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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]

In this paper, we design a tunable phase-modulated metasurface composed of periodically distributed piezoelectric patches with resonant-type shunt circuits. The electroelastic metasurface can control the wavefront of the lowest antisymmetric mode Lamb wave (A 0 mode) in a small footprint due to its subwavelength features. The fully coupled electromechanical model is established to study the transmission characteristics of the metasurface unit and validated through numerical and experimental studies. Based on the analysis of the metasurface unit, we first explore the performance of electroelastic metasurface with single-resonant shunts and then extend its capability with multi-resonant shunts. By only tuning the electric loads in the shunt circuits, we utilize the proposed metasurface to accomplish wave deflection and wave focusing of A 0 mode Lamb waves at different angles and focal points, respectively. Numerical simulations show that the metasurface with single-resonant shunts can deflect the wavefront of 5 kHz and 6 kHz flexural waves by desired angles with less than 2 % deviation. In addition, it can be tuned to achieve nearly three times displacement amplification at the designed focal point for a wide range of angles from − 75 ∘ to 75∘. Furthermore, with multi-resonant shunts, the piezoelectric-based metasurface can accomplish anomalous wave control over flexural waves at multiple frequencies (i.e. simultaneously at 5 kHz and 10 kHz), developing new potentials toward a broad range of engineering applications such as demultiplexing various frequency components or guiding and focusing them at different positions. [ABSTRACT FROM AUTHOR]

Fish swim by flexing their bodies and energy is transferred from the fish to the water. And the plate which imitates traveling wave motion of fish can absorb the energy of fluid under certain motion parameters. By arranging the traveling wave plate in a vertical rectangular flow channel, the flow mode is changed from external flow mode to internal flow mode. This internal flow device is called traveling wave pump-turbine, and it can operate in both pumping mode and turbine mode. The traveling wave plate is simplified into a two-dimensional plate, and the model of the traveling wave pump-turbine is established. The numerical simulation method is used to study its performance characteristics at different wave speeds when it is in pumping mode. The results show that: although it operates in pumping mode by changing the volume of the working chamber, it can output stable mass flow rate. With the increase of wave speed, the volumetric efficiency increases gradually, the time-averaged efficiency first increases and then decreases, and the maximum time-averaged efficiency can reach 0.9. The instantaneous pressure and power are relatively stable when the wave speed is low, while they fluctuate more sharply with the increase of wave speed. [ABSTRACT FROM AUTHOR]

In this paper, we design and fabricate dual-tunable waveguides in a two-dimensional periodic plate with threaded holes. Dual tunability is realized by using rods held with nuts as well as assembly prestress of the nuts. A straight waveguide, a bent waveguide, and a wave splitter are designed by changing the distribution of rods and nuts in different circuits. The experimental and numerical results show that the frequencies of guided waves can be tuned by the assembly prestress. By increasing the amount of prestress, the frequency range of the passing band can be shifted upward. Confinements, guiding, and splitting of Lamb waves are clearly observed in both experimental measurements and numerical simulations. This work is essential for the practical design of reconfigurable phononic devices. [ABSTRACT FROM AUTHOR]

Pumped storage represents an economically viable, highly efficient, and extensive energy storage solution, receiving substantial research attention worldwide. This study utilized the energy dissipation of fish swimming in the water, and the traveling wave plate was arranged in a vertical rectangular flow channel to obtain energy through reverse thinking. Numerical simulation was employed to investigate the performance characteristics of the traveling wave turbine under varying wave number across high and low water head. The results reveal that the efficiency at low water head starts to increase and then decreases with the increase of wave number. There is an optimal wave number N = 4 to maximize turbine efficiency, and the maximum efficiency under low water head condition is 89.35 %. The efficiency of the high head condition shows a continuous upward trend, with a maximum value of 92.27 % in the range of wave number studied. With the increase of wave number, the instantaneous mass flow coefficient and power coefficient become more stable. The advantage of increasing the wave number is that more traveling wave plates can participate in energy capture, which is conducive to reducing the force required by each traveling wave plate and making the pressure distribution in each working cavity more regular. [ABSTRACT FROM AUTHOR]

