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Highlights • Damage localization method by the time-reversed Lamb waves is proposed for plates. • Mode conversion behavior of Lamb waves is considered in the time reversal process. • Localization of a notch-type defect in plates is numerically simulated. • Focused spots appear at the damage location by the proposed time reversal method. Abstract A damage localization method based on the time reversal focusing of the mode-converted scattered Lamb wave is proposed for plate structures with a non-symmetric defect in the thickness direction. Dual transducers are attached symmetrically on the upper and lower surfaces of the plate to selectively emit and receive the lowest-order symmetric (S0) and antisymmetric (A0) modes. The localization of damage is achieved by the numerical time-reversed (TR) simulation of the mode-converted Lamb wave generated at the defect. To investigate the validity of the proposed method, the signals of the Lamb waves in a plate with a partial-thickness notch are numerically simulated by the three-dimensional elastodynamic finite integration technique (EFIT). When the S0 mode is emitted in the damaged plate, not only the S0 mode is scattered but also the A0 mode is generated due to mode conversion at the notch. Similar mode conversion behavior is confirmed when the A0 mode is emitted. The time reversal of the mode-converted scattered Lamb waves creates focused spots at the damage location without using baseline data for the undamaged plate. The proposed method reduces the magnitude of the artifacts compared to the time reversal of the non-mode-converted Lamb wave, and yields the focused spot whose size is associated with the size of the notch and the half wavelength of the time-reversed wave mode. Furthermore, the proposed method is applied to a plate with a notch-type defect adjacent to an a priori known through-thickness hole, demonstrating the damage localization in a relatively complicated structure. [ABSTRACT FROM AUTHOR]

Highlights • The effect of ablation on the shape of laser-induced elastic waves is studied. • The elastic waves were generated and observed by projecting laser pulses. • Two numerical simulations were performed using heat flux and normal stress input. • The effects of thermal expansion and radiation pressure depend on energy intensity. Abstract In this paper, the effect of ablation on the shape of elastic waves generated by laser excitation is studied numerically and experimentally. Laser-induced ultrasound has been widely used in the nondestructive testing (NDT) field because it has the advantage that the sensor does not have to be directly attached to the target structure. In the safety assessment process, low energy excitation is used, and thus the structure is not damaged. Most studies related to laser ultrasound have focused on the method of detecting cracks within the elastic range, and there have been few studies on the effect of ablation. This research consists of experiments and numerical analyses. In experiments, elastic waves were generated in an aluminum plate by projecting laser pulses with different energy intensities. The velocities in the thickness direction were measured using a Laser Doppler Velocimeter (LDV) at a point 135 mm away from the excitation point. In the numerical study, two numerical simulations were carried out using heat flux and normal stress input to mimic laser pulse excitation. A thermo-mechanical simulation by heat flux was conducted to simulate thermal expansion by the laser pulse, and the normal stress was applied to reflect the effect of radiation pressure by ablation, respectively. Waveforms were synthesized by using different magnitude ratios of the obtained numerical responses and were compared with the experiment results. It is found that the effect of radiation pressure should not be neglected if the energy intensity is large although the effect of radiation pressure decreases as the energy intensity decreases. At the energy intensity with which ablation occurs, the effects of thermal expansion and radiation pressure exist simultaneously, and the contribution to the response depends on the energy intensity. [ABSTRACT FROM AUTHOR]

Highlights • Incremental Lamb wave scattering from a notch emanating from a through-hole is analyzed. • Multiple frequencies and hole diameters are considered as well as a notch with no hole. • Numerical results are validated with experimentally measured scattering patterns. • The presence of even a small hole significantly affects notch incremental scattering. • Incremental scattering from off-angle incident waves is a strong function of frequency. Abstract Lamb wave scattering from a crack originating at a through-hole is of practical importance because of the abundance of fastener holes used in engineering structures. Notches are often used to simulate cracks so that Lamb wave methods can be more conveniently investigated in the laboratory. A linear, three-dimensional finite element model is employed in this paper to study incremental scattering of the fundamental anti-symmetric (A 0 ) Lamb wave mode from notches emanating from through-holes. The term “incremental scattering” refers to the change in scattering caused by introduction of the notch and is motivated by structural health monitoring for which transducers are fixed and signal changes are interpreted to detect damage. Far-field angular scattering patterns are generated for multiple incident angles and frequencies, and such patterns are experimentally validated at one frequency by laser vibrometry measurements. Comparisons are made between a vertical notch alone (no hole) and notches located above and below the through-hole. Additionally, holes of different sizes are considered to investigate the effect of hole diameter on incremental scattering patterns. Results show that the presence, location and size of the through-hole affect both the shape and strength of notch incremental scattering patterns. [ABSTRACT FROM AUTHOR]

Abstract This paper deals with the introduction of a spectral-element method, in the time domain, to address problems of ultrasonic wave propagation in the field of the nondestructive ultrasonic testing and evaluation. Two kinds of problems are addressed. The first focuses on guided waves that are often used to control immersed layered defective structures. The second focuses on the use of bulk waves to inspect layered anisotropic media that may contain a defect. This full-wave technique allows for the use of significantly coarser meshes compared to other standard finite-element methods that can be used for non-flat defective or damaged models, as using one element per shortest wavelength is sufficient. It is thus well suited to the simulation of wave propagation phenomena in complex structures at high frequencies. With the goal of first validating our approach for simple cases, we begin by presenting results of simulations for a homogeneous plate. We compare the dispersion curves obtained with the analytical ones. The results are in excellent agreement, and the spectral-element method is fast enough to allow for simulations having a high level of accuracy. We then reproduce and analyze the transmission losses of a quasi plane wave across the immersed plate, and discuss the influence of the finite size of the ultrasonic beam in real physical experiments. We subsequently study the transmission of an ultrasonic wave through an immersed tri-layer composed of two aluminum plates glued together and with a defect, a model that is already not accessible to quasi-analytical calculation techniques. We again determine the dispersion curves, and then study transmission through the structure having a delamination defect compared to transmission through a healthy structure. We then compute the dispersion curves of the same tri-layer structure but with a varying glue thickness. These dispersion curves are compared to those of the healthy structure. We then perform a three-dimensional simulation of a pulse-echo experiment in an immersed medium composed of layers of transversely-isotropic austenitic steel, the axis of symmetry of each layer being tilted differently. This medium represents a very simple model of a weld, in which we include two kinds of defects: a gas bubble resulting from the welding process, and then a branching and crossing Y-shaped crack resulting e.g. from aging. We illustrate their effect on ultrasonic waves by computing the scattered field. [ABSTRACT FROM AUTHOR]

