the boundaries of the plate-resonator finite element model, it is shown how the semi-analytical approach of stitching together analytical Lamb wave expressions to the finite element model can be utilized. The approach is demonstrated for single and double cantilever configurations on a plate. The results have excellent agreement with those of conventional transient finite element simulations. [ABSTRACT FROM AUTHOR]

• Efficient and accurate simulation of PZT-based Lamb wave health monitoring using three-dimensional SBFEM. • Utilizing SBFEM to model the electromechanical vibration of low-profile PZTs based on equivalent force concept. • Simple discretization of complex configurations by linking the SBFEM meshing module to standard finite element mesh generators. • Reduced computational cost for simulation of structural health monitoring techniques based on high-frequency transient wave motion. Exploiting scattering and reflection related data of ultrasonic Lamb wave interactions with damage is a common approach to health monitoring of thin-walled structures. Using thin PZT sensors, the method can be implemented in real-time. Simulation of Lamb wave propagation and its interaction with damage plays an important role in damage diagnosis and prognosis. It is, however, a time-consuming task due to the high-frequency waves that are commonly used to detect tiny damage. The current study employs the Scaled Boundary Finite Element Method (SBFEM) for effective modeling of Lamb wave health monitoring of homogenous thin plates. The electromechanical effects of piezoelectric sensors are included in the model to improve accuracy and make the results comparable to those of laboratory experiments. Simple meshing of complex topologies is possible by converting standard finite elements to scaled boundary elements. The 3D SBFEM wave motion equations are solved in the time domain to capture the sensor's PZT response to a high-frequency tone-burst actuation. The results are validated by pitch-catch and pulse-echo laboratory tests carried out on thin plates. SBFEM is used to study wave propagation in complex configurations, such as a stiffened plate, and the results are compared to their FEM counterparts. According to the findings, SBFEM significantly reduces the computational costs associated with simulation of Lamb wave health monitoring while also providing significant accuracy in comparison to the experimental results. [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]

An accurate description of surface pressure loads imposed by blast waves is crucial for the design of the next generation of blast-resistant structures. However, experimental techniques for non-intrusive, full-field surface pressure measurements are not readily available. To address this challenge, a new application of the Virtual Fields Method (VFM) has been explored to reconstruct the surface pressures acting on thin steel plates using full-field deformation measurements of the plate dynamics. A shock tube facility was used to generate a blast-like loading in controlled, laboratory environments, where the plate dynamics were measured using the deflectometry technique. Different blockages at the shock tube exit allowed for varying the spatial distribution and temporal history of the blast loading. The surface pressures were reconstructed from experimentally measured kinematic fields using the VFM. A nearly non-deformable plate equipped with point-wise pressure sensors was also used to obtain a reference, allowing to assess the performance and reliability of the proposed methodology in capturing the surface pressure distributions. Moreover, visualizations of the different blast wave impacts on the plate were obtained using a background-oriented schlieren setup. Finally, the influence of potential error sources was investigated by means of a virtual laboratory using a finite element analysis to generate synthetic input to the load reconstruction analysis. The proposed methodology provided robust, precise predictions using noise-free input from virtual experiments. The presence of both systematic and random errors during the experimental campaign resulted in a reduced pressure reconstruction accuracy, where the peak pressure amplitudes were approximately 10–20% lower compared to the pointwise transducer data. Pressure reconstructions from experimental data still showed qualitatively good estimates of the pressure distributions that were extrapolated from transducer data. Hence, this work highlights the capabilities of a promising methodology to obtain more insight into the effective action of the loading during blast–structure interaction of plated structures. • Surface pressure on thin steel plates during blast–structure interaction using VFM. • Optical full-field measurements for non-intrusive deformation assessment. • Full-field, non-uniform surface pressure distribution based on load reconstruction. • Load reconstruction based on elastic, small deformations, and thin plate theory. • Virtual experiments for evaluating the load reconstruction methodology performance. [ABSTRACT FROM AUTHOR]

