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Real-time monitoring of structural health status is very important for supersonic aircraft, since the long-term operation in harsh service environments, such as high temperatures, can cause potential damage. Structural health monitoring (SHM) based on lamb wave has been approved to be a critical technique for the reliable operation of supersonic aircraft, but is limited by the operating temperature of conventional piezoelectric sensors, which makes it difficult to meet the high-temperature monitoring requirements. The BiFeO 3 –BaTiO 3 ceramics exhibit excellent piezoelectric properties and high Curie temperature (T c), making them promising for high-temperature SHM of supersonic aircraft. Here, we fabricated a piezoelectric sensor using 0.67BiFeO 3 -0.33BaTiO 3 -0.25%molMnO 2 (BF33BTMN-025) ceramic, which exhibits a piezoelectric charge constant (d 33) of 135 pC/N at room temperature and a T c of 448 °C. The BF33BTMN-025 ceramic sensor operates effectively at temperatures up to 200 °C and maintains stable damage monitoring sensitivity as well. Compared with Pb(Zr, Ti)O 3 (PZT), BaTiO 3 (BTO) and other piezoelectric sensors commonly used in SHM field, the BF33BTMN-025 ceramic sensor has a higher operating temperature and a comparable sensitivity. Our work demonstrates that the BF33BTMN-025 ceramic sensor can be employed as piezoelectric sensor for high-temperature SHM, which has a wide application potential in the field of real-time monitoring of supersonic aircraft. [ABSTRACT FROM AUTHOR]

• The 2022 Tonga eruption confirmed that explosions can produce air-driven tsunamis • Lamb waves from large explosions are one way to generate and drive global tsunamis • Airbursts from comets and asteroids can produce tsunamis by atmospheric coupling • Several mechanisms are proposed for air-driven tsunami generation and amplification • Impact plume collapse, vortices, and steam blowoff are hypothetical tsunami sources [ABSTRACT FROM AUTHOR]

• We characterized the pressure wave produced by the Tonga volcano using cosmic rays. • We believe this is the first time that a Lamb wave is studied using cosmic rays. • Our high-altitude data provides additional information about this unique event. High-energy cosmic rays that hit the Earth can be used to study large-scale atmospheric perturbations. After a first interaction in the upper parts of the atmosphere, cosmic rays produce a shower of particles that sample it down to the detector level. The HAWC (High-Altitude Water Cherenkov) gamma-ray observatory in Central Mexico at 4,100 m elevation detects air shower particles continuously with 300 water Cherenkov detectors with an active area of 12,500 m2. On January 15th, 2022, HAWC detected the passage of the pressure wave created by the explosion of the Hunga volcano in the Tonga islands, 9,000 km away, as an anomaly in the measured rate of shower particles. The HAWC measurements are used to determine the propagation speed of four pressure wave passages, and correlate the variations of the shower particle rates with the barometric pressure changes. The profile of the shower particle rate and atmospheric pressure variations for the first transit of the pressure wave at HAWC is compared to the pressure measurements at the Tonga island, near the volcanic explosion. By using the cosmic-ray propagation in the atmosphere as a probe for the pressure, it is possible to achieve very high time-resolution measurements. Moreover, the high-altitude data from HAWC allows to observe the shape of the pressure anomaly with less perturbations compared to sea level detectors. Given the particular location and the detection method of HAWC, our high-altitude data provides valuable information that contributes to fully characterize this once-in-a-century phenomenon. [ABSTRACT FROM AUTHOR]

• Bending edge wave is studied in a thin transversely isotropic viscoelastic plate supported by a viscoelastic foundation. • The nonlocal elasticity theory is used to extract the micro-scale effect on the travelling edge wave. • The Kirchhoff plate theory and the sinusoidal waveform are considered to obtain the closed-form solution. • It is observed that due to the present of viscosity in the foundation the speed of the wave is reduced. The analysis consists of the dynamic behaviour of a thin semi-infinite functionally graded plate under bending edge wave propagation. The plate is made of transversely isotropic materials supported by an elastic foundation. A quadratic variation of the properties of the materials is used to examine the behaviour of the FGM (Functionally Graded Materials) plate in the bending edge wave propagation. The Eringen differential form is employed to transition from non-local to local elasticity. The parameter entailed in the differential form is used to extrapolate the microstructure's impact on the bending edge wave propagation in the functionally graded plate. Additionally, the surface elasticity theory is included to investigate the surface effects on the dispersion of edge wave. The viscosity effect of the plate and the foundation are taken into consideration while formulating the problem. To develop the kinematics of FGM plates, the Kirchhoff plate theory is considered. The motion of the plate is taken along the transverse direction of the plate. The viscoelastic theory proposed by Kelvin-Voigt is used to formulate the mathematical theory of the proposed model. A coupled equation corresponding to the moments and shear forces has been obtained while formulating the plate equation of motion. The rotatory inertia effect is neglected due to the small deflection of the plate. The numerical simulation presented in the paper shows the influence of the viscosity of the elastic foundation, surface effects, non-local elasticity, and the density of the materials on the natural frequency of bending edge wave propagation. [ABSTRACT FROM AUTHOR]

• Novel mathematical model for wave-particle drag effects in semiconductor plates. • Extended Mindlin method for coupled flexoelectric semiconductor plates. • Explicit coupling mechanisms in different waves (extensional, thickness-stretch, bending, and thickness-shear). • New 3D dispersion relations for Lamb waves in flexoelectric semiconductor plates. • New analytical and simple solutions with high accuracy at small wavenumbers. In this paper, the infinite power series of two-dimensional (2D) differential equations for flexoelectric semiconductor plates are derived by extending the Mindlin method. The zeroth-order and first-order theories are systematically presented and applied to the plate made of cubic crystals of class m3m, identifying explicitly the coupling among wave modes and charge distributions. More specifically, it is observed that the extensional and thickness-stretch waves are coupled with axial motions of free carriers; bending and fundamental shear are coupled with the thickness motions of free carriers, showing the dynamic phenomena of the flexoelectric electronic (flexoelectronic) effect; and the shear-horizontal waves, such as face-shear and thickness-twist, are decoupled. Furthermore, when comparing with the three-dimensional (3D) dispersion relations in the flexoelectric semiconductor plate which are also derived for the first time in this paper, the simple 2D theory is shown to be amazingly accurate. For instance, for an infinite plate of thickness 2 h = 0.2 µm, the extension, bending, and shear modes predicted from the present 2D theory agree well with the 3D benchmarks when the actual wavelength is larger than 1.26 µm, i.e., wavelength is at least more than six times of the thickness. To illustrate the flexoelectricity-based wave-particle drag effects, the wave modes and charge distributions are plotted. Due to its simplicity and computational efficiency, the present 2D theory provides us a powerful tool in understanding the physical mechanism of wave-particle drag in flexoelectric semiconductor plates and analyzing acoustoelectric devices when the flexoelectric effect is involved. [ABSTRACT FROM AUTHOR]