Lamb waves are often used for damage detection in structures. Hot-spot monitoring via wave focusing is considered in this study, which can be achieved by using a plano-concave aspherical lens. Once attached to the plate, the lens modifies the effective plate thickness, and therefore changes the Lamb wave characteristics, such as wavenumber and phase velocity, providing a convenient way of controlling Lamb waves. Another possibility for focusing Lamb waves is to use multiple actuators and design dispersion pre-compensated excitation signals with an embedded time delay. These two approaches can be combined together. We conducted numerical simulations using the time-domain spectral element method based on the 3D elasticity theory and demonstrated that the A0 mode of Lamb waves could be focused on the desired focal point. We compared the efficacies of the plano-concave lens approach, the dispersion pre-compensated focusing approach, and a combination of both approaches, emphasizing the advantages and disadvantages of each method. [ABSTRACT FROM AUTHOR]

A non-periodic multilayer film was analyzed to show that, despite its non-periodicity, the film exhibits full band-gaps and localized modes at its interfaces, as well as in the sonic composites. The film consists of alternating layers of two different materials that follow a triadic Cantor sequence. The Cantor structure shows extremely low thresholds for subharmonic generation of ultrasonic waves, compared with homogeneous and periodic structures. The coupling between the extended-mode (phonon) and the localized-mode (fracton) vibration regimes explains the generation of full band-gaps, for which there are no propagating Lamb waves. The large enhancement of the nonlinear interaction results from a more favorable frequency and spatial matching of coupled modes. A full band-gap that excludes Love waves is also analyzed. [ABSTRACT FROM AUTHOR]

An experimental comparison is reported here between two equivalent resonant subwavelength metasurfaces made of long aluminum beams glued closely together on a thin aluminum plate. One metasurface has a random distribution of the resonator beams, and the other has a regular square lattice of pitch 1.5 cm. The random lattice shows the "resonant" behavior of a typical metasurface, with a wide full bandgap for the first A0 Lamb mode. Instead, the regular square lattice combines Fano resonance with Bragg scattering at the edges of the passband, thus creating anisotropy and a pseudo bandgap. Comparisons with numerical simulations are performed, with good agreement with the experimental data. The multimodal response of the beams is also responsible for double negativity in a narrow frequency band, and the event of a pseudo bandgap around this same flexural resonance. In addition, the scattering regimes for both the random and regular metasurfaces are characterized using coherent and incoherent signal analysis. [ABSTRACT FROM AUTHOR]

The electrostatic precipitator has been widely utilized to control particulate pollutants in industry. It is important to optimize the configurations of the discharge and collecting electrode in order to increase the efficiency of electrostatic precipitators. The purpose of this paper is to investigate the influence of wavy collecting plates in a single-stage electrostatic precipitator. The effects of the applied voltage, volumetric flow rate per unit height, wavelength. h, and amplitude on the particle charging and behavior are extensively analyzed. The numerical model is validated by comparison with previous experimental and numerical data. The wavy collecting plate enhances the charging and transport behavior of the particles. Furthermore, changing the amplitude of the wavy collecting plate leads to a much greater effect on the particle charging and particle behavior in the wavy electrostatic precipitator, resulting in the highest collection efficiency. Unlabelled Image • The wave amplitude of plates is a crucial factor to increase the corona current. • Vortex in the convex section affects the gas flow field and makes flow faster. • The wavy collecting plate enhances the particle charging process. • The wave amplitude of plates significantly affects the collection efficiency. [ABSTRACT FROM AUTHOR]

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]

Structural damage diagnosis is still a challenging task because most current methods are either insensitive to local structural damage or sensitive to measurement noise. A statistical moment–based structural damage detection (SMBDD) algorithm has been proposed to locate and detect damages, revealing superiority in noise immunity. However, it requests that the number of measured responses should be no fewer than that of unknown structural parameters. In this paper, to reduce the number of measurements required in the SMBDD algorithm, an improved method for damage diagnosis is proposed based on the temporal moment of partially measured structural responses. Firstly, structural partial acceleration responses are measured and split into several time segments. Then, the temporal moment in each segment of the measured acceleration response time history is estimated. Finally, an objective error function is established by the temporal moments of measured accelerations and calculated accelerations, and structural stiffness can be identified by minimizing the objective error function. The proposed method is simple and feasible with a robust antinoise property and can identify structural damage when the number of measured responses is lower than that of the structural stiffness. Numerical simulations and an experimental study are conducted, respectively, to verify the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The constantly growing demand for cost-intensive lightweight structures is accompanied by an increased demand for reliable non-destructive testing (NDT) methods. Previous investigations have shown that Lamb Waves (LW) are very well suited for the use in Structural Health Monitoring (SHM) due to their relatively undamped propagation over long distances. Therefore the numerical modelling of a non-contact and automated system for non-destructive testing of C-FRP laminates was investigated in the present study. The method is based on the propagation of LW, which are excited by an actuator-driven piezo transducer and interact with defects in the structure. In contrast to classical Structural Health Monitoring, a scanning inspection system is used instead of a sensor permanently integrated into the structure to detect the measurement signals. For the representation of the wave propagation a three-dimensional simulation model with the commercial FEM software PZFlex was modelled. The wave interactions in defective C-FRP coupons are investigated and visualized through different output formats like A-, B- and C-Scans. Additionally the model allows the determination of the dispersion characteristics of the LW in the examined laminate. The validation of the numerical results was successfully carried out at different levels with the help of experimental investigations. [ABSTRACT FROM AUTHOR]

