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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]

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]

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]

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]

• 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]

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]

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

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

Abstract 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]

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

The time reversal method has been widely used to detect damage in composite plates. Based on the time reversibility of Lamb waves, the waveform of the original input signal will be reconstructed after the time reversal process. However, the time reversal operator of Lamb waves in composite plates is a function of frequency. Thus the shape of the reconstructed signal will not be identical to that of the original input signal. To alleviate the effects of the time reversal operator, an improved time reversal method (ITRM) is proposed. In this method, the response signal in the forward path and the reconstructed signal in the time reversed path are both modulated by a weight vector which is obtained as the product of a window function and the reciprocal of amplitude dispersion. The window function varies with the excitation signal adaptively, and its shape is also determined by a threshold. An experimental example is then introduced to illustrate the effectiveness of the ITRM. The results show that the time reversibility can be greatly enhanced and the impact damage in the composite plate can be exactly located by the ITRM. [ABSTRACT FROM AUTHOR]

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

Estimating the damage state of composites is essential since carbon fibre reinforced polymers (CFRP) have a complex damage and propagation behaviour. Accordingly, a comprehensive study of the effects of matrix cracks – e.g. their size and crack density – and the influence of different damage types and their interaction on the vibro-acoustic modulation (VAM) behaviour of CFRP specimens under fatigue loading was carried out. These effects were evaluated in two sets of commonly employed laminate layups. Firstly, in a cross-ply (CP) layup, the number of 90°layers was varied, with a constant areal weight (268 g/m 2). Secondly, quasi-isotropic (QI) specimens were tested, where the areal weight was reduced from thick-ply to thin-ply (30–268 g/m 2), changing the failure mode from a delamination-dominated to a brittle failure. After damage introduction under tension–tension loading (R=0.1), high-frequency Lamb waves are introduced via piezoceramic actuators and modulated by a low-frequency vibration using a servo-hydraulic testing machine. VAM correlates with the different damage behaviour and damage conditions in CFRP. For this purpose, the evolution of the sideband amplitudes, the modulation index, the signal in time domain and X-ray scans are used. Regardless of the layup, the degree of stiffness-reducing inter-fibre fractures and large-scale delaminations can be detected. • Reliable health monitoring of thin- and thickply CFRP with varied lay-ups via VAM measurements • Modulation envelope and sideband amplitudes provide damage progression during fatigue • Detection of delaminations and the degree of stiffness-reducing inter-fibre fractures, highly sensitive to crack size • Enabling a VAM threshold prior to delamination occurrence to predict failures [ABSTRACT FROM AUTHOR]

This paper proposes a damage assessment approach for composites based on the Lamb wave factorization method. It can exploit both the forward and back-scattering waves and visualize the two-dimensional (2D) defect by solving the inverse scattering problem. When performing the method, a transducer array is firstly adopted, which would take turns to choose one element as the actuator and record the responses using the other transducer elements. On the basis, the scattered waves of defect are then extracted and transformed to the frequency domain to form a transmission-reception matrix. The detection area would be next sampled into numerous points whose pixel values can be determined by calculating the corresponding indicator function using the singular system of the matrix. By visualize the area using all the pixel values, the defect image can be finally obtained. Experiments on different 2D defects are carried out as validations, where both the defect shape and transducer quantity are discussed. Results demonstrates the effectiveness and accuracy of the method for damage assessment in composite laminates. [ABSTRACT FROM AUTHOR]

Lamb waves propagating in a layered structure consisting of layers with equal acoustic impedances, is studied by a variant of Cauchy sextic formalism and the exponential fundamental matrix method. It is shown that the dispersion portraits provide enough information to distinguish individual layers, even if their acoustic impedances are identical, making such layers indistinguishable by the pulse – echo method. It is revealed that the developed methodology is adequate for determining geometrical and physical properties of the individual layers with equal acoustic impedances. [ABSTRACT FROM AUTHOR]