• Thermoelastic Lamb waves in functionally graded nanoplates are studied based on integral form of modified nonlocal theory. • The Legendre polynomial approach is improved based on the proposed iterative solution integration method. • The escape frequency, which appears in differential model of modified nonlocal theory, does not exist in the integral one. • Nonlocal effect on attenuation is more notable than that on phase velocity. • Nonlocal effect strengthens attenuations before their peak frequency, but then weakens them. Based on the integral form of the modified nonlocal theory, the dispersion and attenuation characteristics of thermoelastic Lamb waves in functionally graded material nanoplates are investigated. The improved Legendre polynomial series approach is presented to conquer the difficulty of the very time-consuming integration calculation, which contains the nonlocal factor, the Legendre polynomials, rectangular window function and their derivative. An iteration solution integration method is derived by using the properties of Legendre polynomials and the integration by parts. The proposed approach possesses the advantages of a small scale of the characteristic matrix and can directly obtain the complex wave number solutions without iteration. Comparisons with the available data indicate the validity of the proposed approach. Numerical examples show that the nonlocal effect on attenuation is more notable than that on phase velocity, especially at the frequency of the maximal attenuation. Besides, the nonlocal effect is very weak at the low frequency. Importantly, the escape frequency, which appears in the results from the differential form of modified nonlocal theory, does not exist in the integral form of modified nonlocal theory. In fact, the escape frequency originates from the approximate transformation from integral model to differential model. [ABSTRACT FROM AUTHOR]

The article presents the results of a study of perforated plates effectiveness to ensure safety in case of emergencies on spacecraft and launch pads. The degree of shock wave attenuation by perforated plates with different permeability coefficients has been studied experimentally and numerically. Experiments were carried out with conical shock tube, which was used to produce spherical shock waves of varying intensity, and perforated plate with a permeability coefficient of 41% and. Shock wave reflection and attenuation coefficients were obtained. To study the qualitative picture of a shock wave interaction with perforated plate, a numerical model using Ansys Fluent was created and verified. The numerical model made it possible to qualitatively evaluate the breakup and coupling of a shock wave front after passing perforated plate. The degree of a shock wave attenuation by plates with permeability coefficients of 29% and 20.6% was numerically studied. It was shown that perforated plates can effectively reduce a shock wave overpressure. • Perforated plates effectively reduce shock wave overpressure. • Shock wave attenuation nonlinearly depends on permeability coefficient. • Perforated plates do not change shock wave positive phase duration. • Shock wave breakup and coupling does not depend on characteristic size of hole. [ABSTRACT FROM AUTHOR]

Face-shear deformation was observed in various square piezoelectric transducers prepared using the anti-parallel co-poling method, which led to uniform polarization and the generation and receipt of pure fundamental shear horizontal (SH 0) waves without any Lamb wave. The amplitude of the SH 0 wave was found to be proportional to the piezoelectric charge constant (d 33) of the piezoceramic, suggesting that the d 33 value can be used as a figure of merit for piezoelectric transducers with strong SH 0 waves. The amplitude of the SH 0 wave associated with the square piezoelectric transducer is large at 0° and 90° but almost zero at 45°; therefore, a piezoelectric transducer with good omnidirectional properties needs to be developed. A ring-type omnidirectional piezoelectric transducer was produced using eight trapezoidal piezoceramics. The minimum amplitude of the SH 0 wave generated and received by the ring type-transducer exceeded 70% of the maximum amplitude. Hence, this ring-type transducer is highly omnidirectional and can be used for nondestructive testing and structural health-monitoring applications. [ABSTRACT FROM AUTHOR]

• When a plate is submerged in a fluid, no wave blocking occurs in the plate mode irrespective of compressive force. • Multiple propagating modes occur due to wave blocking when a submerged plate rest on a Winker foundation. • In wave scattering by articulated submerged plate, energy relation depends on wave amplitude and the energy transfer rate. • Discontinuities in the reflection and transmission coefficients occur at blocking and saddle points in wave scattering. • Irregularities in plate deflection occur due to the superposition of multiple propagating waves within blocking frequencies. Surface gravity wave interaction with an articulated flexible submerged plate in the presence of lateral compressive force is investigated within the framework of blocking dynamics, assuming that the submerged plate is resting on a uniform Winkler foundation. The problem is studied considering small-amplitude plate response and linearised water wave theory in finite water depth. The loci of the dispersion relation reveal the occurrence of wave blocking in the submerged plate mode for a range of wave frequency and compressive force when the submerged plate is resting on a Winker foundation. However, in the absence of the Winkler foundation, no wave blocking occurs in the plate mode irrespective of the values of compressive force, and plane wave propagation in plate mode does not happen for particular values of compressive force. Four propagating wavenumbers exist within the blocking frequencies, of which one is in the surface mode, and others are in the submerged plate mode. Thus, to account for the four propagating wave modes, the existing solution approach for a pair of propagating wave modes is modified in an appropriate manner within the limit of blocking frequencies. Consequently, the energy relation is obtained using Green's identity, in which the reflection and transmission coefficients are a combination of the amplitudes of different reflected and transmitted wave modes and the associated energy transfer rates. The reflection coefficient in the submerged plate mode jumps from one to zero for very small values of frequency due to the introduction of the Winkler foundation in the presence of compressive force. In addition, zero wave reflection occurs for a higher frequency range with an increase in the values of the Winkler constant. Removable discontinuities are observed at the blocking and saddle points, whilst jump discontinuities occur in the reflection and transmission coefficients due to the change of incident wave mode within the blocking frequencies. Linear time-dependent displacements in surface and submerged plate modes are presented prior to the blocking limit. [ABSTRACT FROM AUTHOR]

• Semi-analytical method is developed for dispersion of composite plates periodically attached with ABH resonators. • Equivalent dynamic stiffness of the ABH resonator is obtained analytically. • Dynamic coupling between host plate and resonator is described innovatively in terms of waves. • Symplectic method and WKB method are combined to establish the eigenproblem. A semi-analytical method is developed to calculate the wave propagation properties of thin orthotropic composite plate structures periodically attached to acoustic black hole (ABH) resonators. First, the free and forced vibration of the host orthotropic plate is described in terms of the analytical wave shapes obtained by the symplectic method. Second, a wave-based description is provided for the ABH resonators using the higher-order WKB method. Third, the equivalent dynamic stiffness of the resonator is obtained by considering the deformation-force relationship at the connections. Finally, an eigenvalue problem is established for the dispersion of the periodic plate based on wave propagation analysis, where the dynamic coupling between the host plate and resonator is innovatively described in terms of directly excited waves. Because the wave-based derivations in the proposed method are analytical, the calculations highly efficient and accurate, and were validated through several numerical examples. [ABSTRACT FROM AUTHOR]