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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]

Within the second-order perturbation treatment, the present work analyzed the complex second-harmonic generation of Lamb waves in a two-layered solid waveguide by combining the modal analysis method and the nonlinear reflection of acoustic waves at interfaces. The formal solution of the second-harmonic field is obtained on the basis of the modal analysis approach, from which a phase matching modification factor can be determined. The solution of the cumulative second-harmonic displacement field derived from the nonlinear reflection of acoustic waves at interfaces can be modified by multiplying the phase matching modification factor, and then be more general and applicable for the total second-harmonic field of Lamb wave propagation. Numerical computation results show that amplitude of the second-harmonic displacement increases clearly with propagation distance when the phase velocity of a double frequency Lamb wave (DFLW) component is exactly or approximately equivalent to that of the primary Lamb wave propagation, and reveal that the amplitudes of the second harmonics exhibit a decreasing tendency when the relative deviation of phase velocity between the primary Lamb wave and the DFLW component increases. Primary experimental measurements have been performed to verify the results of the numerical simulations in a FeCrNi alloy steel specimen. The research provides a further understanding for the physical process of the cumulative second-harmonic generation in a two-layered solid waveguide. [ABSTRACT FROM AUTHOR]

Water-filled containers placed externally on an armored vehicle offer a potentially low cost, light-weight, and simple technique to mitigate near-field explosive blast, although the use of a gap or standoff between the container and target has not been studied. This paper uses experimental testing and numerical simulations to characterize the influence of this container standoff on the mitigation of near-field blast effects. The addition of the container standoff was not found to generally increase the blast mitigation effect provided by water-filled containers on the deformation caused to a steel target plate. While the container standoff was found to enhance the spreading and shadowing blast mitigation mechanisms provided by the water-filled container, this was offset by an increase in blast loading due to the container being closer to the explosive charge. A new mitigation mechanism was identified as the time delay between the initial loading of the steel plate by the blast wave and the subsequent impact of water ejected from the container. The results from this work provide engineers guidance into the design of water-filled containers for near-field blast protection of armored vehicles. [ABSTRACT FROM AUTHOR]

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

Recent advances in the field of physical acoustics, imaging, and images processing, piezoelectric materials and ultrasonic transducer designs have led to the emergence of new methods and equipment for ultrasound diagnostics, therapy and esthetic medicine, as well as the development of traditional and emerging new application fields. One of the promising applications of piezoceramics and composite materials are ultrasonic transducers, in particular, high intensity focused ultrasound (HIFU) transducers for medical therapeutic devices. This paper describes the results of the development of mathematical models, numerical modeling and optimization of acoustic fields and designs of composite HIFU transducers, which were made from porous piezoceramics. [ABSTRACT FROM AUTHOR]

Abstract This study is focused on the development of a finite difference model to simulate Lamb wave propagation through Single Lap Joints (SLJs). The main advantage of this model is the mathematical ability to easily reproduce the presence of a damage (debonding) as a discontinuity in velocity values. This makes our model suitable for continuous and embedded Structural Health Monitoring (SHM) of a complex structure. Numerical simulations and experimental campaigns are presented in order to validate the proposed model. [ABSTRACT FROM AUTHOR]

In this article, estimation of crack size, shape, and orientation was investigated numerically and experimentally using Lamb waves. A hybrid global–local approach was used in conjunction with the imaging methods for the numerical simulation. The hybrid global–local approach allowed fast and efficient prediction of scattering wave signals for Lamb wave interaction with crack from various incident directions. The simulation results showed the directionality effect of the scattering wave signals and suggested an optimum transmitter–sensor configuration. Two imaging methods were used: one involves the synthetic time reversal concept and the other involves Gaussian distribution function. Both imaging methods show very good agreement during simulations. Experiments were designed and conducted based on the simulated results. A network of eight piezoelectric wafer active sensors was used to capture the scattering waves from the crack. Both the pitch-catch and pulse-echo experimental modes were used. The directionality effect of incident Lamb waves on the imaging results was studied. The effect of summation, multiplication, and combined algorithms for each imaging method was studied. It was found that both methods can successfully predict the crack size and orientation. An attempt was made to use these imaging methods for detecting and sizing smaller sized damage (1- to 3-mm-diameter hole). It was found that these methods can successfully localize the hole, but size estimation was a bit challenging because of the smaller dimensions. The scattering waves for various hole sizes were studied. [ABSTRACT FROM AUTHOR]