We study numerically and experimentally acoustoelastic wave propagation in a two-dimensional phononic metaplate consisting of a periodic array of cups sitting on a thin epoxy plate that is perforated with cross holes. When all cups are filled with water, the metaplate possesses a complete band gap. Reconfigurable coupled-resonator acoustoelastic waveguides (CRAEWs) are created by locally emptying certain cups, thus introducing local resonances that are evanescently coupled. Straight and 90° bent periodic waveguides are considered, together with an aperiodic chain of 11 coupled resonators. The aperiodic chain has no definite spatial periodicity but supports collective resonances resulting from the coupling of nearest resonators. Lamb waves are experimentally excited by a piezoelectric patch and received by a scanning optical vibrometer. Experimental results for acoustoelastic wave propagation along both periodic and aperiodic CRAEWs are compared to a three-dimensional finite element model taking fluid–structure interaction into account. The propagation of confined acoustoelastic waves in the 90° bent waveguides and the collective resonances of the aperiodic chain of defected resonators are observed experimentally. Reconfigurability are realized based on the coupling of acoustoelastic waves in a phononic metaplate. Our results show plenty of potential possibilities for the practical design of reconfigurable and programmable elastic wave devices. [ABSTRACT FROM AUTHOR]

Lamb wave propagating behavior in woven glass fiber reinforced epoxy (WGF/epoxy) composite laminates was studied by simulation and experiment. A 3D finite element (FE) model was built in Abaqus/Explicit software with the help of user material subroutine to capture the wave propagating features in WGF/epoxy composite plate. The Lamb wave tests were carried out by a non-circular array with 8 piezoelectric (PZT) wafers. A defect imaging algorithm was developed using MATLAB based on the ellipse theory in time-dependent domains. Two defect imaging cases were carried out in the WGF/epoxy composite plate to verify the feasibility of the proposed algorithm. A good agreement between the simulated waveforms and experimental data was achieved. This indicates a great potential of the proposed algorithm in the field of damage localization and identification for WGF/epoxy thin-walled laminates. [ABSTRACT FROM AUTHOR]

Arterial stiffness is highly correlated with the functions of the artery and may serve as an important diagnostic criterion for some cardiovascular diseases. To date, it remains a challenge to quantitatively assess local arterial stiffness in a non-invasive manner. To address this challenge, we investigated the possibility of determining arterial stiffness using the guided circumferential wave (GCW) induced in the arterial wall by a focused acoustic radiation force. The theoretical model for the dispersion analysis of the GCW is presented, and a finite element model has been established to calculate the dispersion curve. Our results show that under described conditions, the dispersion relations of the GCW are basically independent of the curvature of the arterial wall and can be well-described using the Lamb wave (LW) model. Based on this conclusion, an inverse method is proposed to characterize the elastic modulus of artery. Both numerical experiments and phantom experiments had been performed to validate the proposed method. We show that our method can be applied to the cases in which the artery has local stenosis and/or the geometry of the artery cross-section is irregular; therefore, this method holds great potential for clinical use. [ABSTRACT FROM AUTHOR]

In this study, a Lamb wave based nonlinear method is used to detect delamination in a composite laminate. For this purpose, an artificial delamination is created in a composite laminate using a thin Teflon sheet. The research includes an experimental study as well as finite element simulation. Two higher harmonics are observed in the Lamb wave response as a result of contact nonlinearity at the delamination. A spectral damage index is extracted from the spectral information of the fundamental and higher harmonics. It is found to be invariant to the sensor location and its values show decreasing trend with increase in the delamination width. Further, a new hybrid method is introduced, wherein the spectral and temporal data are used together in order to locate a delamination. [ABSTRACT FROM AUTHOR]