The application of machine learning techniques based on Lamb waves for structural health monitoring of composite material damage has been widely adopted. However, this method often struggles to acquire sufficient labeled historical Lamb wave signals for training machine learning models. This issue can be addressed through the implementation of transfer learning. This paper proposes a data-driven transfer learning model for locating coordinates of damage areas in plate-like structures with complex geometric features such as rivets and grooves. Based on the differences in conditional distributions in various structural health monitoring scenarios, a Cross-Component Regression for Damage Localization Model (CRDM) is introduced for damage localization across components. Due to the limited availability of labeled target data, CRDM aims to preserve global properties of the target data-dominated conditional distribution, besides considering the predictive accuracy of individual samples. Therefore, a hybrid loss function is constructed by combining Mean Squared Error (MSE) and Conditional Embedding Operator Difference (CEOD). Subsequently, the target model is fine-tuned based on this loss function. Experimental validations are conducted on finite element models and real datasets, confirming the effectiveness of the CRDM model in damage localization for plate-like structures. • Lamb waves are employed to assess the damage in CFRP composite material plate structures. • The influence of external conditions on Lamb waves is taken into consideration. • Transfer learning based on regression problems is utilized to predict the damage location. • The model is fine-tuned from a mapping-based perspective. [ABSTRACT FROM AUTHOR]

• Elastic spin is explicitly derived and experimentally observed for all basic modes in rods. • Unidirectional wave routing is observed due to spin-momentum coupling. • Asymmetric transmission of longitudinal mode requires extremely elliptical chiral source. • Spin-momentum coupling of torsional mode depends on cross-section warping effect. The unusual topology of elastic waves in continuum systems has been recently uncovered. Prominently, the spin of Rayleigh-Lamb waves has been experimentally observed and used to achieve the unidirectional wave propagation. In this work, the elastic wave spin in thin rod systems is theoretically predicted and experimentally observed, for all the three basic wave modes, i.e., the zero-order flexural, longitudinal and torsional modes. By elaborately building up the subwavelength chiral sources, the spin-momentum locking has been achieved for these three modes within broad frequency range. Interestingly and surprisingly, the spin-momentum locking occurs for flexural and longitudinal modes in both circular and non-circular thin rods; the spin-momentum locking for torsional modes only exits in non-circular thin rods, due to the warping effect in non-circular cross-section rods. Our results open up the door for elastic wave spin in thin rod systems, and provide general tips for the research on basic modes in diverse thin rods, based on either the continuum media or the artificial metamaterials in future. [Display omitted] [ABSTRACT FROM AUTHOR]

The process of formation and subsequent propagation of transverse cracks in 90° plies of carbon-fiber laminated composites was studied using modal acoustic emission approach and digital image correlation techniques. The results from modal acoustic emission approach, which included a newly developed processing tool for acoustic emission waveforms, provided information for identification and subsequent characterization or localization of signals originating from transverse cracking by analysis of the separated flexural and extensional Lamb wave modes in terms of their modal parameters. The digital image correlation method served as a verification tool of the acoustic emission data outputs in the terms of activity of significant localized events originating from the formation of the transverse crack in the 90oply. This made it possible to specify more locally the accompanying activity belonging to the formation or propagation of the magistral transverse crack. The manuscript also presents results related to the evolution of flexural/extensional wave modal parameters as the function of loading force for both [0/0/0/90]S and [90/0/0/0]S panels. It can be concluded that the detection of transverse cracks requires the need for applying a more complex acoustic emission data analysis methodology, while the standard parametric analysis, including the waveform peak frequency, is not sufficient. The presented methodology may serve as a basis for development of robust analysis tool capable of detecting the investigated phenomena. [Display omitted] [ABSTRACT FROM AUTHOR]

A jet-jet interaction technique using multiple supersonic fluidic injectors can reduce a shock cell length generated from a supersonic jet. In this study, the effects of the supersonic jet-jet interaction technique on an underexpanded jet impinging on a perpendicular plate was experimentally investigated under an atmospheric pressure. Four supersonic sub jets of Mach 1.5 interacted perpendicularly with a Mach 2.0 main flow at nozzle pressure ratio of 8.5, and the main flow impinged on several plate positions. Jet-jet interaction prevents the development of a central shock wave above the plate surface, which results in a maximum stagnation pressure decrease of 48% on the plate. Instead of the wall pressure decrease, the axial thrust of the main jet is decreased by jet-jet interaction; however, it is less than 6.1%. Jet-jet interaction causes a radial expanded main jet that prevents the flow fluctuation downstream of the jet-jet interaction area as well as in the vicinity of the plate surface because jet-jet interaction negates the oscillating shock cells. Hence, the jet-jet interaction technique is useful for various engineering applications that have issues due to the surface pressure increase and flow fluctuation. • A supersonic jet interaction on the impingement characteristics were investigated. • Jet-jet interaction prevents the surface pressure increase and flow fluctuation. • The axial thrust of a main jet is decreased by jet-jet interaction. [ABSTRACT FROM AUTHOR]

Zero-Group-Velocity (ZGV) guided mode has recently received particular attention because of its highly sensitive to structural changes, such as defects. Parallel to ZGV, in the present work, we introduce a new guided mode so-called Minimum-Group-Velocity (MGV) that is a remarkable phenomenon in defect detection applications. Strictly speaking, when any two phase-velocity dispersion curves of Lamb modes approach each other without overlapping, we observe this new mode in group-velocity dispersion curves. It is worth noting that this MGV mode has never been addressed in literature. We compute the Lamb-like modes in asymmetric viscoelastic sandwich plates using the Stiffness Matrix Method (SMM). We show that the change in properties of this sandwich can dramatically affect frequency ranges associated with ZGV and MGV modes. However, a linear correlation between phase velocities and this change on viscoelastic properties is discussed, where we observed an interconnection phenomenon in the dispersion curves. This interconnection happens between specific points on the dispersion curves, each corresponding to a ZGV- or MGV-mode. On the other hand, we verify that the detection of changes in level of viscoelastic properties or variations in the thickness of the buffer viscoelastic layer is possible within a limited segment of the damped mode curve. [ABSTRACT FROM AUTHOR]

One-dimensional shell chain structures can cause stress wave dispersion, and the dispersion ability is mainly affected by the apparent elastic Poisson ratio of the unit cell composing the chain. In this study, a novel plate–shell chain structure with a variable Poisson ratio is proposed by combining bent plates and arc shells (BPAS). The dispersive wave propagation features in the BPAS chains under striker impact are studied theoretically, numerically, and experimentally. An equivalent continuum model of the unit cell under compression in a linear-elastic deformation regime is established, which contains two material parameters, i.e., the apparent elastic modulus and the apparent elastic Poisson ratio. It is found that the unit cell can reach a relatively large Poisson ratio by adjusting its structural parameters. A continuum-based wave propagation model considering transverse inertia is developed, and an explicit analytical solution is obtained to represent the dispersion wave propagation behavior within a linear-elastic deformation regime. The model can reasonably predict the force and velocity evolution features of the chains and is verified by the numerical/experimental tests. The results show that the BPAS chain with a large Poisson ratio exhibits superior wave dispersion performances. The underlying mechanism is that more impact energy is dispersed by the transverse movement of the cells in the BPAS chain. Increasing the aspect ratio of the unit cell can improve the wave dispersion ability of the BPAS chains, achieving a high attenuation of the stress wave and is helpful for impact mitigation. • A plate–shell chain with a variable Poisson ratio is proposed for tuning wave propagation. • An equivalent continuum model of the unit cell composing the chain is established. • An analytical continuum-based wave model considering transverse inertia is developed. • The plate–shell chain can achieve strong wave dispersion due to its transverse inertia. • The wave dispersion ability of the plate–shell chain is superior to the hollow elliptical cylinder chain. [ABSTRACT FROM AUTHOR]