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

Ultrasonic guided wave is a promising technique for structural health monitoring and nondestructive testing. However, due to the anisotropy and complexity of composite materials, the imaging performance of numerous signal processing methods deteriorates with significant artifacts and unsatisfactory accuracy. To obtain a better damage imaging performance of Lamb waves in noisy and noise-free conditions, a weighted delay-and-sum (DAS) imaging method based on denoising autoencoder (DAE) learning is developed for complex composite structures. The traditional DAS formulation is modified to be more compatible with anisotropic materials. The DAE with feature learning capability is then employed to extract potentially efficient features and remove noise from ultrasonic signals. Several verification experiments conducted on flat or curved and stiffened composite structures have confirmed the ability of the DAE-DAS method to suppress artificial noise and to intensify singularities induced by the anomalies. By comparing with the unweighted DAS methods and the weighted DAS method without feature extraction, the proposed algorithm has satisfactory robustness to achieve higher localization accuracy and fewer artifacts. [ABSTRACT FROM AUTHOR]

In this work, a structural health monitoring system has been implemented with damage identification purpose on a winglet of a general aviation aircraft. Using a pitch-catch approach, guided waves (Lamb waves) have been measured by means of an array of piezoelectric sensors able to excite and record the dynamic response of the structure. The undamaged configuration has been used as reference state (baseline) for the structural health monitoring (SHM) purpose while the damaged configuration (current state) has been obtained by low velocity impact test. The effectiveness of the damage has been verified using a Non-Destructive Inspection (NDI) by means of a C-SCAN Olympus Omni SX. The calculation of the damage index obtained comparing the measured wave propagation data in a reference state and the current state is introduced as a determinant of structural damage. Its calculation in different paths associated with the Probability Ellipse (PE) method has been used to identify the position of the damage. Additional radar graphs have been developed for the measurement of the directionality of the Lamb waves and further considerations have been introduced in order to evaluate the sensitivity of the Lamb waves with the aerodynamic load. [ABSTRACT FROM AUTHOR]

In this paper, the S 0 -mode Lamb wave phase velocity at low frequency range is chosen to characterize fatigue damages accumulated in composite laminates under cyclic loadings. A stiffness/velocity degradation model is proposed based on three distinctive damage mechanisms: fiber breaks, matrix cracks and delamination, all of which are always involved in fatigue damage process. In order to reduce the complexity of the damage model, fiber damage is treated as an independent damage and delamination is coupled with matrix cracks. An approximation of shear-lag model is then proposed to avoid any direct measurements of crack density for it is highly impractical in real applications. Following this, energy release rate for formation of matrix cracks is extracted based on measured phase velocity and damage evolution is realized by incorporating a modified Paris law. Controlled fatigue experiments were conducted where in-situ phase velocity was measured by a laser ultrasonic system. The fatigue damages were then characterized using the proposed damage model with the measured velocity. Finally, the ability of proposed model to predict residual lives was also validated and good accuracy has been obtained. [ABSTRACT FROM AUTHOR]

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

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

The interface debond initiation and evolution of Carbon Fiber Reinforced Polymer (CFRP) T-joints (T700/QY8911) under static load are studied in this paper. The experiment test is conducted by using the active Lamb-wave-based damage non-destructive detection technique associated with piezoelectric sensors to pitch and catch the wave signals as well as a high-speed camera to record the failure behaviors of the specimen. The PZT sensors is arranged sequentially along the predicted path in which the debond damage propagates. Furthermore, the real-time monitoring method is also applied to extract the wave characteristic parameters, including V pp of direct wave, Amplitude Damage Index (ADI) and total wave energy, for efficiently describing the damage status of the T-joint during the tensile process. Of which the Amplitude Damage Index (ADI) is considered to indicate initial damage warning and the damage propagation is qualitatively analyzed by observing the consolidated trends of the wave characteristics. The debond behavior of the T-joint obtained by the high-speed camera is also analyzed accompanied with the Lamb-wave characteristic curves. The Lamb-wave-based detection results could be found in a good coincidence with the load-displacement response as well as the measurement strain-time curve obtained by the Fiber Bragg Grating (FBG). [ABSTRACT FROM AUTHOR]