Highlights • A non-linear Lamb wave technique for assessment of breathing disbonds is proposed. • A clock-like array of PZT consist of dual-transmitter and multi-receiver is employed. • The nonlinear interaction of the wave with the disbond produces higher harmonics. • A filter is designed to extract the second harmonic component from the signal. • Damage index maps and damage quantity curves locate and quantify breathing disbonds. Abstract A non-linear Lamb wave based active sensing technique for identification of breathing type disbonds in stiffened metallic plates has been presented. In this technique, a built-in dual transmitter and multi receivers (DTMR) clock-like array of piezoelectric wafer transducers (PWT) is used. The central part of the array consists of two transmitters, which are placed concentrically on either side of the plate. A Hanning window modulated sine pulse has been used for excitation. Opposite polling is applied to the transmitters to generate a predominant anti-symmetric (A 0) Lamb mode to encourage breathing in the disbond. It is observed that the contact acoustic nonlinearity (CAN) of breathing disbond generates higher harmonics when a primary Lamb wave interacts with the disbond. Since these harmonics carry damage information, a bandpass filter is used to extract the second harmonic component from the received signals followed by wavelet transformation. Finally, the transformed signals are used to device a baseline free damage detection algorithm for locating the disbond. Experimental investigations are carried out to interrogate a full width disbond that is located either at the middle of the T-stiffener or at an offset from its center. The damage index (DI) map obtained for the entire plate show higher values at the disbond location. Once the disbond is localized, a damage quantity (DQ) curve is devised to determine the extent of disbond with a high degree of accuracy. Extensive numerical simulations are carried out using commercially available finite element package, ANSYS 14.0, in an effort to examine the effectiveness of the damage detection algorithm and to further establish the DQ curves. The numerical results are found to be in good agreement with the experimental findings that affirm the robustness of the nonlinear active sensing technique for detection and assessment of breathing type disbond in stiffened aluminium panels without requiring the baseline signals. [ABSTRACT FROM AUTHOR]

Quantitative assessment of the crack position and dimension in metallic structures is critical to nondestructive testing and evaluation. Finite-difference time-domain method was used to simulate the propagation of Lamb waves in a plate and its interaction with cracks. The corresponding reflection and transmission coefficients and power spectral densities were used as the training data for direct search and an artificial neural network model. The estimation accuracy was compared with each other at varied detection thresholds. In addition, detecting multiple cracks was also possible by considering each crack individually, if the echoes from each crack can be separated in the time domain. [ABSTRACT FROM AUTHOR]

The present study focuses on critical analysis of various aspects related to the evaluation of amplitude based material nonlinearity parameter γ amp through the numerical simulations and theoretical justifications. Two different materials with distinct nonlinearities are interrogated using Lamb waves in the numerical simulations. The effects of input force amplitude, number of cycles in the tone burst, tone burst windowing functions, material model parameters, excitation frequency, and wave propagation distance on γ amp are studied in detail. Also, γ amp estimated for the data obtained from the numerical simulations considering S 0 - S 0 mode pair at low frequencies and resonant S 1 - S 2 mode pair at higher frequencies are compared with the physics based material nonlinearity parameter γ phy . The difference between the results is marginal. Thus, the nonlinear Lamb wave technique can be effectively used even at low frequencies to estimate the actual material nonlinearity of the (metallic) plate materials with fair accuracy using the proposed nonlinearity models. [ABSTRACT FROM AUTHOR]