• Mode-entangled resonance (MER) enables mode-preserving full transmission of a Lamb wave. • Symmetric (S) or asymmetric (A) Lamb wave can pass through an asymmetric waveguide using the MER. • MER develops when S- and A-modes individually satisfy specific phase-matching conditions. • Super-enhancement of transduction efficiency for A-mode is achievable using the MER. We report a novel resonance phenomenon called the mode-entangled resonance. It can occur at a specific frequency in a plate carrying both symmetric and antisymmetric Lamb wave modes. We show that at this resonance, a symmetric or antisymmetric Lamb wave can be fully transmitted with its mode preserved from one plate to another through a matching plate geometrically misaligned with the two base plates. This phenomenon appears to be counter-intuitive because both symmetric and antisymmetric waves are generally formed in the reflected and transmitted wave fields when the misaligned plate is used as a matching plate. Our analysis shows that if both symmetric and antisymmetric Lamb wave modes at the resonance are entangled in the misaligned matching plate to satisfy the established phase matching condition, the matching plate fully transmits any wave mode across the two base plates. The discovered phenomenon is related to the classical Fabry-Perot resonance (FPR), but FPR is only applicable for single-mode transmission through a matching plate aligned with two base plates. Numerical and experimental results were presented to validate the discovered mode-entangled resonance phenomenon. Furthermore, we show that the discovered phenomenon can be used for super-amplification of the amplitude of the antisymmetric Lamb wave when it is excited by plate-surface-mounted patch transducers. [Display omitted] [ABSTRACT FROM AUTHOR]

Porous materials contain a network structure composed of distributed pores. Due to the distribution of pores, the volume of the material will be less than the actual dense volume. The material coefficients such as density and elastic modulus can change due to the influence of porosity. Therefore, the characteristics of porosity can be used as a measurement standard in the properties detection. This study explores the dispersion relationship of the wave propagation behaviour of the Lamb wave of porous material plates such as 316 L stainless steel and permeable alumina ceramic with different porosities in water fluid environment. We used laser to excite the ultrasonic wave and used probe receiving as a method to measure the guided wave dispersion relationship. The results of the study show that the effect of porosity of the porous plate on the density and elastic modulus can be shown through the dispersion relationship. The theoretical model and the experimentally measured dispersion relationship results were in good agreement. In addition, by altering the density, Poisson's ratio, shear modulus and independent porosity of the porous plate, the dispersion curve can be shifted, which can be used for the development of ultrasonic measurement in the future. [ABSTRACT FROM AUTHOR]

• The method of multiple scales is applied to analysis of internally resonant guided waves. • The amplitudes of the internally resonant guided waves are nonlinearly dependent to propagation distance. • The propagation-distance dependence of the amplitudes is affected by detuning parameters. Taking into account damping effects, the propagation-distance dependence of the amplitude of nonlinear guided waves arising from an internal resonance was investigated. In the analysis of Lamb-wave propagation, we introduced nonlinear quadratic effects derived from both geometric and material nonlinearities, referred to as classical nonlinearities, and damping effects by expressing the Lamé constants as complex numbers. Assuming a small detuning from the phase matching condition with respect to wavenumber and frequency, and using the method of multiple scales and self-adjointness of the governing equations with boundary conditions, the amplitude equations describing the nonlinear coupling of the two wave modes are derived as solvability conditions. This derivation also provides both physical and mathematical insights into both the phase and group velocity matching conditions. Numerical results obtained from these amplitude equations show that because of the nonlinear coupling an internal resonance between the two modes can occur under the above matching conditions. Without detuning, one mode increases monotonically in amplitude while the other decreases monotonically. Nevertheless, their changes gradually decrease with propagation distance, and each amplitude converges to a finite value. However, with wavenumber and frequency detuning, both amplitudes increase and decrease cyclically with propagation distance, resulting in a weakening of this resonance. Furthermore, results with damping effects show an attenuation of both amplitudes with propagation distance, which limits the distance resonance waves can propagate. [ABSTRACT FROM AUTHOR]

Active Lamb wave detection is an effective and simple monitoring method for structural health monitoring (SHM). Piezoelectric lead zirconate titanate (PZT) ceramics are most commonly used as transducers to excite and receive Lamb wave. However, due to their hardness, thickness and brittleness, PZT ceramics are severely limited in detecting damage on curved surface structures. Herein, we report that flexible PZT film sensor is an excellent candidate for receiving Lamb wave used in Lamb wave-based SHM of aircraft. An improved piezoelectric coefficient d 33 (~130 pm V−1) is obtained in epitaxial PZT films grown on flexible inorganic mica substrates via van der Waals heteroepitaxy. Superior stable ferroelectricity and piezoelectricity retain in flexible PZT film after bending 104 cycles under a radius of 8 mm. As proven by experiment and simulation, flexible PZT film is demonstrated as an ultrasonic sensor with high sensitivity to be bonded on the curved aluminum plate for Lamb wave-based damage monitoring. Our work demonstrates that flexible PZT films have enormous potential for real-time light-weight and high-sensitivity receiver sensors for the SHM of aging aircraft. [ABSTRACT FROM AUTHOR]

Based on the guided wave, a crack detection method for U-shaped flexural plate (UFP) butt welds is established to realize damage localization and quantitative identification. Firstly, the semi-analytical finite element (SAFE) method was used to obtain the dispersion curve in a plate with weld by discretizing the cross-section and then solving the characteristic equation. Subsequently, numerical studies were performed to investigate the mode conversion and energy transfer efficiency in weld bending. Then the method was determined after the wave packet analysis of voltage responses under different damage conditions. Finally, a validation experiment was conducted in an actual UFP butt weld. The result indicates that there exist 10 guided wave modes in a plate with weld at 150 kHz, while only 2 modes in a flat plate; For curvature radius of weld bending substantially greater than the weld thickness, the guided wave energy mainly passes through the bending in the form of transmitted waves and the reflected wave formed in the bending can be ignored; The damage can be located with the defect echo of the specific guided wave and the error distance only accounting for 1.5% of the total weld length. It is effective to take wavelet energy as an indicator to characterize the damage degree. The method significantly improves crack detection efficiency and provides a scientific basis for monitoring similar structures. • Detect weld with bending in the U-shaped flexural plate using the guided waves. • The method proposed can realize the full line scanning of the weld with bending with just one excitation. • The damage location and severity can be further identified by the guided wave echo and the selected damage indicator. [ABSTRACT FROM AUTHOR]

