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• An acoustic source localization technique for composite laminates based on an improved signal energy-based approach is proposed. • The prior knowledge of the orientation of the symmetry axes of the composite laminate is not needed. • Experiments of the proposed technique for a composite laminate are verified. • Discussion of the reasons for errors and suggestions for improving the feasibility are given. Composite laminates are widely used in various fields, but their structural properties make them prone to impact damage. Thus, the real-time localization of the impact source in composite laminates is of great importance. The signal energy-based approach for acoustic source localization, as a new technique proposed in recent years, has advantages in that it can be applied in the case of low signal-to-noise ratio and does not need to have a priori knowledge of material properties. In this paper, an acoustic source localization technique for composite laminates based on an improved signal energy-based approach is proposed. The prior knowledge of the orientation of the symmetry axes of the composite laminate is not needed. The proposed approach is verified by numerical simulation and experiment. The results show that proposed approach has good stability and accuracy. The error analysis and suggestions for improving the feasibility of this technique are also given. [ABSTRACT FROM AUTHOR]

[Display omitted] • A thermo-acoustoelastic coupling model of plate/shell structures is proposed. • The coupling effect of thermal and stress on dispersion features is revealed. • The experimental results are in good agreement with the numerical results. • An efficient strategy to estimate the phase velocity change is confirmed. • A significant improvement in accuracy is gained compared to existing models. Determining the coupling effect of the temperature variation and structural stress on the dispersion characteristics of plate/shell structures is essential for guided-wave detection technology. However, most existing research has focused on the individual effects of stress and thermal fields. Considering this problem, a thermo-acoustoelastic model is proposed to obtain the dispersion characteristics of guided waves in plate/shell structures under temperature-stress coupling. The nonlinear constitutive equation of hyperelastic materials under temperature-stress coupling was derived using the temperature-dependent strain energy density function and incorporated into the model via a user material definition. The solution of the numerical model was divided into two analysis steps using the superposition solution method. The calculated phase velocity changes of different guided wave modes utilizing this model were consistent with the experimental data. Then, the effects of the temperature-stress coupling on the dispersion characteristics of guided waves with different propagation angles in plate/shell structures were systematically analyzed. The results show that the effect of the temperature-stress coupling on the phase velocity of 6061-T6 aluminum plates can be decomposed into the individual effects of stress and thermal fields with a very small error, which provides an efficient strategy to estimate the phase velocity change induced by the temperature-stress coupling effect. The comparative analysis of the experimental results and prediction values from the existing models demonstrates a significant improvement in the accuracy of the proposed model. This work reveals the coupling effect of the stress and thermal field on the guided wave dispersion characteristics and can provide valuable data to develop guided wave environmental compensation algorithms. [ABSTRACT FROM AUTHOR]

It is challenging but meaningful to detect and image the micro-damage in the early stages of engineering structure failure. The nonlinear ultrasonic technique has gained considerable attention for micro-damage detection, meanwhile, the Lamb wave phased array technique has become a practical tool for macro-damage imaging. The combination of these two techniques can pave a promising way for micro-damage imaging in plate-like structures. However, the research on nonlinear Lamb wave phased array imaging is complicated and not reported. The weak nature and hard extraction of the nonlinear features, such as the second harmonic generation, limits the application of the nonlinear Lamb wave phased array. A framework of nonlinear Lamb wave phased array for micro-damage imaging in the plate-like structure is developed. To maximize the second harmonic response of the recorded Lamb waves, two kinds of lead zirconate titanate (PZT) wafers are selected to compose the nonlinear phased array. Furthermore, focusing on the nonlinear feature extraction, the sparse representation approach is introduced for reconstructing the time-domain second harmonic generated by the micro-damage, from which a wealth of waveform information corresponding to the micro-damage can be extracted. The total focusing method (TFM) is used for micro-damage imaging based on the reconstructed second harmonic. Numerical simulation and experimental validation demonstrate the capability of the proposed framework to image the local contact acoustic nonlinearity represented by the barely visible closed fatigue crack with the nonlinear Lamb wave phased array. [ABSTRACT FROM AUTHOR]