In this paper, the attenuation of guided waves in damped multilayered arbitrary anisotropic composite laminates is investigated using a Legendre orthogonal polynomial approach. The laminate is made of the graphite–epoxy composite material with the layering sequence [0/ ϕ /0]. The validity of the polynomial approach is illustrated through a comparison with exact solution obtained from the Stiffness matrix method and a comparison with earlier known Lamb wave solution of pure elastic multilayered plate. The dispersion and attenuation of coupled Lamb waves with different layering sequences are investigated. The viscous effect on dispersion curves is shown. The combination of frequencies and modes with minimum attenuation in high and low frequencies are determined. These frequencies and modes are of great importance for NDT applications since they play a critical role in the selection of the optimal inspection frequencies. [ABSTRACT FROM AUTHOR]

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

The multi-mode and dispersion natures of elastic guided waves bring big challenges for wave manipulation, and also provide more possibilities for smart device design. In this study, a piezoelectric bimorph plate connected to a periodic arrangement of electric circuits is proposed, and both propagating and evanescent guided Lamb waves are investigated through the complex band structure resulting form using different shunting capacitor or inductor distributions. Results show that keeping and breaking the symmetry of the electric boundary condition can, respectively, lead to decoupling or hybridizing of multi-mode Lamb waves. Accordingly, the asymmetric transmission at a desired frequency can be realized by combining the symmetric and asymmetric electric boundary conditions. Harmonic analysis shows that numerical results are in good agreement with the prediction of complex band structure. Actively changing the electric boundary conditions can be used for guided wave propagation in piezoelectric bimorph plates to easily control the system without changing the structure's geometry. The framework presented herein offers enhanced capabilities for controlling guided wave propagation for engineering applications and to develop nondestructive testing techniques. [ABSTRACT FROM AUTHOR]

This work aims to investigate the wave propagation in an exponential-law-based functionally graded (FG) plate with micro-structural defects using a simple exponential-trigonometric quasi-3D HSDT that employs four variables only. In this study, the elastic properties varied across the plate's thickness using a function of volume fraction in the form of exponential power-law. Hamilton's principle is utilized to obtain the governing differential equations, and the analytical solution is processed using the Navier approach. A parametric investigation has been developed to explore the impacts of material exponent, porosity distribution, and plate thickness on the wave propagation of the exponentially graded material (EGM) varying plate. The results showed that the phase velocity in the plate decreases with increasing the percentage of porosity volume of the plate for a specific material power exponent. In contrast, the FG plate's porosity does not affect the primary frequency. [ABSTRACT FROM AUTHOR]

• Lamb wave-based damage monitoring method is proposed for CFRP composite structures. • A resource-constraint LM and QGSA methods are combined to identify damage location. • Convex envelope and maximum inscribed polygon are adopted to quantify damage size. • Area constraint and bivariate normal distribution are used in singular loci removal. • Sensor layers with embedded PZT network are employed to excite and receive Lamb wave. Accurately and quantitatively monitoring the damages in complicated composite structures is very challenging, but essential for assessing their integrity. In this paper, an innovative damage monitoring method based on Lamb wave is proposed for large-scale complicated composite laminates. A combination of a resource-constraint Levenberg-Marquardt algorithm associated with a quantum-inspired gravitational search algorithm is introduced to identify the damage location. A damage contour algorithm using convex envelope of damage reflection points and maximum inscribed n-polygon is developed to determine the quantitative damage size. Meanwhile, a singular loci removal scheme based on an automatically updated boundary region constraint and bivariate normal distribution is utilized to separate the correctly positioned points on the damage periphery from those associated with noise. Experiments on two large-scale composite panels with multiple damage cases are conducted to substantiate the proposed algorithms. Sensor layers with embedded piezoelectric transducer (PZT) sensor networks mounted on the structures are employed to excite and receive Lamb wave signals because of their superiority of flexibility, electrical stability and apt for complex structures. The results demonstrate that the proposed technique is capable of pinpointing both location and size of the damage in the complicated composite structures. [ABSTRACT FROM AUTHOR]