The utilization of corrugated steel plates in concrete filled steel tube (CFST) structures has shown advantageous performance and considerable economy due to their high out-of-plane stiffness and local buckling resistance. A novel composite shear wall composed of multiple CFST components using double horizontally-corrugated steel plates and connected using pure corrugated steel plates with large wave heights was proposed. Fifteen CFST components with double horizontally-corrugated steel plates were designed and statically tested to evaluate the eccentric compression performance. Parameters including nominal eccentricity ratios, nominal sectional aspect ratios, steel plate thicknesses, and diameters of reinforced longitudinal reinforcements were considered. The bearing capacity, stiffness, and ductility of the CFST component with double horizontally-corrugated steel plates were investigated. In addition, the non-uniformities of confinement along with cross-sectional depth was studied through the analysis of measured strains and finite element simulations. Moreover, two capacity models of the eccentrically loaded components were proposed based on eccentricity reduction coefficient and stress balance of the cross-section, respectively. Both models effectively predicted the experimental and simulated results. • The eccentrically loaded behavior of a novel component utilizing double horizontally-corrugated steel plates was studied. • The corrugated steel plates effectively mitigated longitudinal buckling and effectively confined the core concrete. • The novel component showed good performance even though its sectional width-thickness ratio exceeding the specified limit. • Two capacity models were proposed to predict the strength of the novel CFST components. [ABSTRACT FROM AUTHOR]

Monitoring damage in advanced composite structures proves challenging due to insufficient transfer and reuse of diagnostic models. Such limitations fail to meet practical monitoring requirements. This paper proposes an innovative method to monitoring damage via Lamb waves based on transfer learning. Firstly, the proposed method utilizes algorithm-centric transfer learning to extract transferable features from the source domain and a limited amount of data from the target domain, using the domain adaptive feature mapping. The aim of this method is to achieve feature adaptation in both the source and target domains by obtaining approximate distribution patterns in the feature space. Subsequently, a fine-tuning method is presented based on data-centric transfer learning for adaptive damage identification. A screening mechanism utilizing a coarse-to-fine strategy is employed to select suitable samples for diagnosing and evaluating damage in the target domain. The proposed method is verified by targeted monitoring cases of advanced composite structures under four monitoring stages, namely, detection of the damage existence, identification of damage types, damage localization and quantification. The results show that the proposed method can present higher monitoring accuracy in general damages for advanced composite structures than nine state-of-the-art methods. [ABSTRACT FROM AUTHOR]

This paper aims at experimental study of delamination process in multidirectional composite laminate exhibiting elastic couplings by using the acoustic emission (AE) technique. The mode I double cantilever beam (DCB) test was conducted on carbon/epoxy laminate with the bending-twisting elastic couplings according to the ASTM D5528 Standard. Experimental research was supported with visual high resolution registration of propagated crack tip, as well as with measurements of elastic waves generated by developing internal damages by using the acoustic emission set Vallen AMSY-5 equipped with two piezoelectric sensors. Prior to test, values of fundamental Lamb wave modes velocities were estimated and the dispersion curves were calculated. Delamination initiation point was determined by using two different AE criteria. The mode I critical strain energy release rates were calculated by using three different data reduction schemes. Different damage mechanisms during crack propagation process were quantitatively evaluated by fundamental waveform features divided into three characteristic frequency bands groups. In addition, signal energy concentration regions were assessed by using the wavelet transforms. The outcomes revealed various damage mechanisms related with delamination process, even though the matrix cracking found to be the dominating mode of failure. • Application of the acoustic emission technique to examine of delamination process in elastically coupled laminates. • Determination of total fracture toughness by using two different AE criterion. • Classification of damage mechanisms by using waveform characteristic features. • Analysis of failure mechanisms by using wavelet transform. [ABSTRACT FROM AUTHOR]

Orthotropic steel bridge decks (OSDs) find extensive use in bridge engineering worldwide but suffer from fatigue cracking due to vehicle loads. The technology of Lamb wave longitudinal transmission has potential to monitor fatigue cracks. Experimental and numerical investigations were conducted to examine the impact of wave longitudinal transmission on fatigue crack monitoring, utilizing the lightweight arrangements of Lamb wave sensors. Three full scale specimens of OSDs were manufactured and subjected to testing. The sensors utilizing Lamb waves were continuously monitored two types of cracks that were initiated in tests. Standardized wave features were extracted by a wavelet transform algorithm from the received waves in order to evaluate crack growth. Finite element models was also established and verified by the experimental results. Parametric studies were conducted to investigate the effects of crack types, wave transmission distances, and excitation frequencies on wave features. The experimental results show that the wave features exhibit clear variation trends as both the fatigue life and the fatigue crack lengths increase. The findings of the parametric study reveal the proposed sensor arrangements are proved to be suitable for general application. Meanwhile, the different variations in wave features can be observed in the various crack types and lengths, but they are less noticeable when considering different wave transmission distances, as long as the excitation frequencies remain constant. The approach to improve the signal-to-noise ratio in engineering applications arises from the fact that the monitoring frequency of wave features should be chosen as the same as the excitation frequency. • Experimental and numerical investigations were conducted to examine the impact of wave longitudinal transmission on fatigue crack monitoring. • The proposed sensor arrangements are proved to be suitable for general application, even with OSD structures having changeable floorbeam spacing. • The different variations of standardized wave features in response to the various types and lengths of cracks are apparent. • Despite changes in wave transmission distances, the variations of standardized wave features are less noticeable when excitation frequencies remain constant. • The monitoring frequency should choose the same as the excitation frequency to improve the signal-to-noise ratio in engineering applications. [ABSTRACT FROM AUTHOR]

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

• A theoretical method is developed for analytical calculation of spin Chern number. • The effect of surface elasticity on dispersion and topological properties of phononic plate at nanoscale is examined. • Surface effect can be approximately seen as a comprehensive result of surface density and the residual stress. • The topologically protected interface mode is observed in the phononic plate at nanoscale. A new model with analysis for the propagation of flexural waves in a phononic plate at nanoscale is developed. The Gurtin-Murdoch theory for surface elasticity is adopted to model the surface heterogeneity. The Mindlin (or first-order) plate theory is further generalized to establish the governing equations for flexural waves in a phononic plate with surface effect, for which the plane wave expansion method is applied to derive the dispersion relation. A numerical model is developed using the finite element method and very good consistency between theory and numerical solution is observed. It is found that the surface density and the surface residual stress play the main role that affects the band structures. The surface effect can be approximately regarded as the competition between frequency decrease due to surface density and frequency increase caused by surface residual stress, which effectively increases the low-frequency bands but decreases the high-frequency bands. The quantum spin Hall effect is observed in the phononic plate at nanoscale, and the surface effect is studied numerically. By applying the k.p perturbation method, a theoretical framework is established to calculate the spin Chern number, which is an important topological invariant that determines the quantum spin Hall effect. Based on the topological analysis, an efficient waveguide with a zig-zag path is designed, in which a topologically protected wave in the interface state can robustly propagate along the path against disorders. The theory and numerical study developed in this paper will help better understand the size-dependent quantum spin Hall effect in nanostructures and it may also provide guidance for the design of topological wave devices at nanoscale. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Corrugated waveguides are periodic structures that exhibit important acoustic features. Features like acoustic bandgaps exhibit the ability to filter acoustic and ultrasonic frequencies. By varying the mean thickness, corrugation height, and periodicity, one can tune the propagating wave modes in the guide and thus can modify the bandgaps. This study aims to obtain a generalized Rayleigh–Lamb equation for corrugated waveguides such that a single equation is sufficient for both flat waveguides such as plates as well as corrugated and tapered plates. Further, the objective was to understand the effects of the corrugation height, periodicity, and mean thickness such that a physics-based predictive design of wave filters can be achieved. Instead of applying the Bloch–Floquet theorem to the displacement function directly, which is conventional, the theorem is applied to the scalar and vector wave potentials obtained from Helmholtz decomposition. The governing equations from these relationships after applying boundary conditions were then solved using a logical root-finding algorithm. To verify the generalized expressions guided wave band structure for a plate was obtained using Rayleigh–Lamb equation and compared with the generalized form setting the corrugation height to zero. The analytical solutions are then validated through a comparison with the results obtained from a rigorous finite element simulation. Finally, the effects on the propagating and evanescent wave modes due to the corrugation height and periodicity are studied for future reference. [ABSTRACT FROM AUTHOR]