The present work introduces two unsupervised data-driven methodologies for processing Lamb waves (LWs) to localize structural damage, specifically employing convolutional autoencoders (CAEs) and conditional generative adversarial networks (CGANs). Both techniques are capable of processing diagnostic signals without the need for any prior feature extraction. Once all signals are processed, a damage probability map is generated. The performance of the methods was tested using two different experimental datasets. The first derives from LWs obtained from a set of piezoelectric transducers mounted on two different composite panels, made of two different layups. Pseudo-damage and real damage were considered. The second dataset derives from LWs acquired on a full-scale composite wing, where damage was introduced through impacts performed using an air-gun. The results of this study revealed that the proposed unsupervised methods are capable of localizing damage properly, with comparable accuracy. [ABSTRACT FROM AUTHOR]

• A wireless data transmission scheme for multiple cross-space users is proposed. • Lamb waves provide a low-cost and structurally hassle-free solution. • The disadvantage of the limited speed of conventional PPM is overcome. • The inter-access interference in multiple users communication systems is eliminated. • Multiple access design criterion considering local correlation is more reliable. Due to the high cost and safety risk brought about by wire penetration within the aerospace and underwater structure, reliable wireless cross-space multiple access transmission techniques become highly necessary. Lamb waves data transmission is recognized as a feasible solution, since it utilizes the solid structures as the transmission medium without being affected by electromagnetic radiation. However, in the case of synchronous transmission of multiple users, the aggravation of inter-access interference would severely weaken the reliability of the transmission system. Focusing on the issue of anti-interference synchronous independent data transmission for multiple cross-space users, this study proposes a Lamb waves-based pulse code frequency division multiple access (PCF-DMA) scheme. Firstly, a fast pulse position modulation strategy is established to improve the transmission rate. Considering the characteristics of the Lamb waves excitation pulses as the codeword, a PCF-DMA multiple access encoding method is designed. Secondly, by sparsely representing the Lamb waves physical transmission channel, a shift-invariant sparse deconvolution technology is proposed for decoding. Specially, to achieve reliable multiple access transmission with the PCF-DMA scheme, a normalized cross-correlation coefficient is used as the design criterion of the Lamb wave pulses. To validate these concepts, a three-user PCF-DMA data transmission system based on Lamb waves is built. The experimental results illustrate that the proposed design criterion can accurately predict the reliability of the multiple access transmission system. The bit error rate (BER) of the proposed PCF-DMA scheme is 30 % lower than that of conventional FDMA within a signal-to-noise ratio of −12 to 8 dB. Further, the influence factor analysis for the BER and communication rate are conducted. Finally, a synchronous independent data transmission for multiple cross-space users is achieved, with a maximum communication rate of 92.9 kbps per user and a BER below 0.17 %. [ABSTRACT FROM AUTHOR]

Fatigue damage prognosis studies typically involve updating a degradation model based on the collected measurements. The updated model can accurately depict the past degradation process, while it is always assumed to be capable of describing the future damage progression, and thus can be used for remaining useful life (RUL) prediction. However, this assumption cannot always hold true, due to the uncertainties arising from the degradation process. In this context, this paper proposes a scheme of prognostic-aided model updating in a particle filter (PF)-based prognostic framework, which empowers the updated model to capture both the historical and future degradation process, and consequently, enhances the prognostic performance. First, two degradation models are formulated using the physics-based damage state and the data-driven health indicator (HI), respectively. The former is then re-formulated in state space by incorporating a measurement model. Next, the state space model is updated by the latest measurement through PF, providing the RUL and lifetime percentage (LP) distributions. Finally, the particles of damage state and parameters are modified by comparing the LP distributions from the updated model and the latter degradation model. The feasibility and robustness of the proposed method are validated by an experimental study, where an aluminum lug joint subject to fatigue crack growth is monitored by a guided wave measurement system. [ABSTRACT FROM AUTHOR]