This study examines and critically compares various commonly used models for calculating complex wavenumbers in viscoelastic orthotropic multi-layer materials (e.g. fiber reinforced polymer laminates). Both exact 3D elasticity theory based methods and approximate methods are considered for wave propagation analysis: (i) global matrix method (GMM), (ii) stiffness matrix method (SMM), (iii) hybrid compliance-stiffness matrix method (HCSMM), (iv) semi analytical finite element (SAFE) method, (v) Legendre polynomial method (LPM), and (vi) fifth order shear deformation theory (5-SDT). A detailed mathematical formulation is provided for each of these techniques. The accuracy and the computational efficiency for calculating complex wavenumbers over a wide frequency range are analyzed for both homogenized and multi-layered viscoelastic orthotropic media. This information is helpful to select the proper method to be used in inversion schemes for material characterization. A comparative analysis is also performed on the computed through thickness displacement fields within the solid medium. The different implemented methods have been compiled in a toolbox with a graphical user interface, called "The Dispersion Box", and can be freely downloaded from Orta et al. (2022). • A benchmark study for dispersion and attenuation behavior of viscoelastic materials • GMM, as reference, is compared with SMM, HCSMM, SAFE, LPM and 5-SDT • The comparison is performed in terms of accuracy, speed and robustness. • A MATLAB toolbox called 'The Dispersion Box' can be downloaded freely [ABSTRACT FROM AUTHOR]

Adhesively bonded joints between metallic plates and composite laminates are frequently used in various fields including aviation, automobile industry, and marine manufacturing. The edge of adhesive layer is exposed to pollutants in the air, which implies that the joints are inclined to suffer debonding defects along the adhesive bond line. To evaluate the contours of the edge disbond, a method based on propagation characteristic analysis and Lamb wave excitation recovery is proposed. Firstly, the dispersion curves of Lamb waves in bonded area and single-layer plate are obtained via the Semi-Analytical Finite Element (SAFE). Subsequently, the effect of disbond on propagation characteristics of Lamb waves is analyzed and discussed numerically. On this basis, the excitation recovery method is proposed and applied to the received waves. The value of envelope at the initial position is extracted as the index during backward propagation for quantitative evaluation of disband size for every transmitter-receiver propagation path. Finally, the disbond contours are sketched by synthesizing the information of all paths with a hypothesis that disbond exists at the edge of the adhesive layer. The proposed method is applied to a lap joint specimen composed of a quasi-isotropic CFRP laminate and an aluminum plate experimentally, and the result demonstrates the effectiveness of proposed method for the accurate evaluation of disbond contours. [ABSTRACT FROM AUTHOR]

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

As a good information carrier, solitary waves have broad application prospects in the field of non-destructive testing. In this paper, we studied the coupling effect between highly nonlinear solitary waves in the one-dimensional particle chain and composite plate with matrix cracking. We based on the tensor internal variable damage model and the residual stiffness model, obtained the couple differential equation of the one-dimensional particle chain and the composite plate with matrix cracking. The fourth-order Runge-Kutta method is used to solve the differential equations, and the change law of the particle velocity, internal force, and displacement curve is obtained. This paper analyzed the influence of the damage amount and the geometric parameters on the reflected solitary waves of composite plates with different material properties. It is found that the reflected waves are sensitive to the thickness of the plate, the average distance between cracks, and the thickness of the damage layer. The research results of this paper have enriched and perfected the theory of nondestructive testing of solitary waves in the field of composite materials and provided broad application prospects for the practical application of one-dimensional particle chains. [ABSTRACT FROM AUTHOR]

This paper presents a dynamic criterion for improving the performance on fatigue life and failure probability prediction of GFRP laminates using Lamb wave velocity. A statistic damage model is built upon a Lamb wave velocity degradation model by introducing a normal distributed random variable to represent the uncertainties. The posterior distributions of the model parameters are obtained by Bayesian inference and Metropolis-Hastings algorithm with noninformative and informative prior distributions. Most importantly, a dynamic criterion is proposed to obtain a specimen-specific failure threshold value based on an assumption that the critical damage stage (CDS) of each specimen differs due to the randomicity of initial flaws. Experimental validation is conducted to compare the predicted fatigue life with both the dynamic criterion and traditional static criterion. The results show that applications of the dynamic criterion can improve the predicting accuracy and consistency, especially during early stages when available data is limited. [ABSTRACT FROM AUTHOR]