Highlights • The new series of differential equations of free vibration of thick functionally graded plates, including the lateral extension, have been derived. • Based on the dispersion relation and Lamb wave velocity, the validation of the differential equations has been investigated. • Legendre spectral element method is utilized to analyze the thick functionally graded plates subjected to impulsive loading. • The accuracy of the numerical simulation is examined by comparing the group wave velocity of first symmetrical and asymmetrical modes of Lamb waves derived from the dispersion relations and numerical models. Abstract In this paper, a new series of differential equations which can predict the propagation and dispersive behavior of first symmetric and antisymmetric Lamb wave modes in functionally graded material (FGM) thick plates are developed. Then, given the accuracy and cost effectiveness of Legendre spectral element method in combination with central difference method, these methods are utilized to set up the numerical models and solving the dynamic discretized differential equations. Finally, based on the wave front velocity, the percentage of error is estimated and it is shown that considering some limitations which are inherently included in the utilized method of error estimation, the proposed equations and the numerical procedure can predict the behavior of first symmetric and antisymmetric Lamb wave modes in FGM thick plates precisely. Graphical abstract Image, graphical abstract [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]

Abstract We investigate experimentally Lamb wave propagation in coupled-resonator elastic waveguides (CREWs) formed by a chain of cavities in a two-dimensional phononic crystal slab with cross holes. Wide complete bandgaps, extending from 53 to 88 kHz, are first measured in a finite phononic crystal slab sample. A straight waveguide and a wave splitting circuit with 90° bends are then designed, fabricated and measured. Elastic Lamb waves are excited by a piezoelectric patch attached to one side of the phononic slab and detected using a scanning vibrometer. Strongly confined guiding and splitting at waveguide junctions are clearly observed for several guided waves. Numerical simulations are found to be in excellent agreement with experimental results and allow for the identification of the involved resonant cavity modes. The influence on the dispersion of guided waves of the slab thickness and of the hole length is also investigated. The results have implications for the design of innovative phononic devices with strong confinement and tailorable dispersion. [ABSTRACT FROM AUTHOR]

Abstract The orbital angular momentum of photon is one of the key source of quantum information. Polarized light passing through twisted special wave plates show entangled states of both spin angular momentum (SAM) and orbital angular momentum (OAM). The variable circular and linear retarders (VCR and VLR) are studied here from the view point of quantum processing of entangled states. In association with twist dependent gain of OAM, the geometric phase has been identified here both for VCR and VLR. For the application of VLR in an antenna array, we established a correlation between geometric phase and concurrence to explain the gradual flattened shape of the emergent wave by controlling the external twist of wave plates. [ABSTRACT FROM AUTHOR]

Abstract Lamb waves offer an efficient way for monitoring the structural integrity of large carbon fiber reinforced polymer (CFRP) structures. This paper presents a new method that is able to detect and assess delamination's length in anisotropic CFRP plates. It uses a chirp signal, which contains a wide range of frequencies, as excitation. Thus the material is interrogated by Lamb waves with various wavelengths. This method is based on that, for a specific delamination length, only some particular frequency components are disturbed. Simulation and experimental results show that the lowest frequency that is perturbed by a delamination decreases as the delamination length increases. Therefore, by observing which frequency components of the chirp-exited Lamb wave are disturbed, delamination length can be estimated. Wavelet analysis is implemented for better comparison of signals at multiple frequencies. [ABSTRACT FROM AUTHOR]

This paper studies the interaction of the fundamental symmetric Lamb wave mode S0 with delaminations in 8-layered glass fibre reinforced polymer (GFRP) laminated composites, using numerical models validated against experiments. The validation is performed using experiments on the eight layer quasi-isotropic GFRP composite laminate [0/90/45/−45] S . Further numerical studies were performed on the laminates [0/45/−45/90] S, [0/45/90/−45] S, [90/45/−45/0] S, [90/−45/0/45] S, [45/−45/0/90] S, and [45/90/−45/0] S to study the effect of ply layups on the reflection of S0 Lamb waves. The reflection coefficients calculated for these laminates followed a similar trend as the laminate [0/90/45/−45] S . Physical insight was obtained into the reflection behaviour, based on an analysis of wave dispersion in the sub-laminates. It was difficult to detect the delamination situated at the mid plane of the laminate, where the shear stress was found to be zero for all ply layups. The delaminations present at a depth greater than DL 12 (>0.42 mm) from the source of excitation, were found to be accessible for the laminates [0/90/45/−45] S, [0/45/−45/90] S and [0/45/90/−45] S . The sensitivity of the S0 mode reflection depends upon the position of the delamination along the thickness direction and ply layup orientation of the composite laminate. The S0 mode showed higher sensitivity to DL 23 and DL 34 (0.42 mm to 1.26 mm) in all the laminates that were inspected. [ABSTRACT FROM AUTHOR]

The feasibility of fatigue damage evaluation has been investigated using nonlinear Lamb waves with group-velocity mismatching. To choose an efficient mode pair, a parameter is proposed to quantify the efficiency of cumulative second-harmonic generation (SHG) of Lamb waves based on the normal modal analysis. Experiments and simulations are performed to verify the proposed parameter, which demonstrates that whether the matching condition of group velocity is satisfied or not, the efficiency of cumulative SHG increases with the order of Lamb mode for the five low-order Lamb waves investigated. Then, S3-s6 mode pair with group-velocity mismatching is chosen to characterize the fatigue damage of an aluminium alloy for the high efficiency of cumulative SHG. Results show that S3-s6 mode pair is sensitive to fatigue damage evolution and the integrated amplitude of second harmonics increases by nearly 300% with fatigue cycles. Nonlinear Lamb waves with group-velocity mismatching are validated to be a candidate to efficiently evaluate the fatigue damage. [ABSTRACT FROM AUTHOR]