• A technique is presented to unwrap the wave mode spectrum that is obtained from a Bloch–Floquet model and extrapolate it into the extended Brillouin zones. • A method is presented to determine the relative amplitudes of the wave components at different wavenumbers that contribute to the respective eigenmode, for any frequency of interest. • The unwrapping method is illustrated by means of data from a numerical model, supported by experimental results. • The dispersion relationships of a 2D periodic reticulated frame structure can be approximated by the dispersion relationships of a homogeneous plate. Using Bloch–Floquet boundary conditions implies a periodicity in both the spatial and wavenumber domains, causing the numerically computed wavenumbers to be folded or aliased within the Brillouin zone. A methodology is presented to unwrap the wavenumber spectrum to extended Brillouin zones using a Fourier transform method in the spatial domain, thus providing new insights on the physical meaning and relative amplitudes of aliased wave components. In addition, a procedure is presented to identify out-of-plane (Lamb wave type) components, which are usually of interest for thin structures. The proposed methodologies were applied to finite element based simulations of a 2D periodic reticulated structure consisting of interconnected rectangular struts with orthorombic symmetry. This reticulated structure was used as a toy model for the skeleton in porous media, focussing on the out-of-plane waves. Laser ultrasonic experiments were conducted to verify the numerical results. Both the simulations and measurement results indicate that, in spite of bandgap features induced by the periodicity of the structure, the dispersion behavior of the out of plane Lamb wave modes in the considered type of structure is similar to the one of the Lamb waves of a homogeneous plate with thickness equal to the one of the struts, both for wavenumbers within and outside the Irreducible Brillouin Zone. [ABSTRACT FROM AUTHOR]

• A polynomial method is developed for guided wave propagation in piezoelectric laminates. • The proposed method is validated by comparing it with MRRM. • The effect of polarization directions on acoustic wave propagation. • Illustrates the effect of initial stress on the dispersion curves of Lamb and SH waves. Effects of initial stresses on the dispersion curves of Lamb and SH waves in multilayered PZT-4/PZT-5A composites are investigated using the polynomial expansion approach. The piezoelectric layers are considered with arbitrary crystal orientations with a result that only Lamb or SH waves may be transmitted. The problem is solved employing the Legendre polynomial approach that poses the advantages of numerically stability and effectiveness over conventional matrix method. The solution is validated by comparing the wave propagation behavior of piezoelectric materials with those reported in literature, and the convergence properties are examined. Numerical results demonstrate that initial stress has profound influences on the guided wave propagation in multilayered PZT-4/PZT-5A laminates. The phase velocity of Lamb and SH waves increases with initial tensile stresses. In addition, the effects of initial stresses rely on the wave mode and thickness of constituent layers and the stacking sequence of the constituent materials. The results are useful for understanding and optimization of new designs for actuator, electromechanical sensor and acoustic wave devices made of PZT-4/PZT-5A composites. [ABSTRACT FROM AUTHOR]

• Dispersion relation for symmetric and anti-symmetric vibrations. • Frequency equations for plate, flexural and Rayleigh's waves. • Two modes of solution of frequency equations for surface waves. • Velocity ratios of first mode of solutions are greater than second mode. The problem of symmetric and anti-symmetric vibrations in micropolar thermoelastic plate with voids has been investigated. The dispersive frequency equations are obtained for different surface waves propagating in the plate. The velocity curves are depicted for the symmetric and anti-symmetric vibrations, plate, Rayleigh and flexural waves. It is found that there exist two modes in the solution of frequency equation for the surface waves in micropolar thermoelastic plate with voids. We have observed that the first modes of velocity ratios of corresponding surface waves are lesser than those of second mode of vibration. [ABSTRACT FROM AUTHOR]

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

Lamb wave-based structural health monitoring offers excellent potential for real-time delamination detection in laminated structures. It necessitates fast and accurate numerical simulation of guided wave interaction with delaminations. Towards this objective, we present an efficient layerwise zigzag theory (ZIGT) based time-domain spectral element for wave propagation analysis of composite beam- and panel-type structures (strips) containing delamination at arbitrary locations. The ZIGT delivers accuracy like layerwise theories by allowing slope discontinuities in the in-plane displacement field at layer interfaces while maintaining computational efficiency like equivalent single-layer (ESL) theories, making it ideal for such analyses. The high-order elemental nodes at Lobatto points have only four displacement variables, irrespective of the number of layers in the laminate. Delamination is modelled using the region approach, adapting the recently proposed hybrid point-least squares continuity method to satisfy the nonlinear displacement field continuity at the delamination fronts. The model's efficacy is examined vis-à-vis elasticity-based elements, ESL theory-based elements, and the ZIGT-based standard finite element for free vibration and guided wave propagation responses of composite strips featuring multiple delaminations. The present model shows superior overall efficiency, accuracy, and convergence. An application of the model is also explored for identifying delamination locations from Lamb wave velocity fields. [ABSTRACT FROM AUTHOR]

• Proposing a kind of non-resonant metasurfaces to guide and manipulate flexural waves. • Clarifying the difference between the metasurface-guided and conventional flexural waves. • Establishing a theoretical framework to solve the dispersion relations of the guide waves. • Realizing wave manipulations for rainbow reflection, mode conversion and topological interface state. Elastic metasurfaces, being the subject of much attention and discussion, are first designed by phase modulation and now are also considered as a kind of structures with subwavelength size, e.g., layer structures in 3D space or strip structures in 2D space. However, most of elastic metasurfaces, especially those with phase gradient subunits, cannot support elastic wave propagations along them. In this research, we construct non-resonant metasurfaces with an array of slots in plates and explore a unique form of flexural waves propagating along and decaying exponentially with the increasing of the distance from the metasurfaces, which are called metasurface-guided flexural waves. A theoretical framework is established to solve the dispersion relations of these waves by coupling high-order diffraction and waveguide modes. On the basis of this framework, we design three metasurfaces for rainbow reflection, mode conversion and topological interface state, which are realized and verified by finite element simulations and experiments. Our work provides a new idea for the manipulation of flexural waves in plates, which may have wide potential applications in vibration control, structural health monitoring and acoustic device design. [Display omitted] [ABSTRACT FROM AUTHOR]