In engineering practice, failures due to fatigue cracks in metallic structures have always been difficult to predict. In our study, nonlinear Lamb wave-mixing was applied in the detection of micro-cracks in plates. The analysis of the nonlinear interaction of these waves with cracks of various lengths and widths was performed using finite-element simulations. The simulation results showed that the sideband at the sum frequency provide a sensitive means for micro-crack detection. Moreover, the sideband amplitudes show a monotonic increase with micro-crack length, but a decrease with micro-crack width. Experiments using Lamb wave-mixing were conducted on plates with fatigue cracks. The experimental results show that a proposed acoustic nonlinearity parameter related to the sideband at the sum frequency is sensitive to micro-cracks in plate, and is well correlated with the damage degree. [ABSTRACT FROM AUTHOR]

The homogenization of material parameters layer by layer is a mostly sufficient method for the simulation of Lamb wave propagation in carbon fiber reinforced plastics. But since current experimental investigations of the wave behavior in undamaged CFRP structures reveal an effect called “quasi-continuous mode conversion”, which can not be reproduced with this layer-wise homogenization procedure, an improved material model for the finite element analysis of the Lamb wave propagation is presented in this work. The used 2D plane strain state model considers the random distribution of fibers in the matrix material on the microscale by homogenized subdomains and thereby properly capture the “quasi-continuous mode conversion” effect. Numerical investigations of the influence of the material model on the developing wave modes in a unidirectional layer will complete the work. [ABSTRACT FROM AUTHOR]

Lamb waves have been widely explored as a promising inspection tool for non-destructive evaluation (NDE) and structural health monitoring (SHM). This article presents a combined analytical finite element model (FEM) approach (CAFA) for the accurate, efficient, and versatile simulation of 2-D Lamb wave propagation and interaction with damage. CAFA used a global analytical solution to model wave generation, propagation, scattering, mode conversion, and detection, while the wave-damage interaction coefficients (WDICs) were extracted from harmonic analysis of local FEM with non-reflective boundaries (NRB). The analytical procedure was coded using MATLAB, and a predictive simulation tool called WaveFormRevealer 2-D was developed. The methodology of obtaining WDICs from local FEM was presented. Case studies were carried out for Lamb wave propagation in a pristine plate and a damaged plate. CAFA predictions compared well with full scale multi-physics FEM simulations and experiments with scanning laser Doppler vibrometry (SLDV), while achieving remarkable performance in computational efficiency and computer resource saving compared with conventional FEM. [ABSTRACT FROM AUTHOR]

In this paper, the propagation of evanescent Lamb waves in the one-dimensional viscoelastic phononic metastrip is studied. Complex band structures and transmission spectra are calculated by using the finite element method. The effect of viscosity is included according to the Kelvin–Voigt model. Two types (namely H-type and I-type) of metastrips are fabricated in either steel or epoxy. A theoretical model is developed to predict the distribution of the displacements of evanescent waves in the finite metastrip. The effect of different cutting forms on the complex band structure is also investigated. It is found that the spatial attenuation of evanescent waves is clearly observed in both simulation and measurement. Numerical and experimental results agree well for steel metastrips when only the elastic stiffness matrix is considered, whereas good agreement for epoxy metastrips is achieved when viscoelasticity is taken into account. The displacement distribution of evanescent waves can be accurately predicted from the two least evanescent waves identified in the complex band structure. Different slicing forms for the metastrip result in the reconstruction of evanescent waves, leading to the opening or closing of bandgaps. The present work lays the numerical and experimental foundation for practical application of phononic metastrips. [Display omitted] • Complex band structure for evanescent Lamb waves is calculated by using finite element method. • Numerical and experimental distributions of evanescent waves agree well and can be accurately predicted. • Evanescent waves are reconstructed for different cutting forms, leading to the opening or closing of bandgaps. [ABSTRACT FROM AUTHOR]