• A novel method to achieve sub-wavelength focusing is proposed, and validated via FEM and experiments. • A critical angle distinguishing sub-wavelength focusing from normal focusing is proposed. • The design scheme is general, with the phase distribution not unique. • The sub-wavelength focusing is broadband and can be artificially controlled. The flexural wave focusing in plates and shells is of great importance for energy harvesting and vibration control. Traditional plane lens based on gradient index and metasurface usually exhibits normal focusing, i.e. , the focusing spot size is larger than half wavelength. In order to improve its working performance, increase energy at the focal point, reduce the focusing spot size and further achieve subwavelength focusing, a novel design scheme is proposed, i.e. , splitting one traditional plane lens into two halves with a controllable inner angle. Firstly, the transmission and phase are calculated by using the Timoshenko beam theory, utilizing which the lens design is finalized via changing beam thickness perpendicular to wave propagation. After that, the working performance of two sub-lenses is compared with that of one single lens via numerical simulations, including the focal position, energy distribution, and spot size, which are further validated through experimental measurements. It is demonstrated that the flexural wave can exhibit subwavelength focusing after traveling across two adjacent halves, with the focusing spot size smaller than half wavelength. Specifically, the subwavelength focusing is broadband, and still occurs in a frequency region centered at the designed frequency. The proposed design scheme is expected to provide an innovative approach for the lens design in acoustics and optics. [ABSTRACT FROM AUTHOR]

• A novel method for accurately detecting small-size defects of in-service CFRP plates is proposed and verified. • The developed nonlinear defect index using the relatively nonlinear coefficient of air-coupled Lamb wave is confirmed to be more sensitive to small-size defects than the traditional linear defect index. • The proposed adaptively weighted(AW) imaging algorithm takes the beam width of the Lamb wave into account so that imaging resolution no longer depends on the scanning step length. • For the small-size defect with a diameter of 10 mm, the area detection error of the proposed method is only 7.8 %, compared with 22.4 % of the traditional method. The air-coupled ultrasonic technique is capable of detecting defects in in-service CFRP plates and ensuring their application safety. Traditional air-coupled ultrasonic methods often rely on the linear defect index, which is not effective in characterizing small-size defects. Additionally, the scanning step length completely limits their imaging spatial resolution, resulting in the contradiction between imaging spatial resolution and detection efficiency. To address the above problems, the nonlinear defect index and the adaptively weighted imaging algorithm are proposed. The nonlinear defect index uses the relatively nonlinear coefficient of the Lamb wave to enhance the ability to detect small-size defects. Considering the beam width, the adaptively weighted imaging algorithm constructs the relationship between any imaging point and all scanning paths. At this time, the imaging spatial resolution can be set arbitrarily, eliminating the dependence on the scanning step length. The experimental results demonstrate that the proposed nonlinear defect index can characterize the small-size defect more accurately than the linear defect index. When the scanning step length is increased to improve detection efficiency, the adaptively weighted imaging algorithm can achieve better imaging spatial resolution compared to the traditional defect probability imaging algorithm. For the small-size defect with a diameter of 10 mm, the proposed method has an area detection error of only 7.8 %, whereas the traditional method has an error of 22.4 %. The proposed method is promising to detect small-size defects of in-service CFRP plates in the aircraft skin. [ABSTRACT FROM AUTHOR]