Highlights • We propose a modified design of the omnidirectional electromagnetic acoustic transducer (EMAT) for antisymmetric (A0) Lamb wave generation. • Numerical simulation based on FEM is performed to characterize the magnetic configurations and determine the coil parameter of the EMAT. • The double-turn coil is used instead of a single spiral coil. • The proposed EMAT can generate and receive pure single A0 mode Lamb wave signal with high signal to ratio. Abstract This work presents a modified design of electromagnetic acoustic transducer (EMAT) that generates only antisymmetric (A0) omnidirectional Lamb wave. The new magnetic configuration includes a solid cylindrical magnet and a ring-shaped magnet surrounding the outside of the solid cylindrical magnet. The diameter of the solid cylindrical magnet is slightly small than the inner diameter of the ring-shaped magnet. The double-turn coil is used instead of a single spiral coil. Numerical simulation is applied to characterize the magnets and determine the coil parameters. Experimental results on a 3 mm-thick aluminum plate show that the modified EMAT we proposed can generate and receive pure single A0 mode Lamb wave with high signal to noise ratio and can effectively meet the imaging requirements. [ABSTRACT FROM AUTHOR]

This paper investigates the propagation of Lamb waves in thin plates with quadratic nonlinearity by one-way mixing method using numerical simulations. It is shown that an A 0 -mode wave can be generated by a pair of S 0 and A 0 mode waves only when mixing condition is satisfied, and mixing wave signals are capable of locating the damage zone. Additionally, it is manifested that the acoustic nonlinear parameter increases linearly with quadratic nonlinearity but monotonously with the size of mixing zone. Furthermore, because of frequency deviation, the waveform of the mixing wave changes significantly from a regular diamond shape to toneburst trains. [ABSTRACT FROM AUTHOR]

The validation of structural health monitoring (SHM) systems for aircraft is complicated by the extent and number of factors that the SHM system must demonstrate for robust performance. Therefore, a time- and cost-efficient method for examining all of the sensitive factors must be conducted. In this paper, we demonstrate the utility of using the simulation modeling environment to determine the SHM sensitive factors that must be considered for subsequent experiments, in order to enable the SHM validation. We demonstrate this concept by examining the effect of SHM system configuration and flaw characteristics on the response of a signal from a known piezoelectric wafer active sensor (PWAS) in an aluminum plate, using simulation models of a particular hot spot. We derive the signal responses mathematically and through the statistical design of experiments, we determine the significant factors that affect the damage indices that are computed from the signal, using only half the number of runs that are normally required. We determine that the transmitter angle is the largest source of variation for the damage indices that are considered, followed by signal frequency and transmitter distance to the hot spot. These results demonstrate that the use of efficient statistical design and simulation may enable a cost- and time-efficient sequential approach to quantifying sensitive SHM factors and system validation. [ABSTRACT FROM AUTHOR]

In recent research on Structural Health Monitoring (SHM) guided waves, especially nonlinear Lamb waves, turned out to be a suitable means for monitoring material deterioration in thin-walled structures. In the corresponding numerical simulations on wave propagation the nonlinear elastic theory by Murnaghan is often implemented, which requires 14 material parameters for transversely isotropic materials. Enhancing an existing linear strain energy potential, a new nonlinear hyperelastic transversely isotropic material model is introduced which reduces the number of independent material parameters to six. In order to verify the applicability of the presented material model with respect to the simulation of nonlinear wave propagation in composite structures, and the generation of higher harmonic wave modes, the existence of a power flux from the fundamental to the higher harmonic mode is investigated analytically and numerically. Analytical considerations show that this power flux exists like in Murnaghan’s theory. For the numerical validation the S 0 – S 0 mode pair in the low frequency range is used. Therefore, the amplitude of the second harmonic wave mode is oscillating with increasing propagation distance. This behavior is in excellent agreement with the theoretical prediction. It is shown further, that even for an oscillating behavior the amplitude of the second harmonic mode can be approximated by a linear curve fit over a considerably propagation distance and hence shows a quasi cumulative behavior. Therefore, the introduced material model is an advantageous alternative to Murnaghan’s theory to simulate the second harmonic Lamb wave generation due to in composite structures. [ABSTRACT FROM AUTHOR]

Lamb wave-based structural health monitoring is one of the most widely used damage detection techniques. For quantitatively identifying the damage, damage features that Lamb waves carry may need to be carefully studied by numerical simulation. In this paper, spectral element method (SEM) is used to simulate Lamb wave interaction with open and closed crack. Cracked spectral element models are established for open and closed cracks, respectively. Results calculated by SEM are compared with the conventional finite element method to verify the proposed model. Some simulations are conducted to study different damage features between open and closed crack models. Wave reflection and transmission ratios with different crack depths are also quantitatively analyzed. Damage features obtained are used to conduct a simple experiment to identify the location and size of the crack. [ABSTRACT FROM AUTHOR]