For an elastic medium containing a homogeneous distribution of micro-cracks, an effective one-dimensional stress-strain relation has been determined with finite element simulations. In addition to flat micro-cracks, voids were considered that contain a Hertzian contact, which represents an example for micro-cracks with internal structure. The orientation of both types of micro-cracks was fully aligned or, for flat micro-cracks, totally random. For micro-cracks with Hertzian contacts, the case of random orientation was treated in an approximate way. The two types of defects were found to give rise to different degrees of non-analytic behavior of the effective stress-strain relation, which governs the nonlinear propagation of symmetric (S0) Lamb waves in the long-wavelength limit. The presence of flat micro-cracks causes even harmonics to grow linearly with propagation distance with amplitudes proportional to the amplitude of the fundamental wave, and gives rise to a static strain. The presence of the second type of defects leads to a linear growth of all harmonics with amplitudes proportional to the power 3/2 of the fundamental amplitude, and to a strain-dependent velocity shift. Simple expressions are given for the growth rates of higher harmonics of S0 Lamb waves in terms of the parameters occurring in the effective stress-strain relation. They have partly been determined quantitatively with the help of the FEM results for different micro-crack concentrations. [ABSTRACT FROM AUTHOR]

Human skull poses a significant barrier for the propagation of ultrasound waves. Development of methods enabling more efficient ultrasound transmission into and from the brain is therefore critical for the advancement of ultrasound-mediated transcranial imaging or actuation techniques. We report on the first observation of guided acoustic waves in the near field of an ex vivo human skull specimen in the frequency range between 0.2 and 1.5MHz. In contrast to what was previously observed for guided wave propagation in thin rodent skulls, the guided wave observed in a higher-frequency regime corresponds to a quasi-Rayleigh wave, confined mostly to the cortical bone layer. The newly discovered near-field properties of the human skull are expected to facilitate the development of more efficient diagnostic and therapeutic techniques based on transcranial ultrasound. [ABSTRACT FROM AUTHOR]

Abstract The design of orthotropic panels, in which some mechanical properties are associated to high degrees of uncertainty, such as for the natural fibres, is very arduous and often techniques that give local information are needed. In this paper a unidirectional flax–polyethylene panel is experimentally and numerically investigated. Experimentally, for a given distance source–receiver, the group velocities are obtained by processing the guided waves time-transient signals via a time-frequency transform leading to an estimation of the dispersion curves for different fibre directions and frequencies. Test results are then compared and used to validate numerical ones, obtained by means of both Finite Element and Wave Finite Element models. [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]

We propose two-dimensional reconfigurable phononic crystal slabs that support topologically protected edge states for Lamb waves. It consists of a triangular lattice of triangular-like air holes perforated in a slab and two sandwich cylinders are placed in the matrix of each unit cell. The pseudospin–orbit coupling is achieved by shifting the sandwich cylinders up or down. The robust transport of Lamb waves along reconfigurable interfaces is further demonstrated. The dynamically reconfigurable design provides an excellent platform for tuning the frequency range of topological edge states and steering efficient transport of Lamb waves along arbitrary pathways, which can be employed for elastic-wave communications, signal processing, and sensing. [ABSTRACT FROM AUTHOR]

Abstract We develop an optically controlled, rewritable half-wave plate based on a twisted nematic liquid crystal cell and present the relevant theory. The cell is bistable, with switching between states controlled by one-step illumination of a single PAAD azobenzene alignment layer with visible light. The photo-alignment properties of the layer enable reversible switching between two perpendicular alignment states at the cell surface, resulting in controllable polarization manipulation. The resulting changes in transmission are observed for four different probe beam wavelengths in the visible and near infrared (NIR) spectral regime. We demonstrate a reproducible modulation of the transmitted polarization with an optical contrast of up to 90–100%. This work represents a first step towards creating LC modulators and phase shifters which can be controlled remotely using only external light sources. Highlights • A dynamic system capable of forming rewritable twisted LC structures using a photoaligning azobenzene dye layer is presented. • The system is all-optically controlled. • Changes in transmission are observed for four different wavelengths in the visible and near infrared spectral regime. • We demonstrate a reproducible modulation of the transmitted polarization with an efficiency of up to 90–100%. [ABSTRACT FROM AUTHOR]

The limited knowledge of scratch hardness in ultrasonic vibration-assisted scratch (UVAS) impedes a comprehensive understanding of the abrasive wear process with ultrasonic vibration. The main challenge is the complex kinematic-geometric characteristic of instantaneous contact induced by ultrasonic vibration and the abrasive particle. This paper proposed a novel method to describe the contact behavior exerted by ultrasonic vibration in multiple directions. The wear behavior of the indenter in the UVAS test was analyzed, and the indenter surface was measured to calculate the instantaneous contact area. Based on the developed model and the UVAS test, the effects of ultrasonic vibration on scratch hardness with varying scratch velocity and cutting depth are evaluated. It could be found that the scratch hardness for UVAS was reduced compared to that of common scratch owing to the changed contact states. This study provides an effective tool to capture the instantaneous contact state with a complex moving path and predicts the scratch hardness under ultrasonic vibration through common scratch tests. • A novel analytical model is presented to describe the dynamic contact behavior during scratch process. • The model discloses the special instantaneous contact states induced by the high-frequency reciprocating motion. • The scratch hardness reduction mechanism due to ultrasonic vibration is revealed. • The effect of the processing parameter on the effective contact area and the scratch hardness is studied. • The wear behavior of the indenter is analyzed, and the measured surface is used to calculate the contact area. [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]