High-speed trains inevitably experience the impact of complex temperature loads during operation, which can easily result in cracks. Structural health monitoring methods based on Lamb waves are affected by the impact of temperature on their propagation mechanism, leading to difficulties in constructing accurate damage diagnostic models. This paper proposes a crack damage identification method based on adaptive multi-scale sample entropy under variable temperature environment. A sliding window method is proposed to obtain multiple wave packets in a Lamb wave, which avoids the disadvantage of insufficient information of a single wave packet and improves the robustness and reliability of diagnosis. A variance-based multiscale transform method is proposed to process Lamb wave signals, which reduces the sensitivity of Lamb wave signals to temperature compared to the traditional mean-based multiscale transform method. Lamb waves in different sliding windows are transformed at multiple scales to extract damage Multi-scale sample entropy (MSE) at different scales. The Quantum genetic algorithm (QGA) is introduced to optimize sliding windows and MSE, achieving adaptive MSE extraction under variable temperature environments. Based on the advantage of adaptive multiscale entropy, a quantitative crack diagnosis model is established. To verify the effectiveness of the proposed method, crack detection experiments under variable temperatures were conducted. The results show that the proposed adaptive MSE has good resistance to temperature changes and can accurately identify crack length. [ABSTRACT FROM AUTHOR]

Researchers have recently gained interest in ultrasonic guided waves (UGWs) for structural health monitoring (SHM). This interest has mainly grown thanks to the high sensitivity of UGWs to changes in the mechanical and material properties of the medium they travel in. Among the several types of UGWs, Lamb waves (LWs) have been successfully employed to detect, localize and quantify damage in thin-walled structures. To date, localization has been mainly performed through tomographic algorithms. Although those algorithms represent consolidated methods, they come with unsolved issues, such as artifacts generation in damage probability maps and strong dependency on the network of sensors installed to excite and sense diagnostic signals. Hence, more recently, supervised data-driven approaches have been propounded in the literature for LW-based damage diagnosis. Despite the good performance of such methods, a strong limitation is represented by the need for large high-fidelity datasets for training, which are generally not available for real-life structures. A possible solution is represented by unsupervised machine learning methods. Such methods have already been employed for damage detection in the SHM field, but no fully unsupervised approach to damage localization has been proposed yet. Hence, in this work, a framework combining an unsupervised data-driven method and an in-house tomographic approach is presented to process LWs to localize damage. Specifically, convolutional auto-associative neural networks (CAANNs) are employed to process diagnostic signals without requiring any prior feature extraction process. Furthermore, CAANNs predictions are processed to generate damage probability maps. The performance of the proposed method was tested against a numerical case study involving an Aluminum plate and against two experimental datasets of LWs acquired on a full-scale composite wing and a composite plate. Results showed that the proposed method outperformed classic damage diagnosis algorithms in terms of damage localization accuracy. [ABSTRACT FROM AUTHOR]

This paper presents a new framework for stochastic updating of a finite element model for a composite plate, considering the influence of temperature on Lamb wave propagation. The framework involves deterministic updating to optimize mechanical properties and stochastic updating to derive probability density functions for key parameters. It utilizes sensitivity analysis and Bayesian inference with Markov-Chain Monte Carlo simulations and the Metropolis–Hastings sampling algorithm. This paper proposes a machine learning surrogate model based on artificial neural networks to improve computational efficiency. This surrogate modeling approach allows parallelized Monte Carlo simulations, reducing updating time significantly without compromising the accuracy of the resulting probability density functions for model parameters. These advancements show a promising way to enhance composite plate modeling and Lamb wave propagation studies, providing a more efficient and accurate approach to verify and validate finite element models with potential applications in engineering simulations. [ABSTRACT FROM AUTHOR]