Lamb waves (LWs) are widely used to achieve structural health monitoring of aeronautic composite structures. Composite materials are however anisotropic and LWs propagation characteristics depend on their propagation direction. Within one composite ply, when this direction coincides with the principal axis of the ply, they are not coupled with shear waves (SHWs) but become coupled with SHWs for any other direction. As composite materials are built up with layers at various orientations LWs and SHWs are coupled for some layers and uncoupled for some others when studying an arbitrary propagation direction. Transfer Matrix Method (TMM), Global Matrix Method (GMM), and Stiffness Matrix Method (SMM) are all methods allowing to predict guided waves (LWs and SHWs) behavior in composite materials. However those methods suffer from an incompatibility issue preventing them to manage cases when SHWs and LWs are coupled for some plies but uncoupled for some other plies. This issue is particularly frequent when dealing with metallic-composite plates or with composite plates made up with isotropic, orthotropic, and triclinic materials. In order to solve this incompatibility issue, a hybrid matrix strategy (HMS) is proposed here on the basis of SMM. The core idea of the HMS is to re-couple LWs and SHWs into hybrid guided waves when they are uncoupled in order to make them compatible with the general coupled waves cases. The numerical stability of HMS is proved theoretically and its effectiveness is validated through numerical investigations and using experimental data from the literature. The SMM-HMS framework can thus be considered as a state of the art benchmark approach for evaluating the performance of numerical methods dedicated to the computation of guided waves dispersion curves and can be confidently applied to any arbitrary composite material used in aeronautic and aerospace industries. • The coupling issue of Lamb and SH waves is studied via the Stiffness Matrix Method. • Hybrid Matrix Strategy is proposed to deal with the matrix incompatibility issue. • SMM-HMS is naturally extended to metallic-composite laminate case. • The numerical stability of SMM-HMS is theoretically proved. • The effectiveness of SMM-HMS is validated by numerical and experimental data. [ABSTRACT FROM AUTHOR]

• The tensile and shear elastic moduli of the plates can be estimated using the S 0 and A 0 modes. • The phase velocity of the S 0 mode depends only on the equivalent tensile elastic modulus and in-plane Poisson's ratio, whereas the phase velocity of the A 0 mode depends almost only on the equivalent shear elastic modulus. • The in-plane Poisson's ratio of cross-ply laminates changes sufficiently small to neglect its effect on the phase velocities of the S 0 mode. This paper proposes a simple and effective Lamb wave-based measurement method for the elastic moduli of cross-ply composite plates, considered equivalent to a single-layer orthotropic plate with nine independent elastic constants. The effects of theses constants on the phase velocities of the fundamental S 0 and A 0 modes are investigated. The phase velocity of the S 0 mode depends only on the equivalent tensile elastic modulus and in-plane Poisson's ratio in the low-dispersive frequency-thickness product regime, whereas the phase velocity of the A 0 mode depends almost only on the equivalent shear elastic modulus. Moreover, the in-plane Poisson's ratio of cross-ply laminates changes sufficiently small to neglect its effect on the phase velocities of the S 0 mode. Therefore, the tensile and shear elastic moduli of the plates can be estimated using the S 0 and A 0 modes, and the mapping relations between the phase velocities of these modes and the elastic moduli are established. The proposed approach has been numerically and experimentally tested on cross-ply carbon fiber reinforced laminates, and validated by theoretical values and conventional tensile tests. [ABSTRACT FROM AUTHOR]

• A meta-plate with high-static-low-dynamic stiffness local resonators is proposed. • Mathematical modeling of the meta-plate is established and solved by plane-wave expansion method. • The band gaps are predicted analytically and validated by numerical simulations. • The meta-plate with HSLD resonators can generate very low-frequency band gaps. Aiming to attenuate very low-frequency flexural wave in a thin plate, this paper proposes a meta-plate model by periodically attaching high-static-low-dynamic-stiffness (HSLDS) resonators onto the thin plate. The HSLDS resonator consists of a linear spring with a negative stiffness mechanism (NSM) in parallel, and thus its stiffness can be adjusted to any low values within a range from zero to the stiffness of the linear spring. Using the plane-wave expansion (PWE) method, and considering the linearized stiffness of the resonator, the dispersion relation of the meta-plate can be derived and the band structure can be obtained. A dynamic model of the meta-plate with the original nonlinear stiffness under external excitations is also established. The numerical simulations are carried out by the Galerkin method to evaluate the band structure and study the propagation of the flexural wave along the meta-plate with different azimuth angles. The analytical results exhibit good agreement with the numerical ones, which indicates that the proposed meta-plate can create a very low-frequency band gap. [ABSTRACT FROM AUTHOR]

In order to monitor the structural conditions, an SHM technology is necessary for long-term aircraft storage tanks under cryogenic conditions. In this paper, a PZT-based Lamb waves SHM technology is developed for such storage tanks. In order to determine the survivability, durability of different PZT-epoxy sensor systems and functionality of the damage diagnosis method under cryogenic conditions of long-term storage tanks, a series of tests have been conducted. First, the durability of PZT-epoxy sensor systems under cryogenic environment was considered by cryogenic durability tests. Simultaneously, performance tests of different PZT-epoxy sensor systems were performed, include high strain performance test and Lamb waves propagation tests under different temperature environments. The high strain performance of different epoxy adhesives under cryogenic environments was investigated by lap shear strength tests. The functionality of different PZT-epoxy sensor systems was investigated by Lamb waves propagation tests. At last, the damage diagnosis ability of the SHM technology was evaluated in a composite damage diagnosis experiment under cryogenic temperature. Experimental results demonstrated that the developed SHM technology can withstand operational levels of high strain and long-term under cryogenic/room temperature on cryogenic storage tanks, and is functional in the cryogenic environment. [ABSTRACT FROM AUTHOR]

Motivated by problems in behavioural finance, we provide two explicit constructions of a randomized stopping time which embeds a given centred distribution μ on integers into a simple symmetric random walk in a uniformly integrable manner. Our first construction has a simple Markovian structure: at each step, we stop if an independent coin with a state-dependent bias returns tails. Our second construction is a discrete analogue of the celebrated Azéma–Yor solution and requires independent coin tosses only when excursions away from maximum breach predefined levels. Further, this construction maximizes the distribution of the stopped running maximum among all uniformly integrable embeddings of μ. [ABSTRACT FROM AUTHOR]