In this paper, a new kind of wave-plate mist eliminator using open-celled porous silicon carbide foam layers as enhanced structure has been firstly proposed. The effects of foam layer geometry parameters (foam layer thickness, porosity, PPI) on droplet performance (separation efficiency and pressure drop) were experimentally investigated. Re-entrainment mechanism influenced by flooding state in the foam layer was also discussed. Downstream droplet diameter distribution under the re-entrainment conditions were also investigated by color-developing method. The results show that the sample with smaller foam pore size has larger volume mean diameter of secondary droplets in the outflow, due to its weaker liquid drainage ability. Comparisons between the experiment and our earlier numerical simulation demonstrate the average relative errors of the separation efficiency and the pressure drop are within 10%, meanwhile trends relating demister performance to foam geometry parameters are the same between the two sets of results, indicating the validity of the simulation method. In addition, two empirical correlations were obtained by fitting the experimental data, which can be used for predicting separation efficiency and pressure drop in the selected experiment conditions. [ABSTRACT FROM AUTHOR]

Wave-plate mist eliminator with porous foam layers as enhanced structures on vanes to improve separation efficiency for tiny droplets was proposed in this paper. Overall separation efficiency, grade separation efficiency and pressure drop of the mist eliminators were systematically studied by using Computational Fluid Dynamics (CFD) method. The Shear Stress Transport (SST) k-ω turbulence model and Discrete Phase Model (DPM) were adopted to describe the motion of gas phase and liquid phase, respectively. The computational field was simplified by introducing an assumption that the real 3D structure of porous foam can be equivalent to 2D layout with randomly arranged circles. The CFD results showed that the separation efficiency was enhanced by increasing the foam layer thickness. Moreover, a gradual increase in the foam porosity at fixed PPI (pores per inch) value made the separation efficiency increased to a maximum value, then fall to a lower level, while by decreasing the PPI value of the foam at fixed porosity, the separation efficiency increased. Pressure drop was increased both with the increase in the foam layer thickness and PPI value as well as the decrease of the porosity. Furthermore, the flow field and the droplet trajectories were also analyzed. [ABSTRACT FROM AUTHOR]

A nonlinear Lamb-wave-based method for fatigue crack detection in steel plates with and without carbon fibre reinforcement polymer (CFRP) reinforcement is presented in this study. Both numerical simulation and experimental evaluation were performed for Lamb wave propagation and its interaction with a fatigue crack on these two steel plate types. With the generation of the second harmonic, the damage-induced wave nonlinearities were identified by surface-bonded piezoelectric sensors. Numerical simulation revealed that the damage-induced wave component at the second harmonic was slightly affected by the existence of CFRP laminate, although the total wave energy was decreased because of wave leakage into the CFRP laminate. Due to unavoidable nonlinearity from the experimental environments, it was impractical to directly extract the time-of-flight of the second harmonic for locating the crack. To this end, the correlation coefficient of benchmark and signal with damage at double frequency in the time domain was calculated, based on which an imaging method was introduced to locate the fatigue crack in steel plates with and without CFRP laminates. [ABSTRACT FROM AUTHOR]

In this paper, the propagation of Lamb waves in the functionally graded piezoelectric plates discretized into sub-layers is studied. In this presentation, a Legendre polynomial method is proposed to solve the wave equation. This method has three specificities in comparison to traditional methods. It directly incorporates the boundary conditions into the constitutive equations and it allows a resolution by a non-iterative system to the eigenvalues ​​and eigenvectors. Furthermore, this method takes full account of converged results nature. The convergence of this method is discussed a thorough manner. Here, the converged results are obtained with very low truncation number M. In addition, the effect of the gradient coefficients on the Lamb phase velocities is illustrated. Consequently, the different results also reveal differences between the properties of Lamb waves propagation in the functionally graded piezoelectric plates and the corresponding properties in homogeneous plates. Furthermore, the effect of constituent volume fraction is also considered through gradient models. All the developments performed in this work were implemented in Matlab software. [ABSTRACT FROM AUTHOR]

Cumulative nonlinear Lamb waves have been widely reported in experimental studies. These studies report on cases where nonlinear Lamb waves will increase in amplitude with propagation distance because the entire specimen is responsible for generating harmonics. The applications for nonlinear Lamb wave propagation include distributed damage such as fatigue and even identifying one type of metallic alloy from another. In this paper, nonlinear elasticity is presented as the key mechanism responsible for cumulative second order Lamb wave generation. This is followed by a series of numerical simulations which will shed light on how to achieve efficient generation of cumulative nonlinear Lamb waves. The effects of excitation amplitude and frequency-thickness will be explored through numerical modelling. The numerical results presented in this paper will support rules prescribed by other authors. It is found that both factors are crucial in producing a measurable nonlinearity. [ABSTRACT FROM AUTHOR]