Realistic structures with complex features have been intricate challenges for structural health monitoring (SHM) with permanently installed transducers. Poor conformability of conventional rigid and brittle piezoceramic wafers is a typical issue of applications on surfaces with complex geometry. Moreover, the accompanied high-stress concentration requires high-sensitivity defect detection which is difficult to achieve with large-area monitoring methods like lamb waves at tens to hundreds kHz. Previously, in-situ conformally fabricated piezopolymer coating-based array transducers (PCATs) have been developed to build large-area, lightweight, flexible, and tunable Lamb wave networks. In this study, two novel local monitoring methods were investigated with PCATs, namely ultrasonic bulk wave array inspection and local ultrasonic resonance spectroscopy. For thick structures, ultrasonic bulk waves were generated and detected by PCATs with broadband operating frequencies (1–10 MHz) and flexible array parameters. Simulation tools and imaging algorithms of array inspection in non-destructive testing (NDT) were well implemented based on the analogous directivity pattern and normal-pressure coupling mechanism. As proof of concept, PCATs were applied on example structures with flat, concave, and convex surfaces for internal defect imaging. For thin-walled and/or multilayer structures, PCATs were used to measure local ultrasonic resonances, comparable to conventional non-contact methods. With negligible influence on local mechanical properties and broadband frequency response, multiple resonance peaks from 0 to 25 MHz were identified as zero group velocity (ZGV) Lamb modes and thickness vibration modes of host structures, which can be used as damage indices for local monitoring of corrosion, delamination, stiffness degradation, etc. Through embracing advanced NDT techniques with PCATs, high-sensitivity and quantitative local monitoring could be achieved with conformal networks, offering the possibility to integrate with large-area monitoring as multi-scale SHM for complex structures. [ABSTRACT FROM AUTHOR]

• Achromatic ribbed elastic meta -structures for ultra-broadband manipulation of flexural waves are proposed. • The transmitted amplitude and phase shift are analytically solved by combining the equivalent ribbed plate theory and transfer matrix method. • The "frequency-phase shift" relation is derived to reveal the intrinsic physics between the non-dispersive functionality and dispersive phase shift of subunits. • The ultra-broadband flexural wave deflection and focusing functionalities within 1–10 kHz, with a high relative bandwidth of 163.6 %, are methodically demonstrated using numerical simulations and experiments. Elastic metasurfaces have shown remarkable ability in exotic wavefront manipulations based on the generalized Snell's law (GSL), stimulating great promise for many potential applications, such as elastic wave signal processing, vibration control, and energy harvesting. However, elastic metasurfaces suffer from strong narrow-band confinement and dispersion modulation due to the inherent velocity-dispersive properties and the dispersion of the accumulated phase during propagation. To address this issue, we propose a novel conceptual design paradigm of achromatic ribbed elastic meta -structures (AREMs) beyond the narrow-band limit for the ultra-broadband manipulation of flexural waves in thin plates. We establish an analytical model for ribbed subunits to accurately solve the phase shift and amplitude of the transmitted waves. In addition, we reveal the relationship between broadband achromatic functionality and subunit phase shift dispersion. We theoretically design and numerically demonstrate three flexural wave manipulation functionalities in an ultra-broadband frequency range of 1–10 kHz, including flexural wave deflection, focusing, and splitting, with a relative bandwidth of 163.6 %. Finally, we experimentally verify the ultra-broadband flexural wave manipulation capabilities of AREMs. Our study may open new horizons for ultra-broadband high-efficiency achromatic and structurally simplified flexural wave-based devices, with promising extensions to other elastic wave modes. [ABSTRACT FROM AUTHOR]

• A new compounding imaging algorithm integrating amplitude and phase information of signal based on Lamb wave phased array is proposed. • Sigmoid function with variable gradient is used for establishing the nonlinear mapping between imaging index and phase difference. • The proposed imaging algorithm is validated on plates of different materials and dimensions with multiple types of defects. • The proposed imaging algorithm can effectively improve the signal-to-noise ratio of the images and suppress artifacts, implementing imaging without threshold. Aimed at improving the imaging performance of Lamb wave phased array based on Delay-and-Sum (DAS) of signal amplitude, a compounding imaging algorithm integrating amplitude and phase information of signal is proposed in this paper. By applying back propagation dispersion compensation, the pulse echoes caused by defects will be in-phase with the excitation signal, implementing signal focusing. In the proposed imaging algorithm, the amplitude imaging index is calculated by classical total focusing method (TFM) and the phase imaging index is calculated by phase coherence factor (PCF), which establishes the nonlinear mapping between imaging index and phase difference by sigmoid function with variable gradient. The final imaging results are the compound of two imaging indexes. The experimental validation is carried out on plates of different materials and dimensions with multiple types of defects based on uniform linear array. Compared with the imaging results of conventional TFM, the proposed imaging algorithm can effectively improve the signal-to-noise ratio (SNR) of the images and suppress artifacts, implementing threshold-free imaging. [ABSTRACT FROM AUTHOR]