In this study, we numerically investigated the interaction between a solitary wave and a bottom-mounted, submerged elastic plate. The fluid structure interaction (FSI) solver (solids4Foam), the stabilized k - ω SST turbulence closure model, and the geometric volume of fluid approach (isoAdvector) were combined to simulate the dynamic process, including the plate movement and the flow field. The model package was first validated by using experimental data, and the effects of plate height, plate thickness, and the Young's modulus of plate material on the hydrodynamics were further investigated. Moreover, the Euler-Bernoulli beam theory was used to calculate the plate displacement. Lengthening the plate height, reducing the plate thicknesses, or reducing Young's modulus can all increase the plate displacement. Among them, lengthening the plate height has the greatest impact. Finally, the effects of the wave nonlinearity and plate material property on wave reflection, transmission, and dissipation were also analyzed. The results show that while the height of the plate has the most significant effect on the reflection coefficient, the plate thickness and the plate material have little influence on the reflection coefficient. The more flexible the plate is, the smaller the transmitted coefficient, and the larger the dissipation coefficient are. • The two-dimensional fluid-solid interaction solver, a stabilized k - ω SST turbulence closure model, is combined with the geometric volume of fluid approach. • Effects of the plate material on the flow fields, surface elevations, and forces are studied. • The material conditions for the numerical simulations are varied according to the variables in the cantilever beam theory; the height of the plate has the most significant effect on the reflection coefficient. [ABSTRACT FROM AUTHOR]

• Wave-propagation characteristics are depicted between Lamb waves and various orientation crack. • A propagation distance calculation method considering dispersion effects was achieved using Sparse Decomposition. • A novel algorithm to quantify various orientation and length crack without baseline signals has been developed. Crack damage is one of the significant factors that may accumulate at the stress concentration area of engineering structures and cause catastrophic accidents. In this paper, we proposed a novel approach to identify the crack location and size by exploiting the reflections and diffractions of Lamb waves. The interaction mechanism between the crack and Lamb wave has been analyzed thoroughly, our analysis of the interaction between the crack and Lamb wave revealed that both reflections and diffractions carry valuable damage information that reflects the size and orientation of the crack. As the interaction coefficients between these two components and the crack are different, there are supposed to exist differences between them in the amplitude value. We implemented a threshold to classify the signals received from all paths into two groups: reflections and diffractions. Then we constructed an overcomplete dictionary of waveforms corresponding to different propagation distances to extract the damage information. Using sparse decomposition, the received signals were mapped to their corresponding propagation distances without the use of baseline signals. The diffractions allow us to determine the crack's tip points, while the reflections provide information about the edge points. The kinked crack's size and orientation were visualized based on the time-domain signals acquired in our experiment. We provided a comprehensive description of our algorithm and verified it through numerical simulation and experimental data. Our results show high agreement with actual cracks, demonstrating the efficacy of our proposed method. [ABSTRACT FROM AUTHOR]

• A new multi-GPU-based spectral element formulation for ultrasonic wave propagation. • Novel direct GPU-to-GPU message exchange strategies enabled by CUDA-aware MPI. • Significant accelerations over a multi-CPU core-based counterpart formulation. • Achievable accelerations and model sizes scalable with the number of GPUs. • Practicality shown through simulating real-life ultrasonic inspection scenarios. In this paper, we introduce a new multi-GPU-based spectral element (SE) formulation for simulating ultrasonic wave propagation in solids. To maximize communication efficiency, we purposely developed, based on CUDA-aware MPI, two novel message exchange strategies which allow the common nodal forces of different subdomains to be shared between different GPUs in a direct manner, as opposed to via CPU hosts, during central difference-based time integration steps. The new multi-GPU and CUDA-aware MPI-based formulation is benchmarked against a multi-CPU core and classical MPI-based counterpart, demonstrating a remarkable acceleration in each and every stage of the computation of ultrasonic wave propagation, namely matrix assembly, time integration and message exchange. More importantly, both the computational efficiency and the degree-of-freedom limit of the new formulation are actually scalable with the number of GPUs used, potentially allowing larger structures to be computed and higher computational speeds to be realized. Finally, the new formulation was used to simulate the interaction between Lamb waves and randomly shaped thickness loss defects on plates, showing its potential to become an efficient, accurate and robust technique for addressing the propagation of ultrasonic waves in realistic engineering structures. [ABSTRACT FROM AUTHOR]

In this paper, we propose an innovative design of an elastic network, which is capable of channelling the energy supplied by an external source towards any of its endpoints, that can be chosen arbitrarily and in advance. This system, named Mechanical Switching Network (MSN), consists of an interconnected array of branches, each of which is represented by a lattice strip endowed with gyroscopic spinners. The latter make the system non-reciprocal and, hence, are responsible for the preferential directionality exhibited by the network. We formulate and solve the forced problem for the gyro-elastic strip in the analytical form and compare the derived solutions with the results of independent finite element simulations, showing an excellent agreement. Additionally, we carry out a parametric analysis to evaluate the influence of the main parameters of the system on the uni-directional wave propagation of Lamb waves. We envisage that the proposed model can have important implications in many engineering applications, where control and tunability of guided waves play a key role. • A gyro-elastic network is designed to channel the energy to any desired endpoint. • Each branch of the gyro-elastic network has a triangular lattice micro-structure. • The response of a gyro-elastic strip to external forcing is obtained analytically. • The analytical results show an excellent agreement with finite element simulations. • A parametric analysis is used to reveal parameters crucial for one-way propagation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The phase winding of topological vortex modes with high quality factors in pillared plates are observed experimentally. • The topological vibration modes are strongly robust against the random perturbation of resonant frequencies of pillars. • The frequency of the topological vortex mode is sensitive to added small-mass objects. Localized topological modes with high robustness to various perturbations are receiving increasing attention. Recently, zero-order topological vortex modes have been designed in phononic structures, in analogy with zero-energy fermionic states modulated by Jackiw-Rossi binding mechanism. Such localized modes may have potential applications for biosensing, bioimaging and on-chip communication. In this work, we propose a pillared phononic plate with a Kekulé distortion of pillars position to bind topological modes at a vortex core via dispersion engineering. The phase winding and amplitude diagrams of the topological vortex mode are observed experimentally. It is found that existence of vibration peaks and corresponding mode patterns are strongly robust against the random perturbation of resonant frequencies of pillars at the vortex core. We further design a topological resonant sensor for mass sensitivity. The frequency of the topological vortex mode is almost linearly sensitive to added small mass at the vortex core in terms of mass values and positions. The proposed pillared plates provide a platform for potential applications such as energy localization and harvesting, remote health monitoring. [ABSTRACT FROM AUTHOR]