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

On the basis of reverberation-ray theory, this article presents the development of a new mode tracing method which can be used to efficiently calculate dispersion curves of Lamb waves in composite laminates. The dispersion relations are built by reverberation-ray formulas in three-dimensional Cartesian coordinate, and the transcendental equations are numerically solved by the improved mode-tracing method. In contrast to conventional mode-tracing method, a slope control algorithm is adopted in the new approach to avoid the switch of tracer from one mode to another around the cross point of dispersion curves, which solves the problem of solution loss. Numerical verifications are conducted to assess the performance of the proposed method and the good agreement with the existing experiment results are demonstrated to show that the proposed method is accurate, efficient and robust for the dispersion analysis of Lamb waves in composite laminates with arbitrary lay-up. [ABSTRACT FROM AUTHOR]

In this paper an application of Artificial Neural Networks and Probability Ellipse methods for damage detection using Lamb waves is presented. The wave propagation data are used to determine the location and degree of damage in metallic and composite plates. The structural components are to be instrumented with array of actuators and sensors (PCB) able to excite and record its dynamic response. A damage index, calculated from the measured wave propagation data in a reference state (baseline) and the current state, is introduced as a determinant of structural damage. The index is a relative measure comparing the two states of the structure under the same ambient conditions. The damage index increase its intensity when the damage size increase too. Moreover, its calculation in different paths associated with the Artificial Neural Networks or Probability Ellipse methods allow to identify position and entity of the damage. Two panels made by aluminum and fabric composite material have been used for tests. A damage entities simulated as through-thickness hole with diameter equal to 6 mm has been investigated for aluminium panel while three damage entities (through-thickness hole with size equal to 2.5 mm, 4.5 mm and 9 mm) have been investigated for that in composite material. In order to get a training, needful for the application of the ANN method, finite elements analysis have been performed for the aluminum panel taking into account several damage paths. Simulations have been performed in order to characterize the propagation path of the Lamb waves in composite plate changing the damage entities from hole diameter equal to 2.5 mm up to 9 mm. Damage index for each damage size for both simulations and tests will be compared. The potential applications of these methods in developing health monitoring systems in defects-critical structures and in composite material structures are discussed. [ABSTRACT FROM AUTHOR]

This paper focuses on quantification of attenuation coefficient of fundamental anti-symmetric (A o ) Lamb wave when it propagates through hybrid structural composites with nanomaterials. Investigations were carried out on nanosilica and carbon nanofiber hybridized-glass and carbon fiber reinforced epoxy composites in comparison to their respective control composite laminates with no nanofillers. Composite specimens were fabricated through Resin Film Infusion (RFI), ensuring near zero void fractions. Effective and actual attenuation characteristics of A o mode were evaluated at three different frequencies – 100 kHz, 200 kHz and 500 kHz in hybrid glass and carbon based composites. It is observed that effect of the nanomaterials (nanofillers) on attenuation of A o mode is more significant at high frequencies. Furthermore, attenuation behavior of A o mode depends on the type of nanomaterial. It is also observed that A o mode undergoes more attenuation in glass fiber reinforced composites compared to their carbon fiber based counterparts, irrespective of the nanomaterials. [ABSTRACT FROM AUTHOR]

The present work focuses on the structural health monitoring of an aluminium vertical helicopter stabiliser with a pre-introduced crack which was repaired with an adhesively bonded composite patch. The structure was monitored under bending fatigue and its performance was evaluated with Lamb waves, lock-in thermography and ultrasonic testing. Outlier analysis of Lamb waves captured the onset and progress of the damage in the form of patch debonding, enabling the identification of five damage-severity regions. Principal component analysis showed distinctive clusters that corresponded to different damage levels while the application of principal curves on the selected features proved to be an additional damage detection index. Amplitude and phase lock-in images accurately captured the onset and evolution of the damage in the form of patch debonding and honeycomb/skin debonding in agreement with the damage indices obtained from Lamb waves. C-scan further validated the damage mechanisms that were captured by the other methods. [ABSTRACT FROM AUTHOR]