• A particle filter-based prognostic method combining multiple features is shown. • An online feature selection criterion is proposed based on bias estimation. • The method is tested by fatigue testing of an Aluminum lug joint with Lamb wave monitoring. • The robustness of the proposed method is tested on different specimens. Damage prognosis generally resorts to damage quantification functions and evolution models to quantify the current damage state and to predict the future states and the remaining useful life (RUL). The former typically consists of a function describing the relationship between the damage state and a statistical feature extracted from the measured signals, thus the prognostic performance will strongly depend on the selection of a proper feature. Given the best feature may vary for different specimens or even at each time instant for the same specimen during damage progression, such selection is a challenging task but has received little investigation so far. In this context, this paper proposes a particle filter-based damage prognosis framework, which involves an online feature fusion and selection scheme. A prognostic model is considered for each feature, with a multivariate process equation, formulated using both a damage degradation function and a bias parameter, and a measurement equation linking the damage state and that feature considering a data-driven model and the bias. One PF is used to estimate the damage state, its evolution parameters, and the bias for each model. Then, at each step, the feature with the smallest estimated bias is selected as the best feature providing the most likely state vectors and is used to select the most likely samples of the damage state and growth parameters for predicting the RUL and for calculating the prior at the next step. The proposed prognostic framework is demonstrated by an experimental study, where an aluminum lug structure subject to fatigue crack growth is monitored by a Lamb wave measurement system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Modified dynamic time warping method is proposed for non-uniform temperature compensation of Lamb waves. • Warping adjusting mechanism is introduced to resolve the temperature over-compensation problem. • A modified dynamic time warping-based imaging is developed to realize a triple-suppression for the complicated effects of non-uniform temperature variations on Lamb wave damage identification. In practical applications of Lamb wave monitoring, the environmental temperature fields usually vary non-uniformly over the monitored structures, which can inconsistently affect the wavepackets of every diagnostic signal and reduce the reliability of final detection results. A number of temperature compensation methods have been reported. However, under the complicated non-uniform temperature variations, many methods are deteriorated in their performance or require large amount of Lamb wave data. To effectively improve the reliability of Lamb wave damage detection under non-uniform temperature variations, modified dynamic time warping (MDTW) is presented in this paper. In MDTW, with the flexible time alignment ability of dynamic time warping (DTW), the non-uniform temperature variation effects on Lamb wave signals can be compensated just requiring one set of baselines. Furthermore, to preserve the damage information in the compensated signals, the warping adjusting mechanism is introduced to resolve the temperature over-compensation problem of traditional DTW. This makes MDTW a robust non-uniform temperature compensation method for high reliability Lamb wave detection. After the basic principle of DTW is analyzed, MDTW is proposed and numerically validated. Then, a MDTW-based Lamb wave imaging method is developed for high reliability damage imaging, in which a triple-suppression of non-uniform temperature variations can be accomplished. Finally, the efficiency of MDTW and MDTW-based imaging is demonstrated by the temperature experiment of A 0 mode Lamb wave damage imaging on an aluminum plate with its temperature field non-uniformly varied by 13℃∼43℃. [ABSTRACT FROM AUTHOR]