• The displacement field of the 2D theory is designed according to the wave structure. • A higher accuracy in high frequency simulation can be achieved compared with FSDT. • The modeling error of A0 mode can be controlled within 1.43 % up to 2500 kHz·2 mm. • Combined with FEM, the method can simulate the 2D wavefield of A0 and A1 accurately. • More computational resources can be saved compared with the 3D FEM. For the modeling of Lamb wave propagation, the accuracy of some two-dimensional (2D) plate theories like the first order shear deformation theory (FSDT) may decline at a high frequency, since their displacement fields cannot match those complicated Lamb wave structures. To address this issue, this paper proposes an improved 2D thin plate theory which has a more accurate high frequency modeling ability of Lamb wave than the FSDT. This is done by designing the displacement field according to the high frequency Lamb wave structure. Since the shear stress curves of the Lamb wave are complicated and would vary with the frequency, the initial displacement field takes a combination of polynomial and trigonometric forms, which has two coefficients that need to be determined by fitting the shear stress curve at a certain frequency. Then, a strategy is applied to determine the eventual displacement field by searching the high frequency range for the optimum coefficients that enable a best modeling accuracy. The relative error of its modeling result of A0 mode can be controlled within 0.03 % below 800 kHz·2 mm, and keeps the highest error of 1.43 % up to 2500 kHz·2 mm. To verify its high frequency modeling performance, a finite element method program is finally built. Results show that both the wavefields of A0 and A1 can be accurately simulated with the improved two-dimensional thin plate theory. [ABSTRACT FROM AUTHOR]

The use of guided waves to identify damage has become a popular method due to its robustness and fast execution, as well as the advantage of being able to inspect large areas and detect minor structural defects. When a travelling wave on a plate interacts with a defect, it generates a scattered field that will depend on the defects geometry. By analysing the scattered field, one can thus characterize the type and size of the plate damage. A Bayesian framework based on a guided waves interaction model for damage identification of infinite plate for the first time is presented here. A semi-analytical approach based on the lowest order plate theories is adopted to obtain the scattering features for damage geometries with circular symmetry, resulting in an efficient inversion procedure. Subsequently, ultrasound experiments are performed on a large aluminium plate with a circular indentation to generate wave reflection and transmission coefficients. With the aid of signal processing techniques, the effectiveness and efficiency of the proposed approach are verified. A full finite element model is used to test the damage identification scheme. Finally, the scattering coefficients are reconstructed, reliably matching the experimental results. The framework supports digital twin technology of structural health monitoring. • A fast physics-informed damage identification Bayesian framework in 2D structures. • A semi-analytical approach for modelling wave damage interaction is presented. • Physical ultrasound experiments are performed to extract scattering coefficients. • Numerical, experimental case studies are conducted to verify the proposed framework. • The computational efficiency of the proposed framework can be greatly improved. [ABSTRACT FROM AUTHOR]

In this paper, a new online uniaxial stress monitoring methodology has been developed. Based on the acoustoelastic effect of Lamb waves, multimodal information at a single frequency, in which the approximate linear fitting between group velocity ratio of two modes and stress is obtained, is fused to characterize stress. The transceiver distance can be eliminated, which is essential knowledge for existing methods, in stress coefficient calibration and practical application by using the ratio of acoustic time. The semi-analytical finite element method(SAFE) is adopted to calibrate the stress coefficient. Next, the time–energy assignment algorithm derived from the smoothed pseudo-Wigner–Ville distribution(SPWVD) is evaluated on the ideal dataset obtained by time-domain finite element(TDFE) simulation. The arrival times of different modes are extracted accurately, and the fitting results are consistent with the theoretical analysis. Finally, the laboratory experiments employing a pair of piezoelectric wafer active sensors(PWAS) are conducted to calibrate unknown coefficients. The same calibration process is carried out at three different distances, and almost the same calibration results are obtained, indicating the distance-dependent nature of the stress coefficients. According to the completed calibration, the actual measurements from the same batch show the maximum absolute error of stress measurement is 4.2MPa within the applied stress range from 20MPa to 100MPa and the standard deviation is less than 3MPa, demonstrating good measurement accuracy and consistency. It shows the great potential of the proposed strategy to measure stress at unknown propagation distances. • A new online stress monitoring based on dual mode Lamb wave is proposed. • This method can improve the measuring ability without reducing the measuring sensitivity. • The method adopted Wigner–Ville time–frequency distribution to obtain the group delay of dispersive Lamb wave signal. [ABSTRACT FROM AUTHOR]