Appearing the zero group velocity (ZGV) point belonging to the first quasi flexural branch (A 1) of Lamb waves propagating in the multilayered traction-free plates containing homogeneous isotropic layers is observed and analyzed; in contrast to the previously studied homogeneous isotropic plates admitting ZGV points that may belong to the higher flexural modes (A 2 , A 3 , ...) and higher symmetric modes. The detected ZGV point in a multilayered traction-free plate leads to the transverse quasi resonance occurred at the lower frequency than the previously studied resonance associated with the first symmetric mode (S 1) and especially higher symmetric and flexural modes. The analysis is done by the combined method comprising Cauchy sextic formalism and the exponential fundamental matrix method. [ABSTRACT FROM AUTHOR]

Lamb wave is widely studied in the non-destructive testing of composite materials due to its wide detection range and high sensitivity to defects. In order to solve the problem of extracting the effective signal of damage in a strong noise environment to improve the accuracy of damage location determination. A damage location imaging method based on power spectral density and Lamb wave tomography is proposed in this paper. The damage location imaging of composite materials under strong noise environment is realized through simulation and experiment respectively. Firstly, the simulation analysis the propagation characteristics of Lamb waves in composite materials. The circular sensor array is evenly arranged on the composite material. Each sensor is used as an actuator to generate Lamb wave in turn clockwise. Other sensors are responsible for collecting signals. Strong noise is added to the collected signal to simulate the signal collected in the strong noise environment. Finally, the damage information is characterized by power spectral density to determine the damage factor, and by using the probability imaging algorithm, the damage location imaging is totally realized. The experimental results show that the maximum error of imaging positioning for single damage and multiple damage under strong noise environment is 5.10 mm and 8.04 mm, respectively. This method does not need to pretreatment the signal with strong noise, and can directly image the original signal. At the same time, the extraction process of complex reflected signals is avoided, and it has a great potential in the location and identification of composite materials damage in a strong noise environment. [ABSTRACT FROM AUTHOR]

In elastic systems, the researchers have observed the topological valley edge states. However, the topological valley edge states can only enable elastic waves to propagate along the narrow interfaces. Here, we obtain the topological valley waveguide states (TVWSs) by constructing the sandwich-like plates. By virtue of the TVWSs, the transport of elastic waves in the wide straight and bent waveguides is achieved. In addition, we realize the splitting effect and converging of elastic waves according to the valley-locking property. Our research may promote the design of innovative devices about energy harvesting, sensing, and information processing. [ABSTRACT FROM AUTHOR]

Lamb waves are useful for detecting changes in material properties, as their propagation characteristics are sensitive to changes in the medium through which they travel. Defects and damage in materials/structures can be detected by analyzing the difference between the phase and/or group velocity amplitude and mode changes of Lamb waves’ propagation in damaged versus un-damaged specimens. The propagation characteristics of Lamb waves are described in the form of dispersion curves, relating phase and/or group velocities to the product of frequency and thickness, with these curves being generated by solving the Lamb wave equations. In this article, the effect on the Lamb wave propagation/dispersion due to changes in the material properties such as E 11 , E 22 , G 12 , and density were analyzed for three different composite laminate types – unidirectional, cross-ply, and quasi-isotropic, all constructed out of 16-plies of unidirectional carbon-fiber epoxy prepregs. The analysis was performed by obtaining the characteristic Lamb waves’ phase velocity dispersion and slowness curves for each laminate type. This was done by reducing E 11 , E 22 , and G 12 by 5%, 10%, 15% and 30%, and density by 1%, 2%, 5% and 10% with the intent of representing manufacturing and in-service defects. Since the Lamb waves’ velocity in a composite laminate varies with the propagation direction, the laminates were analyzed at 0°, 20°, 45°, 70°, and 90° propagation angles, with all the aforementioned variations. A program developed in MATLAB was used to generate the dispersion curves for the fundamental anti-symmetric and symmetric Lamb wave. The information provided in this paper can be used to select the proper Lamb wave modes, excitation frequency, which in turn governs sensor selection, data acquisition hardware, and damage detection algorithm for structural health monitoring of composite structures. [ABSTRACT FROM AUTHOR]