• Guided-wave propagation is characterized in 3D-printed plates with different infills. • A method to calculate phase velocity from the wave's time-spatial plot is introduced. • A method to experimentally determine dispersion curves and identify 5+ modes is shown. • Dispersion, slowness, tuning, and attenuation are highly affected by infill density. • Reference propagation characteristics for a wide range of infills are made available. Thanks to its many advantages, additive manufacturing (AM) is witnessing a gigantic shift from small applications to the production of complex industrial components, including safety–critical applications like aerospace and civil construction. This arises the need to develop accurate and robust technologies for evaluating and monitoring the structural integrity of AM components. The objective of this study is to comprehend how ultrasonic guided waves (GWs) propagate in 3D-printed poly(lactic acid) (PLA) plates of different infill densities. The experimental investigation involves five intact PLA plates printed with infill densities ranging from 20 to 100% at a step of 20%. A scanning laser Doppler vibrometer was used to visualize GW propagation by performing full-wavefield area-scans. Spatial-time, frequency-wavenumber, and wavenumber-wavenumber analyses were performed to scrutinize the available GW modes and their slowness profiles within the anisotropic structure. In addition, the tuning of the fundamental Lamb-wave (LW) modes at different excitation frequencies was examined, and the amplitude attenuation of the generally most pronounced mode (A 0 mode) was investigated for different infill densities at an excitation of 50 kHz. A reference methodology is presented to experimentally identify more than five propagating modes in the frequency-wavenumber domain then obtain their phase-velocity dispersion curves. Significant variations are observed in the dispersion characteristics when lowering the infill density leading to the appearance of more modes at lower cutoff frequencies and drops in both the group and phase velocities in most of the cases. A complex behavior of the guided waves was observed in the dispersion curves of the 20%-infill plate, with possible mode interruptions/disturbance at an excitation frequency beyond 70 kHz. The appearance of shear-horizontal modes is noticed/strengthened at oblique propagation directions which may be attributed to mode conversion of the LW modes after obliquely interacting with the internal structure of the plates. Though high wave attenuation is a characteristic of the used polymer material, this attenuation is further increased at lower infill densities. The tuning behavior of the fundamental LW modes is also shown to be significantly affected by the infill density. The presented results can serve as reference propagation and dispersion characteristics for researchers working on similar materials and geometries. Such knowledge and understanding can be particularly useful for GW-based structural healthy monitoring of AM structures. [ABSTRACT FROM AUTHOR]

In this work, a novel approach for the topology optimization of phononic crystals based on the replacement of the computationally demanding traditional solvers for the calculation of dispersion diagrams with a surrogate deep learning (DL) model is proposed. We show that our trained DL model is ultrafast in the prediction of the dispersion diagrams, and therefore can be efficiently used in the optimization framework. The main novelty of the proposed approach relies on the use of non-uniform rational basis spline (NURBS) curves instead of pixels and/or mesh elements to control the shape of the unit cells of phononic crystals. The surrogate DL model is combined with a genetic algorithm serving as a topology optimization tool. The validity of the approach is shown in the case of phononic crystals made of a continuous matrix with cavities. Several objective functions have been tested as an alternative to the most common gap to mid-gap ratio. This allowed us to obtain interesting phononic crystal geometries which can be easily additively manufactured. The proposed method applies to problems involving inverse design and can open new avenues in the design of computer-assisted periodic structures. [ABSTRACT FROM AUTHOR]

Bolts are commonly applied to lap joints in various industrial fields. Harsh environment, low quality of operation, and other unfavorable conditions may result in bolt looseness. Due to the structural complexity, conventional Lamb wave-based methods just extract statistical characteristics for looseness detection, while neglecting the information contained in the propagation process of Lamb waves. To take full advantage of the information on bolt states carried by array signals during the propagation, this study proposes a detection procedure based on propagation model and imaging methods of Lamb waves. Firstly, the contact status on the macro and micro scales is investigated with theoretical analysis and finite element simulation, and the theoretical model of Lamb wave propagation across bolted joints is established deriving from the analysis of contact. Subsequently, three imaging methods, including delay-and-sum, minimum variance, and sparse reconstruction, are adapted for the identification of bolt state. Since Lamb waves are mainly transmitted into another plate through the contact area near the bolts based on the proposed propagation model, pixel values are inclined to be concentrated around the bolts. On this basis, imaging intensity near each individual bolt can reveal its fastening state given that the transmitted energy around its location is associated with its torque. Finally, experiments are carried out in both single-bolt and double-bolt joints. The proposed propagation model is verified in the single-bolt joint and loose bolts can be identified in the imaging results of the double-bolt joint, which demonstrates the effectiveness of the proposed method for bolt state identification. [ABSTRACT FROM AUTHOR]