Guided waves, especially Lamb waves, propagating in plate-like structures with constant thicknesses have been investigated intensively in the past decades in the context of structural health monitoring (SHM) and non-destructive testing (NDT). However, structures with variable thicknesses are also very common in practice, such as ship hulls whose thickness varies for satisfying various load conditions, and defect identification for such structures is also of importance. In this paper, we compare the propagation characteristics of both Lamb modes and SH 0 mode in variable thickness plates and propose to use the SH 0 mode for defect identification in such waveguides because of its non-dispersive property. First, the non-dispersive property of the SH 0 mode in waveguides with variable thickness is demonstrated in theory. Then, a wave prediction method is adopted to illustrate the differences between the Lamb modes and the SH 0 mode when propagating in waveguides with constant or variable thickness. Finally, experiments are conducted on a variable thickness plate to verify the non-dispersive property of the SH 0 mode and further compare the results with those of the Lamb modes. Recently developed thickness-poled omnidirectional shear horizontal piezoelectric transducers (OSH-PTs) are used to produce pure SH 0 waves. It is observed that the velocity of the Lamb modes may change significantly and the group velocity of the S 0 mode even decreases by 55.1% during propagation, which hampers Lamb modes from defect identification in variable thickness plates. By contrast, the SH 0 mode is non-dispersive thus promising for damage detection, and its effectiveness for defect localization in such structures is validated experimentally. [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]

• A eulerian compressible fluid model combines the isentropic single fluid model to solve shock wave propagation and water cavitation of water-filled double plate model. • A freestanding or softly supported outer plate has a minimal effect on the impulse transmitted to the inner plate and its motion. • Stiff support spring of the outer plate can reduce the shock wave transmitted to the gap water and also the motion of the inner plate. • Cavitation initiates in gap water near the inner plate and spreads towards the external water. • Stiff support spring of the inner plate will suppress water cavitation. Water-filled double hulls are a prevalent design for underwater warships. Studying the propagation of underwater shock wave in such structures is essential to comprehend their shock resistance. The aim of this paper is to investigate the propagation of underwater shock wave and water cavitation in a water-filled double plate. To achieve this objective, the paper establishes a Eulerian compressible fluid model that combines the isentropic single fluid model and Newton's second law. The accuracy of the numerical solver is confirmed through validation tests. The reflection and transmission characteristics of shock wave, the dynamic response of the plate, and the cavitation in external and gap water are analyzed. It also examines the effects of the supporting springs of outer and inner plates and the area density of the outer plate on the shock wave propagation and cavitation evolution. The findings indicate that the transmitted wave in the gap water is influenced by the area density and supporting spring of the outer plate. A thicker outer plate can reduce the peak value of the transmitted wave, and a stiffer supporting spring can decrease the transmitted impulse. On the other hand, if the outer plate is freestanding or softly supported, it has minimal effects on the response of the inner plate. Moreover, the water cavitation is determined by the supporting spring of the inner plate in the case of a softly supported outer plate, and the cavitation initially develops in the gap water near the inner plate before expanding towards the external water. When the supporting spring of the outer plate is stiff, it significantly reduces the occurrence of cavitation in the external water, while the cavitation in the gap water is still determined by the support spring of the inner plate. These research findings are valuable for analyzing the shock resistance of typical double-hull submarines. [Display omitted] [ABSTRACT FROM AUTHOR]

• The time–frequency diagrams are obtained based on the Normal Time-Frequency Transform, and the characterization effect on Lamb waves is discussed. • A method to characterize signal differences by Singular Value Decomposition and Dynamic Time Warping is presented. • A new damage index is developed to quantify the damage in composite structures, and the matching performance of damage trends is investigated using experimental tests. Damage index (DI) is of vital importance for the health monitoring of composite structures. However, most of the existing DI construction methods cannot reveal the rich information hidden in Lamb waves, such as the time-varying characteristics of the signal amplitude and phase, and thus cannot well characterize the evolutionary trend of damage. To address this limitation, this study proposes a novel DI construction method for damage identification in composite structures. The normal time–frequency transform is first applied to obtain the unbiased time–frequency distribution of the received Lamb wave signal, and then the singular value decomposition method is used to obtain the singular value sequence of the two-dimensional time–frequency matrix to characterize its matrix sparsity. Next, the dynamic time warping method is used to measure the distance between singular value sequences of the current and baseline signals. A quantitative DI is finally defined using these distances. Two experimental studies are utilized to validate the effectiveness of the proposed method. Results show that the proposed method can reveal the rich information hidden in the Lamb wave signals and has better trend-matching performance in fitting the actual damage propagation trend than traditional methods. [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 real time-reversal operation based on the Lamb wave to detect the damage rather than virtual. • A multi-modal superposition finite difference time domain method is proposed to simulate the Lamb waves propagation with little calculation cost, which facilitates damage imaging output quickly. • A maximum energy frame method is presented to automatically determine the focusing time from wavefield animation, and enabling the detection of multiple damage points. • A detailed comparison between the proposed method and other four Lamb wave-based damage detection methods is evaluated. This study proposes a wavefield reconstruction method with a time-reversal operation (W R -TR) based on the Lamb wave to detect the damage in the plate. Currently, it is challenging to implement the wavefield reconstruction method for damage detection because of two issues. One is how to quickly simulate the Lamb wavefield. The other is how to determine the focusing time for searching the desired frame from wavefield animation, which can show the location and size of damage. In response, this study introduces a multi-modal superposition finite difference time domain method (MS-FDTD) to simulate the Lamb waves propagation with little calculation cost, which facilitates damage imaging output quickly. In addition, a maximum energy frame method (MEF) is presented to automatically determine the focusing time from wavefield animation, enabling the detection of multiple damage points. The simulations and experiments have demonstrated good noise robustness, anti-distortion ability, and broad applicability with dense or sparse array layouts. Moreover, a detailed comparison between the proposed method and other four Lamb wave-based damage detection methods is evaluated in this paper. [ABSTRACT FROM AUTHOR]