• A full 3-D analytical model for interpreting nonlinear aspects of guided ultrasonic waves modulated by a fatigue crack of "breathing" behavior. • An analytical insight into the generation of contact acoustic nonlinearity from fatigue crack. • A nonlinearity index for quantitatively evaluating fatigue crack including crack length. • A capacity of predicting initiation of a fatigue crack at an embryonic stage. • Rigorous experimental validation and numerical validation. Previously, a two-dimensional (2-D) analytical model for interpreting the modulation mechanism of a "breathing" crack on guided ultrasonic waves (GUWs) is developed [1]. Based on the theory of wave propagation in three-dimensional (3-D) waveguides and using an elastodynamic analysis, the 2-D model is extended to a 3-D regime, to shed light on the nonlinear aspects of GUWs disturbed by cracks with "breathing" traits. With the model, generation of contact acoustic nonlinearity (CAN) embodied in GUWs, subjected to the key parameters of a "breathing" crack (e.g., crack length), is scrutinized quantitatively. On this basis, a nonlinearity index is defined to link crack parameters to the quantity of extracted CAN. In virtue of the index, initiation of an undersized fatigue crack in a 3-D waveguide can be delineated at its embryonic stage, and, in particular, the crack severity can be quantitatively depicted. This facilitates prognosis of imminent failure of the monitored structure. Experimental validation is performed in which a hairline fatigue crack is evaluated, and the results well corroborate the crack parameters predicted by the 3-D analytical model. [ABSTRACT FROM AUTHOR]

• Sinusoidal wavy plate fins with phase shift of 0°, 90° and 180° are investigated. • Thermal performance of SW-PFHS and CCSW-PFHS are compared. • The phase shift has significant effect on the thermal performance. • The proper working conditions of sinusoidal wavy fins heat sinks are reported. This article presents new experimental data on the influence of phase shift, air velocity, heat sink base surface temperature on heat transfer coefficient, and pressure drop of airflow through sinusoidal wavy plate fin heat sinks (SW-PFHS) and crosscut sinusoidal wavy plate fin heat sinks (CCSW-PFHS). Sinusoidal wavy plate fins with a wave length of 18.68 mm, amplitude of 2.0 mm, and phase shift of 0°, 90°, and 180° are used. For CCSW-PFHS, the sinusoidal wavy plate fin is cut transversely at crests and troughs with a 2 mm length. The test runs are performed at an air velocity ranging between 1 and 5 m/s and a heat sink base surface temperature of 70 °C, 90 °C, and 110 °C. The results show that the higher phase shift and air velocity lead to the enhancement of the heat transfer coefficient and pressure drop. Conversely, the heat sink base surface temperature has a slight effect on the heat transfer coefficient and pressure drop. The heat transfer coefficient of SW-PFHS is enhanced when increasing the phase shift compared with a phase shift of 0°. Under the same phase shift, the Nusselt number of CCSW-PFHS is higher than that of SW-PFHS by about 5.9–19.1%. The CCSW-PFHS with a phase shift of 180° yields the highest TPF. [ABSTRACT FROM AUTHOR]

• A hybrid numerical-analytical model is presented for elastic-plastic plate impact. • The model has been verified by the experiment of sphere-plate impact. • The impact-induced waves have been detected by the experiments and the model. • The accuracy and efficiency are verified by three-dimensional finite element simulations. The aim of this study is to present an efficient model for the analysis of complicated nonlinear transient dynamics of an elastic-plastic plate subjected to a transversely eccentric low-velocity impact. A mixed numerical–analytical model is presented to predict the transient dynamic behaviours consisting of either plate impact responses or wave propagations induced by the impact in a plate with an arbitrary shape and support. This hybrid approach has been validated by comparison with results of laboratory tests performed on an elastic-perfectly plastic narrow plate eccentrically struck by an elastic sphere, and results of a three-dimensional finite element (FE) analysis for an elastic-perfectly plastic simply-supported rectangular plate eccentrically struck by an elastic sphere. The advantages of this hybrid approach are in the simplification of local contact force formulation, computational efficiency over the FE model, and convenient application to parametric study for eccentric impact behaviour. The hybrid approach can provide accurate predictions of the plate impact responses and plate wave propagations. [ABSTRACT FROM AUTHOR]

Abstract This work reports a study aimed at quantifying damage severity in composite materials by an in-field inspection technique known as Acousto-Ultrasonics (AU) where Lamb waves are transmitted through a damage zone by exciting a pulse onto the composite surface and capturing the received waves after their interactions with the damage. It builds on a previous study that proposed as quantitative descriptors of the received signal a set of stress wave factors (SWFs) extracted from the power spectral density (PSD) distribution of the AU signal. The SWFs were previously shown to detect the spatial extent of impact damage, but the severity of the damage within the impact damage zone was not quantified. The present study takes a step in the direction to achieve that goal. Lamb waves are transmitted through a cross ply laminate in which transverse ply cracks of increasing density (number of cracks per unit axial length) are induced. The PSD distribution is first analyzed for the case of an undamaged laminate and then the SWFs are correlated with the crack density. The implications of the correlations obtained and the future prospects of quantifying the damage severity by such correlations are discussed. [ABSTRACT FROM AUTHOR]

Abstract The application prospect of a permanently installed structural health monitoring (SHM) system for composite structural prognosis may be determined by the reliability, cost and the added weight of the system. In this paper, we aim at exploiting information encoded in multipath scattering Lamb wave signals, so that damage imaging could be achieved with as few as possible sensors. Firstly, dispersion compensation is applied to the residual signal between two measurement sequences, and thus the propagation distance of each wave packet could be identified. Assuming that acoustic rays propagate according to Fermat's principle, the multipath scattering between the known features (e.g. edges, stiffeners, lap joints, and rivets) and the damage is equivalent to adding a virtual transducer distributed in a symmetrical position. Thus enlarges the element-number and the aperture of the sensor array. On this basis, a modified elliptical method is established to accommodate for these multipath scattering signals for damage imaging. An experimental example is introduced, where the specimen is a carbon fiber-reinforced polymer (CFRP) laminate and the damage is simulated by an added mass. The results show that the damage could be correctly detected and accurately localized even with a single transmitter-receiver pair. [ABSTRACT FROM AUTHOR]

Highlights • Dynamic fundamental solutions are derived for the first time for the Kane–Mindlin plate theory. • First BEM formulation based on the higher-order plate theory for extensional motion of plates. • The formulation is efficient for modelling high-frequency S0 Lamb wave mode. • The formulation is able to handle thickness-stretch displacement of the S0 mode and out-of-plane loads. Abstract This paper presents a new boundary element formulation for modelling the fundamental symmetric Lamb wave (S0 mode) propagating in the high frequency range. At such high frequencies, the S0 mode reveals significant dispersive character. Conventional BEM formulations governed by the generalised plane stress theory fail to accurately represent the dispersive properties of the S0 mode and to handle out-of-plane loads because the effects of thickness-stretch are not considered. Therefore, a new BEM formulation is proposed based on the dynamic fundamental solutions which are derived for the first time for a higher-order plate theory (Kane–Mindlin) taking into account coupling between extensional motion and the first mode of thickness vibration. Only plate edges are required to be discretized using simple line elements in the proposed BEM formulation. Three benchmark examples are presented where the solutions from the new BEM formulation are shown to be in excellent agreement with analytical and three-dimensional finite element results. Furthermore, the advantage of the proposed formulation is demonstrated through comparisons with the BEM results based on the generalised plane stress theory. [ABSTRACT FROM AUTHOR]