Lamb wave sensors performing well in liquid medium are good choices for monitoring viscosity. Despite the advances in the biosensing and chemical application of Lamb wave viscosity sensors, the analysis of the dependence of the sensors on liquid morphology is still needed. Therefore, a Lamb wave viscosity sensor has been fabricated. A novel two-dimensional (2-D) array model for viscosity sensing is put forward based on the finite element analysis (FEA) method. The dependence of the resonant frequency of the sensor on liquid morphology has been simulated and experimentally tested. By comparing simulations and experiments, the frequency responses are dependent on the liquid loading position, thickness and coverage ratio on the Lamb wave sensor. In addition, the frequency response of the sensor to viscosity has been conducted using glycerol-water mixtures on the basis of controlled liquid morphology, showing a linear relationship with the square of viscosity, with 238.92 ppm/(mPa s)0.5 in simulation and 153.16 ppm/(mPa s)0.5 in the experiment. Our work potentially allows for reducing the random error of the experiments and for more exploration of Lamb wave viscosity sensors in the future study. [Display omitted] • A Lamb wave viscosity sensor with a higher-order mode has been developed. • A novel 2-D array model for viscosity sensing is proposed to describe the behavior of the sensor in liquid medium. • The dependence of sensors on liquid morphology has been studied from simulations and experiments. • The sensor exhibits a strong relationship with the liquid morphology, including the position, thickness and coverage ratio. • This work allows for reducing the random error of the experiments and for more exploration of Lamb wave viscosity sensors. [ABSTRACT FROM AUTHOR]

An improved understanding of the wave propagation in incompressible soft structures with a pre-deformation, especially the grasp of viscoelastic effect, could fundamentally catalyze technical developments in many areas including medicine, and rubber. Based on the nonlinear elasticity and linear incremental theories, a fractional-order hyper-viscoelastic model is established to derive dynamic equations of Lamb waves in functionally graded pre-deformed viscoelastic soft plates. The fractional-order Kelvin-Voigt model is applied to concern the viscoelasticity. An improved Legendre polynomial method is employed to analytically solve wave equations. The complex wave solutions are obtained without the iterative. The correctness of the present method is validated by a comparison with the finite element method. The influence of viscoelasticity and pre-formation on dispersion and attenuation curves is investigated. Some new wave phenomena are revealed: Phase velocity and attenuation of Lamb waves can be controlled by altering stretch ratios λ i , and the pre-stretching restrains the viscoelastic effect; The minimum attenuation frequency can be adjusted by altering the material component distributions. Results provide the foundation for the design of biomaterials with the controllable viscoelasticity to greatly promote their applications in the regenerative medicine. [ABSTRACT FROM AUTHOR]