Anisotropy of optical parameters of TaTlS 3 and TaTlSe 3 is investigated through first principles calculations. Indirect bandgap quasi-one-dimensional semiconductors TaTlS 3 and TaTlSe 3 have bandgaps of 1.15 eV and 0.84 eV, respectively. Important optical parameters are found to be strongly influenced by the radiation direction of the incident beam, as indicated by large birefringence and dichroism. Large optical anisotropy suggests promising nonlinear and linear optoelectronic applications for these materials, including wave plates, polarizers and phase-matching devices. • Large optical anisotropy is reported for quasi 1D TaTlX 3 (X = S, Se) materials • The optical properties are investigated through the DFT method using HSE06 functional • TaTlX 3 exhibit large birefringence values in the energy range 1−6 eV • TaTlX 3 exhibit large absorption coefficient in the visible and UV regions • Optical parameters of TaTlX 3 are suitable for optoelectronic applications [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]

• The mechanism of the action of corrugated plates on solitary waves was studied. • A predicted model for the film thickness under turbulent conditions is proposed. • The prediction model considers wall structural parameters and Re l. The effect of wall distortion of the scrubbing-cooling tube on turbulent falling film is significant. The wall distortion was simplified by using corrugated plates; the effect of structural parameters (amplitude and wavelength) of corrugated plates on the action mechanism of solitary waves and the axial distribution of liquid film thickness were investigated. The results show that the Coanda effect decelerates and accelerates the flowing liquid film attached to the wall, which leads to the generation and extinction of solitary waves on the liquid film. The intensification of the local perturbation easily promotes the solitary waves to form slender water columns. Under the effect of Plateau-Rayleigh instability, the tails of the slender water columns break up to form tiny droplets. The mean liquid film thickness prediction model can accurately predict the mean liquid film thickness on corrugated plates (amplitude/wavelength = 0.02 ∼ 0.08) and the relative errors within ± 15%. [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]

• Legendre Orthogonal Polynomial Expansion (LOPE) method mapping the relationship between the wave velocity and initial stresses was developed to generate the input characteristics of linear and non-linear variation for deep neural network. • A Deep Neural Network (DNN) was combined with LOPE for the inverse of initial stress based on Lamb wave dispersion date rather than objective function obtained from wave equations. • The performances of ReLU activation function and Sigmoid activation function were illustrated in detail for current inverse work. The prediction of the initial stress in composites is essential for the non-destructive testing (NDT) and structural health monitoring (SHM) of carbon fibre reinforced polymer (CFRP). This paper examines the potential of Lamb waves in the inverse of initial stress by calculating the influence of initial stress on the dispersion characteristics of Lamb waves propagating in multilayered CFRP laminates. By introducing the mechanics of incremental deformation into the linear three-dimensional elasticity theory, the Legendre orthogonal polynomial expansion (LOPE) method is used to mathematically model the Lamb wave propagating in multilayered CFRP laminates subjected to horizontal and vertical homogeneous initial stresses. Then, a three-hidden-layers Feed Forward Deep Neural Network (DNN) with Back Propagation (BP) algorithm is constructed to invert the magnitude and direction of the initial stresses. The input features are the phase velocities of fundamental Lamb wave A 0 mode at five different frequencies. Both training and testing samples are obtained by LOPE forward calculation. An ablation experiment is presented to compare the two different activation functions. Finally, the accuracy of the inverse is verified by comparing with the available outcomes of LOPE forward calculation. [ABSTRACT FROM AUTHOR]

• Measurement of the reflective function of highly attenuative material by using a pair of ultrasound transducers. • Transducer-pair method is proposed to acquire V(z) by using conventional ultrasound transducers with a high f-number and small aperture. • By comparing the results obtained from both the single transducer and the transducer-pair approach, the feasibility is demonstrated. Ultrasonic reflectivity using a V(z) technique is a powerful characterization method in acoustic microscopy to measure the elastic properties of materials. Conventional techniques generally use a low f-number with high frequency; however, to measure the reflectance function of the highly attenuative material, a low frequency is essential. In this study, the transducer-pair method based on Lamb waves is used to measure the reflectance function of a highly attenuative material. The results demonstrate the feasibility of the proposed method using a commercial ultrasound transducer with high f-number. [ABSTRACT FROM AUTHOR]

• A new method for computing RIs from Lamb waves with extended data length to accommodate extra indirect waves. • The damage missed by the direct wave may be exposed by the extra indirect waves. • The modified RIs demonstrate excellent sensitivity to damages and ensure a low threshold for the healthy condition. The Lamb wave reciprocity-based method is an effective candidate for the health monitoring of composite plates. However, if the damage is located symmetrically between the transmitter–receiver pair, the reciprocity still holds true and the method misjudges its presence. In the present work, a new method for computing the reciprocity index (RI) from Lamb wave signals with extended data length is proposed. This method exploits extra indirect waves that bounce one or more times between the damage and other reflectors. These waves probe the damage through different paths and from different directions. Thus, the damage missed by the direct wave may be exposed by the indirect waves. Benefiting from that, two modified RIs are defined and their performance is validated by two experimental examples. As expected, both indices demonstrate excellent sensitivity to damage even in the middle of the transmitter–receiver pair and ensure a low threshold for the pristine condition, illustrating an excellent ability to distinguish between health and illness. [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]