The asymmetric propagation of the first order antisymmetric (A) Lamb wave in a tapered plate respectively carved with sharp bottom corner and round bottom corner is theoretically investigated. Through numerical simulation of A Lamb wave in time domain, we find that when the thickness of the waveguide abruptly decreases to below the cut-off thickness, about half of the A mode is converted into the fundamental symmetrical S and antisymmetrical A modes to pass through the defected region. Furthermore, the transmitted modes A and S are completely apart from each other and can be quantitatively evaluated. Conversely, when the thickness change is very smooth, most of the energy of A Lamb wave is reflected back. It is the unique mode conversion behavior that leads to great transmission difference value of A Lamb wave along the opposite directions. Finally, the influence of geometrical parameters on the transmission coefficient is also studied. The higher efficiency and proper working frequency range can be realized by adjusting the slope angle θ, height h and h . The simple asymmetric systems will be potentially significant in applications of ultrasound diagnosis and therapy. [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]

A plate system, composed of flat array is proposed to be one type of breakwater. At present, little research has been done on the wave elimination of the plate under long-period waves. The Reynolds-averaged Navier-Stokes (RANS) equations are treated based on three-dimensional closed RNG k-ε turbulence model and the governing equations are solved by VOF method. A numerical model of the interaction between wave and flat array breakwater is established to study the wave elimination performance of different wave period ang steep. The flow and vortex fields are solved to research the wave dissipation mechanism of breakwaters. Results show the wave transmission coefficient tends to increase with the increase of wave period. Wave transmitted coefficient are all less than 0.4 in this test, indicating that the flat array plate breakwater maintains a certain wave dissipation capacity for long-period waves. At the same period, the wave steepness change mainly affects the amount of water passing through the upper part of the flat plate, which has some influence on the way the breakwater dissipates energy. The plate spacing is the main wave energy dissipation area, which provides space for water exchange and effectively eliminate waves while reducing the amount of board. The results preliminarily demonstrated the potential application of flat plate array as a wave energy breakwater, but the flat plate parameter design and the interaction of waves and flat plates needs further research. • The hydrodynamic characteristics of the flat plate array breakwater under different long period wave action are studied by the numerical wave flume based on the RANS equation with VOF free surface tracking method. • The wave dissipation performance of a new non-uniform horizontal four-plate arrangement of breakwater under long period waves is studied. • The velocity and vortex distribution of the water column when the waves interact with the flat plate array breakwater structure under different wave steepness conditions are explored. [ABSTRACT FROM AUTHOR]

A coordinated theoretical, numerical and experimental study is carried out in an effort to interpret the characteristics of propagating guided Lamb wave modes in presence of a high-density (HD) core region in a honeycomb composite sandwich structure (HCSS). Initially, a two-dimensional (2D) semi-analytical model based on the global matrix method is used to study the response and dispersion characteristics of the HCSS with a soft core. Due to the complex structural characteristics, the study of guided wave (GW) propagation in HCSS with HD-core region inherently poses many challenges. Therefore, a numerical simulation of GW propagation in the HCSS with and without the HD-core region is carried out, using surface-bonded piezoelectric wafer transducer (PWT) network. From the numerical results, it is observed that the presence of HD-core significantly decreases both the group velocity and the amplitude of the received GW signal. Laboratory experiments are then conducted in order to verify the theoretical and numerical results. A good agreement between the theoretical, numerical and experimental results is observed in all the cases studied. An extensive parametric study is also carried out for a range of HD-core sizes and densities in order to study the effect due to the change in size and density of the HD zone on the characteristics of propagating GW modes. It is found that the amplitudes and group velocities of the GW modes decrease with the increase in HD-core width and density. [ABSTRACT FROM AUTHOR]

Nonlinear ultrasonic Lamb waves are popular to characterize the nonlinearity of materials. However, the widely used nonlinear Lamb mode suffers from two associated complications: inherent dispersive and multimode natures. To overcome these, the symmetric Lamb mode (S0) at low frequency region is explored. At the low frequency region, the S0 mode is little dispersive and easy to generate. However, the secondary mode still exists, and increases linearly for significant distance. Numerical simulations and experiments are used to validate the nonlinear features and therefore demonstrate an easy alternative for nonlinear Lamb wave applications. [ABSTRACT FROM AUTHOR]