• A semi-analytic method to explain the interaction between lamb waves and bolt joint. • The power transmission coefficients change periodically with contact area. • The first S0 wave packet in the received signal is used to monitor bolt torque. • The semi-analytic method is helpful for frequency selection in bolt monitoring. Bolt joints are extensively used in aerospace and civil engineering. Online monitoring of bolt looseness is crucial to ensure the function and safety of these structures. Understanding the interaction between guided waves and the joint interfaces can aid in the design and optimization of such monitoring systems, and hence becomes important. However, the influence of contact area on guided wave energy passing through these jointed plates is still not clear. In this paper, a semi-analytic method, which combines finite element models with a wave superposition method, is proposed to predict the transmission of guided waves in joint interfaces. The wave mode conversions in the contact area are included. The model was validated by comparison with independent finite element models. In addition, the influence of the contact length on transmission coefficients of guided waves passing through the joint interface is investigated. The results were experimentally validated by wire electrical discharge machined specimens with various contact areas. Then, the transmitted guided wave and the contact area in a bolt jointed plate under different torques were measured experimentally and compared with the present method. The results of the two different experiments agree well with those predicted using the semi-analytic method, respectively. In particular, the experimental results of the bolt jointed plate show that the direct S0 wave packet energy decreases with the bolt preload, which can be explained by the proposed model. Thus, the proposed semi-analytic method demonstrates efficiency in the prediction of the power transmission coefficient and has potential applications in the selection of excitation frequencies in the online monitoring of jointed structures. On this basis, a normalized power transmission coefficient can be directly calculated by the energy of the first S0 wave packet in the received signal to monitor bolt torque. [ABSTRACT FROM AUTHOR]

• Investigating an innovative approach to determine the damping property of a viscoelastic coating using ultrasonic Lamb wave in a metal-polymer waveguide. • Dropping amplitude signals as well as changing the phase of Lamb wave when travels through the coated waveguide. • Exploiting the obtained damping property in FEM software, ABAQUS. • Reducing computational cost and time by taking advantage of the experiment-based fitting method. • Verification of amplitude ratio as well as group velocity dispersion curves related to the FEM simulation by experimental results with an error of less than 1%. • Confirming the excitability of piezo discs in a range of frequency with tangible precision. This work presents an innovative approach according to an experiment-based fitting method to determine the damping property of a viscoelastic coating layer, in a simple, low-cost, and time-effective manner. In this experiment, symmetric and asymmetric ultrasonic Lamb waves were applied to two coated plates with different thicknesses, and the waves were generated using piezo discs. A viscoelastic coating influences the signal amplitude as well as the wave phase. By comparing the amplitude ratio (AR) of the transmitting signals between the coated and bare plates, the damping property of the viscoelastic coating was experimentally determined. Similar to the experiments employing the finite element method (FEM) software, in this experiment, ABAQUS, was employed to verify the conformity between numerical and experimental AR. By selecting a non-dispersive Rayleigh damping β for the coating layers at all frequencies, the computational cost reduced significantly to one-tenth the original cost. Apart from corroboration by AR matching, the numerical dispersion curves of the group velocity were also validated by experimental curves. The FEM dispersion curves in the frequency range of the tests were found to be highly reliable, with an average error of less than 1% for the first experimental setup and 10% for the second setup. Furthermore, in coated waveguides, the proposed technique could precisely estimate the damping property of the viscoelastic coating layers, where excitability in a wide range of frequencies is required. However, this precision strongly relies on the selected mode, frequency range, PZT quality, and waveguide thickness. [ABSTRACT FROM AUTHOR]

Electro-acoustic sensors proving an adequate operation in liquid media are appropriate candidates for biosensing or liquid monitoring applications. In this context, electro-acoustic devices based on Lamb waves have been widely used for such purpose during the last years. More particularly, S 0 mode Lamb wave resonators (S 0 -LWRs) have shown promising in-liquid performance. However, a theoretical background describing their in-liquid sensing mechanisms has only been published recently. In this work we present the experimental verification of the previously developed theoretical background based on a finite element model. We discuss about similarities and discrepancies between model and experiments, stating a final model correctly describing the in-liquid sensitivity features of S 0 -LWRs. These devices show appreciable different sensitivities of the resonant frequency to liquid viscosity and density, being more sensitive to the latter. Additionally, when they are not electrically isolated, the influence of the liquid electrical properties is superimposed to the mechanical ones and can be correctly extracted. [ABSTRACT FROM AUTHOR]