In this paper, we study Lamb waves propagating in a discrete strip, whose microstructure is represented by either a monatomic or a diatomic triangular lattice. In considering the in-plane vector problem, we derive an analytical solution for the dispersion relation of Lamb waves. Additionally, we investigate the main features of the eigenmodes of the system, which describe how the lattice strip vibrates at different frequencies. Further, we discuss how the dispersion properties depend on the number of the lattice's rows and on the chosen boundary conditions. For heterogeneous systems, we focus the attention on the internal stop-band and on the flat bands appearing in the dispersion diagram. Different asymptotic models are employed to approximate the low-frequency behaviour of the lattice strip, starting from the classical Euler–Bernoulli beam. The effective behaviour of a lattice strip with dense microstructure is also investigated, and we present a comparative numerical analysis with the analogous continuum for which the classical Lamb wave problem is posed. The theory developed is exploited here to design a structured medium capable of manipulating wavemodes, and, through conversion and selection, generating uni-directional wave phenomena. We envisage that the present work can fill a gap in the research field related to the analytical study of dispersive waves in microstructured media, whose dynamic performance is influenced by the presence of multiple external boundaries. • Propagation of Lamb waves in discrete elastic strips is studied. • Wave dispersion in both homogeneous and heterogeneous strips is considered. • The dispersion relations and the corresponding wavemodes are derived analytically. • Effective continuous models working in different dynamic regimes are also obtained. • A mechanical device supporting uni-directional elastic waves is proposed. [ABSTRACT FROM AUTHOR]

• A WDR method optimized tomography algorithm is developed for robust localization using Lamb waves. • WDR excitation is designed to alleviate the frequency selection problem in defect localization. • Reconstruction independent component analysis is adopted for mode separation. • Time-frequency index is calculated as the damage index in the tomography algorithm. Tomography imaging of defects using ultrasonic Lamb waves has attracted much attention in nondestructive testing of plates. However, there are two challenges for robust localization of defects: frequency sensitivity to defects, and multi-mode interference on damage index (DI) extraction. Aiming for that, a wideband dispersion reversal (WDR) method optimized tomography is developed. Pre-dispersive wideband excitations of a certain Lamb wave mode are generated based on the configuration of transducers, and reconstruction independent component analysis is used for wave mode separation. According to the acoustic reciprocity principle, self-compensation phenomenon of mode signals can be recorded on the intact path, where the signal energy concentrates at a fix self-compensation point in the time–frequency domain, yet such compensation effect would be impaired encountering defects. Thus, a time–frequency index can be used as the DI, which is calculated based on the weighted Euclidean distance from the self-compensation point to the pixel points in the time–frequency domain. Experimental results show the WDR optimized tomography has a robust performance for magnet-simulated defects imaging without excitation frequency optimization. [ABSTRACT FROM AUTHOR]

The effects of laser-induced heat on the performance degradation of liquid crystal (LC) devices are critically important in applications involving laser systems. In this study, we investigated the influences of 1064-nm continuous-wave (CW) laser loading on an LC variable retarder (LCVR). The results show that thermal deposition initially causes an overall downward shift in the electronically controlled phase retardation curve. This phase retardation reduction is restored under a certain voltage and laser power by reducing the driving voltage. For common nematic LC wave plates that do not require voltage application, the decrease in phase retardation can be pre-compensated by increasing the corresponding LC cell thickness. For electrically controlled LC devices, the phase retardation under a high-power CW laser can be preconfigured by plotting phase response curves versus voltages at different laser powers. Based on these calibration curves, the LC devices can still service high-power CW lasers as long as the resulting temperature does not reach the clear point of the LC material. These results clarify the fundamental mechanisms of phase modulation variation induced by thermal deposition and could be useful for the fabrication optimization and effective implementation of LC devices in high-power laser systems. • Thermal effect makes the LCVR electronically controlled phase retardation curve shifts down overall. • Without voltage, the degradation of LCVR performance is attributed to the change in the refractive index of LC material. • Applying a range of voltage, thermal effect leads to an additional increase in the deflection angle of LC molecules. • For nematic passive LC waveplates, phase retardation decrease can be pre-compensated by increasing LC cell thickness. • For active LC devices, phase retardation under CW lasers can be preconfigured by calibrated phase-voltage response curve. [ABSTRACT FROM AUTHOR]

In this paper, a phase unwrapping-based local wavenumber imaging method is proposed for delamination detection in the composite plate. Local wavenumber imaging has shown significant advantages in damage localization and quantification. However, a dense of spatial measurements and a high requirement of sampling intervals are crucial for conventional spatial Fourier transform methods. To address these challenges, a phase unwrapping method adopting low-sampled wavefields for local wavenumber estimation is proposed for damage imaging. Specifically, the wavefield sampling process can divide the detection area into various grids while making each grid consist of four neighbor measurements. Under the far-field assumption, the wave path distances of grid measurements can be approximately obtained by projecting their real distances along the wave vector direction. Meanwhile, by revealing the phase differences of grid measurement pairs with the low-sampled wavefield data, the frequency-related wavenumbers are estimated subsequently. Hence, the local wavenumber can be recovered robustly by averaging the estimated wavenumbers from all measurement pairs in the grid. On this basis, the structural damage can be geometrically imaged and quantitatively evaluated. Both the simulation study and the experimental investigation have shown the efficiency of the phase unwrapping-based method for local wavenumber imaging, which demonstrates the strong capability of damage localization and quantification in composite structures with low-sampled wavefield. • An improved phase unwrapping method is proposed for local wavenumber estimation with less measurements. • The proposed method can provide a high-quality wavenumber image with low-sampled wavefields. • The location, size, and depth can be accurately evaluated for the composite delamination. [ABSTRACT FROM AUTHOR]