Detection of transverse crack (TC) is important in establishing reliable structural health monitoring (SHM) and nondestructive inspection (NDI) for warning the potential occurrence of large-scale damage in carbon fiber reinforced plastic (CFRP) structures at an earlier stage. In this research, a diagnostic method that is sensitive to a single TC was established by visualizing laser-induced guided ultrasonic waves in a cross-ply CFRP laminate. First, the phenomenon of TC-induced mode conversion in the laminate was clarified using finite element analysis (FEA) of harmonic oscillations. This FEA method is explicit and precise for investigating the conversion behavior of individual modes in a cracked plate. Then, the conversion phenomenon was also validated by another FEA-based pulse laser ultrasonic simulation. Finally, to observe the mode conversion phenomenon in practice, we conducted an experiment with a laser ultrasonic visualizing inspector (LUVI). The LUVI system could visualize the wave propagation in a two-dimensional (2D) wavefield that was formed on the surface of the CFRP cross-ply laminate with a single TC. By observing the image results of wave propagation, we verified that the wave component of the A 0 mode was converted from the S 0 mode at a TC even though the converted A 0 mode had a much smaller amplitude than the incident S 0 mode in the cross-ply CFRP laminate. Thus, those visualization results could be used to detect a single TC in the CFRP cross-ply laminate reliably. [ABSTRACT FROM AUTHOR]

• A new finite element method reproduced the impact damage in composite laminate. • The model describes the geometric properties of the actual impact damage. • Simulation of acousto-ultrasonic sensing was conducted to manifest the change of the Lamb wave. • Investigating the change of the Lamb wave which interacts with impact damage. • The simulation results ware compared with another finite element method and validated by the experiment. It is difficult to investigate the interaction between the Lamb wave and impact damage within a composite laminate if the interfaces caused by impact-induced delamination and cracks are not modeled precisely. In this study, a novel finite element modeling method was proposed to virtually reproduce multiple damage after observing the distribution and form of different impacts in a cross-ply carbon fiber reinforced plastic laminate. Further, a numerical simulation of acousto-ultrasonic sensing was performed to manifest the tone-burst Lamb wave that propagated in the laminate. The calculated stress and displacement contours distinguished the dispersed modes and exhibited the wave attenuation after it interacted with multiple interfaces. The change in amplitude provided a reference for impact damage evaluation. The proposed method was compared with another existing modeling method and validated by the acousto-ultrasonic sensing experiment, which demonstrated its accuracy and capability to study the phenomena in acousto-ultrasonic sensing for low-velocity impact damage. [ABSTRACT FROM AUTHOR]

Lamb waves are ultrasonic guided waves that propagate between two parallel free surfaces and their use for damage detection has been widely explored and demonstrated. Damage in materials/structures can be detected by analyzing the difference between the phase/group velocity and the loss of amplitude of Lamb waves on damaged and un-damaged specimens. The propagation characteristics of Lamb waves are described in the form of dispersion curves, which are plots of phase/group velocities versus the product of frequency-thickness generated by solving the Lamb wave equations. Lamb waves’ dispersion behaviors for isotropic materials are well established in the literature; however, such is not the case for the laminated composites. The most common methods for solving the Lamb wave equations in composites consist of using laminated plate theory or 3D linear elasticity by assuming an orthotropic and/or higher symmetry. This assumption may not be true, if the actuators and sensors in an orthotropic or transversely isotropic laminates are installed in a non-principle direction or the layup is symmetric but not balanced. This paper presents a full derivation of Lamb wave equations for n-layered monoclinic composite laminates based on linear 3D elasticity by considering the displacement fields in all three directions using the partial wave technique in combination with the Global Matrix (GM) approach. In the partial wave technique, the principle of superposition of three upward and three downward travelling plane waves are assumed in order to satisfy the associated boundary conditions. The bounded upper and lower surfaces reflect the waves and the combination of these reflections going towards the upper or lower interfaces results in the propagating guided waves. The GM approach is used to assemble all the equations from each layer to form a global, unified matrix that describes the displacement and stress fields along the entire laminate associated with the wave propagation. A robust method for numerically solving the Lamb wave equations is also presented. The presented method was verified experimentally by analyzing the propagation of Lamb waves in two different composite panels constructed out of unidirectional carbon-fiber epoxy prepreg and fiber-metal laminate (GLARE 3-3/4). The panels were instrumented with lead zirconate titanate (PZT) piezoelectric sensors, which were excited at different frequencies ranging from 20kHz to 500kHz to generate and acquire the waves. The waves were excited and gathered at three different propagation angles of 0°, 45°, and 90° for the carbon-fiber epoxy laminate panel and at six different angles of 0°, 20°, 45°, 70° and 90° for the fiber-metal laminates (GLARE). The phase and group velocities of the fundamental symmetric (So ) and anti-symmetric (Ao ) Lamb waves were extracted by tracking the peaks of each individual wave phase and the wave envelope respectively using an in-house code developed in MATLAB. It was found that the presented 3D linear elasticity model followed the experimental data closely for both symmetric and anti-symmetric Lamb modes. The analytical method presented in this paper was able to predict the Lamb wave dispersion for both the carbon-fiber epoxy laminate and the hybrid fiber-metal laminate proving the robustness and versatility of the solution method. [ABSTRACT FROM AUTHOR]