Antisymmetric and symmetric Lamb-type feature guided waves (FGW) have recently been shown to exist in small angle plate bends. This paper reports Semi-Analytical Finite Element (SAFE) method simulations revealing the existence of a new family of Shear Horizontal ( SH B ) type of FGW mode in 90° bends in plate structures. Mode shapes and velocity dispersion curves are extracted, demonstrating the SH -like nature of a bend-confined mode identified in studies of power flow across the bend. The SH B mode is shown to have reduced attenuation in the higher frequency range, making it an ideal choice for high-resolution inspection of such bends. Further modal studies examine the physical basis for mode confinement, and argue that this is strongly related to FGW phenomena reported earlier, and also linked to the curvature at the bend region. Wedge acoustic waves discussed widely in literature are shown as arising from surface-limiting of the SH B mode at higher frequencies. The results are validated by experiments and supported by 3D Finite Element (FE) simulations. [ABSTRACT FROM AUTHOR]

Air-coupled ultrasonic inspection using leaky Lamb waves offers attractive possibilities for non-contact testing of plate materials and structures. A common method uses an air-coupled pitch–catch configuration, which comprises a transmitter and a receiver positioned at oblique angles to a thin plate. It is well known that the angle of incidence of the ultrasonic bulk wave in the air can be used to preferentially generate specific Lamb wave modes in the plate in a non-contact manner, depending on the plate dimensions and material properties. Multiple reflections of the ultrasonic waves in the air gap between the transmitter and the plate can produce additional delayed waves entering the plate at angles of incidence that are different to those of the original bulk wave source. Similarly, multiple reflections of the leaky Lamb waves in the air gap between the plate and an inclined receiver may then have different angles of incidence and propagation delays when arriving at the receiver and hence the signal analysis may become complex, potentially leading to confusion in the identification of the wave modes. To obtain a better understanding of the generation, propagation and detection of leaky Lamb waves and the effects of reflected waves within the air gaps, a multiphysics model using finite element methods was established. This model facilitated the visualisation of the propagation of the reflected waves between the transducers and the plate, the subsequent generation of additional Lamb wave signals within the plate itself, their leakage into the adjacent air, and the reflections of the leaky waves in the air gap between the plate and receiver. Multiple simulations were performed to evaluate the propagation and reflection of signals produced at different transducer incidence angles. Experimental measurements in air were in good agreement with simulation, which verified that the multiphysics model can provide a convenient and accurate way to interpret the signals in air-coupled ultrasonic inspection using leaky Lamb waves. [ABSTRACT FROM AUTHOR]

In this paper, we present the theoretical investigation of Lamb wave propagation in one-dimensional radial phononic crystal (RPC) plates with periodic double-sided corrugations. The dispersion relations, the power transmission spectra, and the displacement fields of the eigenmodes are studied by using the finite element method based on two-dimensional axial symmetry models in cylindrical coordinates. Numerical results show that the proposed RPC plates with periodic double-sided corrugations can yield several band gaps with a variable bandwidth for Lamb waves. The formation mechanism of band gaps in the double-sided RPC plates is attributed to the coupling between the Lamb modes and the in-phase and out-phases resonant eigenmodes of the double-sided corrugations. We investigate the evolution of band gaps in the double-sided RPC plates with the corrugation heights on both sides arranged from an asymmetrical distribution to a symmetrical distribution gradually. Significantly, with the introduction of symmetric double-sided corrugations, the antisymmetric Lamb mode is suppressed by the in-phase resonant eigenmodes of the double-sided corrugations, resulting in the disappearance of the lowest band gap. Furthermore, the effects of the geometrical parameters on the band gaps are further explored numerically. [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]