In this paper, a modified fisher discriminant criterion is used to identify and rank damage sensitive features for debonding classification. For this purpose, the analysis of the correlation between features (ACF) based on Pearson correlation coefficient and the Fisher discriminant ratio (FDR) criterion were combined to select the most relevant features. The debondings classification was evaluated in a multiclass classification by considering three different classes of damage and one class of undamaged. The nondestructive inspection was performed in a composite-adhesive-composite lap-joint using guided Lamb wave testing with angle beam transducers mounted in a pitch-catch configuration. From the collected data, twelve features were extracted by using statistical functions and signal processing techniques. From the extracted features, the most damage sensitive features were selected based on the proposed method (ACF and FDR) and the results were compared using the conventional FDR method. For three different classifiers, the feature selection strategies were tested and their performance evaluated using the metrics: accuracy, precision, recall and F1 score. For the dataset collected in this study, the results support that the proposed method yields an improvement of the performance of the classifiers in comparison with the use of the conventional FDR-based feature selection. [ABSTRACT FROM AUTHOR]

Numerous Lamb wave dispersion curve estimation methods have been developed to support damage detection and localization strategies in non-destructive evaluation/structural health monitoring (NDE/SHM) applications. In this paper, the covariance matrix is used to extract features from an ultrasonic wavefield imaging (UWI) scan in order to estimate the phase and group velocities of S0 and A0 modes. A laser ultrasonic interrogation method based on a Q-switched laser scanning system was used to interrogate full-field ultrasonic signals in a 2-mm aluminum plate at five different frequencies. These full-field ultrasonic signals were processed in three-dimensional space-time domain. Then, the time-dependent covariance matrices of the UWI were obtained based on the vector variables in Cartesian and polar coordinate spaces for all time samples. A spatial covariance map was constructed to show spatial correlations within the full wavefield. It was observed that the variances may be used as a feature for S0 and A0 mode properties. The phase velocity and the group velocity were found using a variance map and an enveloped variance map, respectively, at five different frequencies. This facilitated the estimation of Lamb wave dispersion curves. The estimated dispersion curves of the S0 and A0 modes showed good agreement with the theoretical dispersion curves. [ABSTRACT FROM AUTHOR]

This paper describes a completely non-contact ultrasonic method of gas flow metering using air-coupled leaky Lamb waves. To show proof of principle, a simplified representation of gas flow in a duct, comprising two separated thin isotropic plates with a gas flowing between them, has been modelled and investigated experimentally. An airborne compression wave emitted from an air-coupled capacitive ultrasonic transducer excited a leaky Lamb wave in the first plate in a non-contact manner. The leakage of this Lamb wave crossed the gas flow at an angle between the two plates as a compression wave, and excited a leaky Lamb wave in the second plate. An air-coupled capacitive ultrasonic transducer on the opposite side of this second plate then detected the airborne compression wave leakage from the second Lamb wave. As the gas flow shifted the wave field between the two plates, the point of Lamb wave excitation in the second plate was displaced in proportion to the gas flow rate. Two such measurements, in opposite directions, formed a completely non-contact contra-propagating Lamb wave flow meter, allowing measurement of the flow velocity between the plates. A COMSOL Multiphysics® model was used to visualize the wave fields, and accurately predicted the time differences that were then measured experimentally. Experiments using different Lamb wave frequencies and plate materials were also similarly verified. This entirely non-contact airborne approach to Lamb wave flow metering could be applied in place of clamp-on techniques in thin-walled ducts or pipes. [ABSTRACT FROM AUTHOR]

Nonlinear ultrasonic is known to be a promising technique to characterize the microstructural degradation in engineering materials. This work demonstrates the use of nonlinear Lamb wave mixing technique to assess the localized deformation in modified 9Cr 1Mo steel during creep. Two Lamb wave modes of different frequencies (ω 1 & ω 2 ) are allowed to mix within the material under certain resonant condition to generate third type of harmonic waves of frequencies (ω 1 ±ω 2 ). This new generated wave carries the information of material nonlinearity from the mixing site and independent of the other extraneous nonlinear factors. Amplitude of the generated third wave depends on the third order elastic constants of the material. This study reveals that nonlinear Lamb wave mixing technique could be used to assess the localized deformation much prior to its failure. [ABSTRACT FROM AUTHOR]

Pyrotechnic release devices are widely used in various missions in order to reliably separate the structural parts of the system. However, the pyroshock induced by an explosive pyrotechnic event can lead to fatal malfunctions of the adjacent electrical components. In order to mitigate these pyroshock issues, an accurate understanding of the pyroshock propagation in structures is essential. In this study, an experimental setup for the pyroshock propagation experiments is developed with the pyroshock excitation using pyrotechnic initiators. The pyroshock propagation analysis environment with a commercial hydrocode is established and verified through comparison between the analysis and the experimental results. The pyroshock propagates through the thin plates in the form of flexural waves (or anti-symmetric Lamb waves). Using the established numerical and experimental techniques, the effects of the pyroshock attenuation by the joints and washers are investigated. The plates connected by joints with different materials and the plates connected by joints with inserted washers made of different materials and in different thickness are considered. The experimental and numerical results are in good agreement: the pyroshock attenuation is highly effective when the joints are made of higher density and stiffness materials and when the thickness of the washers is increased. The primary reason for the pyroshock attenuation due to the joints and washers is the flexural wave reflection at the discontinuities caused by acoustic impedance mismatching. [ABSTRACT FROM AUTHOR]

We propose a phase-shifting digital holography (DH) method that uses a liquid crystal (LC) retarder for use in the measurement of moving objects. Because LCs are birefringent, they can be used as waveplates for phase-shifting operations. In addition, the phase retardation can be adjusted by varying the LC drive voltage. Mechanical operations are unnecessary in phase shifting when using LCs, but fast interference fringe measurements are possible. The proposed method can be realized by incorporating the LC retarder into a conventional interferometer and is both simple and inexpensive. We performed experiments to verify the effectiveness of the proposed method and confirmed that the method is effective for imaging of the complex amplitude distributions of moving objects. [ABSTRACT FROM AUTHOR]