Ultrasonic guided waves-based Structural Health Monitoring (SHM) is a promising solution to perform damage diagnosis in plate-like structures. In this field, the Reconstruction Algorithm for Probabilistic Inspection (RAPID) is one of the most widely used tomographic algorithms to perform active damage detection and localisation. Even though this algorithm is easily implementable, very versatile and satisfactorily accurate, it presents some disadvantages, such as the production of artefacts determined by non-uniform distributions of the sensing network density. In the present article, a processing and spatial filtering technique is proposed, which strongly mitigates the aforementioned problem. The proposed methodology is validated using a numerical database created for a large carbon fibre reinforced polymer (CFRP) with a through-the-thickness hole. [ABSTRACT FROM AUTHOR]

Many structural health monitoring systems are being developed to detect the impact damages in carbon fiber reinforced plastic (CFRP) structures, which are widely adopted in the aviation industry. In this study, a refined finite element model is established to simulate a Lamb wave propagating through impact damage in a CFRP skin-stringer structure. The accurate stiffness constants are determined for toughened CFRP laminates, which include the resin layers. The impact damage of toughened CFRP laminates is modeled as a truncated-cone-shaped region in which the rigidity is homogeneously reduced. Furthermore, the directive generation of the Lamb wave through a macro fiber composite (MFC) actuator is incorporated in the model. Simulations are performed for several configurations of the MFC and impact damage. The observed tendency of the delay in the arrival time of the lowest antisymmetric Lamb wave mode is noted to be consistent with the experimental results. The proposed model can facilitate the determination of the optimized configuration of MFCs in CFRP structures. [ABSTRACT FROM AUTHOR]

• Acoustic emission wave is studied in honeycomb sandwich panel on different scales. • Honeycomb cells were found to act as conduit of AE wave energy. • Honeycomb cells is allowing AE wave to transmit from one skin to the other. • Lamb waves are bound in top and bottom skins of the honeycomb sandwich panel. • The honeycomb ribbon direction affects the S o mode velocities. • Concept of insertion loss is introduced as a measure of effect of cell on AE waves. This paper studies acoustic emission (AE) wave propagation in a glass fibre aluminium honeycomb sandwich panel (HSP). Of particular novelty is the quantification of the through-thickness propagation of AE from one surface of the HSP to the other, which is a real-world monitoring consideration for applications where AE instrumentation is only be permissible on one surface, i.e. aerofoils. Complexity was introduced to the specimens in stages to enable a thorough understanding; first investigating propagation in a large glass fibre laminated plate (GFLP) alone; then in bespoke sandwich specimens with a limited number of honeycomb cells; and, finally, in a large HSP. The results of this paper demonstrate that, whilst some energy is transmitted through the honeycomb core, AE propagating in an HSP becomes bound in the outer plates. Despite this, propagation in these outer plates differs to that in an equivalent plate of the same material in isolation due to the complex interactions with the structural elements of the HSP. Further, propagation of AE transmitted from one surface to the other was quantified for the first time, giving insight into expected attenuation rates and characteristics for practical applications. [ABSTRACT FROM AUTHOR]