• Sinusoidally-shaped phononic beam structures are proposed. • A multi-parameter designing technique is proposed for metasurfaces. • Mass of functional units does not varies with the change of design parameters. In this work, we propose a method to design subwavelength metasurfaces for flexural waves in thin plates based on sinusoidally-shaped phononic beam structures. In the proposed method, the effective bending stiffness of sinusoidally-shaped phononic beam structures is tuned, and the effective phase velocity of the flexural waves is modulated subsequently by adjusting two key geometry parameters, namely, the amplitude of the sinusoidal profiles and lattice constants of the phononic beam structures. Compared with the previous methods, which modulate a single design parameter, the proposed method is a two-parameter modulating method. Thus, it not only maintains the functionality of metasurfaces, but also is much more flexible when obtaining a high wave transmittance. In addition, the mass of functional units of metasurfaces designed by the proposed method is not relevant to the design parameters, which is particularly important in weight-critical applications. To validate the feasibility of the proposed design method, the transmission spectra of sinusoidally-shaped phononic beam structures are modeled using the finite element method, and results are compared with experiments of 3D-printed samples. In addition, two representative metasurfaces, refractive and focusing metasurfaces, are designed with an available minimum transmission coefficient of 0.93 for functional units. Numerical evaluation of these metasurfaces' performances confirms the effectiveness of the proposed method. This work paves a novel way to design elastic metasurfaces to manipulate flexural waves in thin plates and demonstrates a potential new direction to design metasurfaces for other wave types. [ABSTRACT FROM AUTHOR]

This work explores the generation of Lamb waves and their higher harmonics using a pattern source. An array of laser sources, when used with a short-pulsed laser, generate multiple Lamb modes in a plate-like component. The variation in duty ratio of the pattern source is found to influence the generation of fundamental modes and their harmonics. A two-dimensional finite element model is developed to simulate the narrowband Lamb wave generation process, and validated against targeted experiments. For a 'square-like' spatial profile of energy distribution, the second harmonic of the fundamental wave modes is fully suppressed at the duty ratio of 0.5, and the third harmonic of the fundamental wave modes is fully suppressed at duty ratios of 1/3 and 2/3. As the duty ratio increased from 0.2 to 0.8, the variation in amplitude of the fundamental modes is greater than that of the higher order modes. For duty ratio values less than 0.25 and greater than 0.75, the second harmonic component of the fundamental waves had approximately the same amplitude as the fundamental wave modes. Similarly, the third harmonic of the fundamental wave modes had the same amplitude or slightly higher amplitude than the higher order Lamb modes for duty ratio values less than 0.25 and greater than 0.75. [ABSTRACT FROM AUTHOR]

Defects that propagate from the inside of a structure can be difficult to detect by traditional non-destructive inspection methods. A non-contact inspection method is presented here that uses the near-field interactions of ultrasonic Lamb waves to detect defects propagating into a 1.5 mm thick aluminium sheet from the opposite side to that which is inspected. Near-field interactions of the S0 Lamb waves with the defects are shown to give rise to a characteristic increase in the wave magnitude, which is used to position and characterise these hidden defects. It is shown that such defects are difficult to detect from a study of their influence on ultrasonic transmission alone. Single defects of different depths, and systems of multiple defects with varying separations and relative depths, are successfully detected in both experimental trials and FEM simulations. Reliable single defect detection is achieved for defects with a minimum depth of 30 % of the plate thickness, and resolution of multiple defects is achieved for defect separations of 5 mm. [ABSTRACT FROM AUTHOR]

Influence of attenuation on acoustic emission (AE) signals in Carbon Fiber Reinforced Polymer (CFRP) crossply and quasi-isotropic panels is examined in this paper. Attenuation coefficients of the fundamental antisymmetric (A 0 ) and symmetric (S 0 ) wave modes were determined experimentally along different directions for the two types of CFRP panels. In the frequency range from 100 kHz to 500 kHz, the A 0 mode undergoes significantly greater changes due to material related attenuation compared to the S 0 mode. Moderate to strong changes in the attenuation levels were noted with propagation directions. Such mode and frequency dependent attenuation introduces major changes in the characteristics of AE signals depending on the position of the AE sensor relative to the source. Results from finite element simulations of a microscopic damage event in the composite laminates are used to illustrate attenuation related changes in modal and frequency components of AE signals. [ABSTRACT FROM AUTHOR]