Typically, numerical simulations of Lamb wave propagation are done using material properties which originate from tensile testing. This approach is well established in relation to isotropic homogenous structures such as aluminium plates. However if this approach is used for woven composites such as carbon fibre reinforced plastics (CFRP), inaccuracies can arise that stem from vastly different stress distributions, strain rates and amplitudes during Lamb wave propagation. In order to account for this, an approach is presented where the elastic properties in a numerical Lamb wave model are optimised to achieve good correlation between model predictions and experimental observations. Since the material properties are determined under a Lamb wave propagation regime, the strain rates and amplitudes are consistent with the intended modelling application. The approach is validated with an experimental case study involving a M18/G939 carbon-epoxy system. The methodology is shown to yield property estimates that furnish simulations that closely match observed behaviours. The optimised properties were significantly different to those supplied by the manufacturer, as much as 52% for the in-plane stiffness. The findings demonstrate that large errors are possible if elastic properties determined using conventional quasi-static testing are used in Lamb wave simulations pertaining to woven composite materials. [ABSTRACT FROM AUTHOR]

The integrity of safety-critical structural composites can be enhanced by the use of innovative ultrasonic nondestructive evaluation techniques. Among the various existing techniques, guided wave methods provide a good promise in terms of sensitivity to a variety of damage types or defects and the extent of the area that can be monitored, given the ability of these waves to travel relatively long distances within the structure under investigation. In comparison with isotropic metallic structures, wave propagation in composite structures presents additional complexity for effective damage identification. The material inhomogeneity, anisotropy, and the multilayered construction of composite materials lead to significant dependence of wave modes on laminate layup configurations, direction of propagation, frequency, and interface conditions. In this article, a specific structure will be analyzed with different levels of complexities in an effort to determine the propagation characteristics of the waves. The investigated structure is a stiffened composite plate of finite thickness and infinite lateral dimensions. The work is carried out using theoretical analysis, numerical modeling, and laboratory experiments. Numerical (finite element) simulations are used for more realistic models, where the geometric and material complexities present practical difficulties in direct analysis using theoretical constructs only. [ABSTRACT FROM AUTHOR]

Lamb wave technique has been widely used in non-destructive evaluation (NDE) and structural health monitoring (SHM). However, due to the multi-mode characteristics and dispersive nature, Lamb wave propagation behavior is much more complex than that of bulk waves. Numerous numerical simulations on Lamb wave propagation have been conducted to study its physical principles. However, few quantitative studies on evaluating the accuracy of these numerical simulations were reported. In this paper, a method based on cross correlation analysis for quantitatively evaluating the simulation accuracy of time-transient Lamb waves propagation is proposed. Two kinds of error, affecting the position and shape accuracies are firstly identified. Consequently, two quantitative indices, i.e., the GVE (group velocity error) and MACCC (maximum absolute value of cross correlation coefficient) derived from cross correlation analysis between a simulated signal and a reference waveform, are proposed to assess the position and shape errors of the simulated signal. In this way, the simulation accuracy on the position and shape is quantitatively evaluated. In order to apply this proposed method to select appropriate element size and time step, a specialized 2D-FEM program combined with the proposed method is developed. Then, the proper element size considering different element types and time step considering different time integration schemes are selected. These results proved that the proposed method is feasible and effective, and can be used as an efficient tool for quantitatively evaluating and verifying the simulation accuracy of time-transient Lamb wave propagation. [ABSTRACT FROM AUTHOR]

A new hybrid computational system for material identification (HCSMI) is presented, developed for the identification of homogeneous, elastic, isotropic plate parameters. Attention is focused on the construction of dispersion curves, related to Lamb waves. The main idea of the system HCSMI lies in separation of two essential basic computational stages, corresponding to direct or inverse analyses. In the frame of the first stage an experimental dispersion curve DC exp is constructed, applying Guided Wave Measurement (GWM) technique. Then, in the other stage, corresponding to the inverse analysis, an Artificial Neural Network (ANN) is trained ‘off line’. The substitution of results of the first stage, treated as inputs of the ANN, gives the values of identified plate parameters. In such a way no iteration is needed, unlike to the classical approach. In such an approach, the “distance” between the approximate experimental curves DC exp and dispersion curves DC num obtained in the direct analysis, is iteratively minimized. Two case studies are presented, corresponding either to measurements in laboratory tests or those related to pseudo-experimental noisy data of computer simulations. The obtained results prove high numerical efficiency of HCSMI, applied to the identification of aluminum plate parameters. [ABSTRACT FROM AUTHOR]