The vibroacoustic modulation (VAM) is a nonlinear ultrasonic testing method that utilizes the modulation of a high-frequency/low-amplitude probe wave with a low-frequency/high-amplitude pumping vibration, resulting in high sensitivities to damages in the structure. However, applying the method outside of the laboratory as a structural health monitoring system for actual structures is challenging, since the amplitude and frequency of the ambient vibrations – which is ideally utilized as pumping vibration – fluctuates over time. To circumvent this, we present a synthetic generation of the VAM signal using only the probe measurements , acquired at two (or more) stress levels of the structure when a steady state is reached. We could show that only 16 values (8 measured values of only two stress levels with a sampling frequency of 1/20 of the Nyquist frequency) are required to generate the often calculated Modulation Index with a mean deviation of 0.97% to the expected dynamic measured values for glass fiber reinforced composites and 1.86% for the aluminum specimens, which is negligible compared to a typical increase of the Modulation Index of 10–30 dB in case of severe damage. Even undersampled measurements at each stress level can be used without sacrificing accuracy, which reduces the sensing requirements for the sensor nodes. Moreover, this method decouples VAM from the actual need for a constant recurrent frequency and amplitude of the natural vibration in order to reliably compare measurements throughout the lifetime. Hence, this work aims to open the possibility of ultimately applying VAM to assess the structural health of complex structures. • Generation of VAM from static ultrasonic measurements for a robust and independent SHM. • Decoupling VAM from actual ambient vibration makes measurements comparable over time. • Data reduction of 99.99% compared to traditional VAM evaluation. • Feasible with undersampled signals resulting in reduced hardware requirements. • Enabling the application of VAM for actual structures. [ABSTRACT FROM AUTHOR]

• Impact testing was done under simulated environmental and operational conditions. • Different conditions generated significant variation in impact generated lamb waves. • Using known Normalised Threshold method gives inconsistent Time of Arrival due to variation. • Proposed Normalised Smoothed Envelope method gave more consistent Time of Arrival. • Proposed method resulted in better localisation accuracy under real life variation. A novel feature extraction method is developed for impact localisation based on Artificial Neural Networks (ANNs) in sensorized composite structures subjected to environmental and operational conditions. Impact induced lamb waves are investigated for the first time for different impact scenarios (angle, mass and energy) on flat and curved plates under environmental (temperature range) and operational (vibration) conditions. The Time of Arrival (TOA) is significantly influenced by these conditions hence complicating the impact localisation. To overcome this complication, a novel and robust TOA extraction method is proposed. It is based on Normalised Smoothed Envelope Threshold (NSET) coupled with a high pass filter to remove vibration noise prior to TOA extraction. Localisation ANNs were trained with data from a single baseline impact condition and were tested under impacts with varying conditions. It was shown that by using the proposed method for TOA extraction, the trained ANN is able to better predict the location of impacts compared to an ANN trained with data from common TOA extraction methods (detection area 0.99–56.08% of sensing region versus 0.28–1.55% for NSET). The developed method gives consistent accuracy and significantly reduces the required training data, making ANN based impact localisation more feasible for real life application. [ABSTRACT FROM AUTHOR]

• Elastic waves interference is proposed as a novel SHM method. • A single lap joint made of two thin plates bonded over an assigned area was tested. • Interference between generated and reflected signals can be constructive or destructive depending on the status of the joint. • A debonding length (deb) can be uniquely related to a specific attenuation frequency. • Destructive interference for health monitoring of single lap joints is novel and the approach is baseline-free. In the present paper a method for the Structural Health Monitoring (SHM) of bonded lap joints is presented. This method is based on the interference of elastic waves generated by piezo sensors and travelling along thin bonded plates through the adhesion area. Tone bursts of different extension were generated and, when they encountered the debonded area, the wave speed changed. This affected the wave reflection at the boundary of the disbond and the subsequent interference of the reflected wave with the main wave travelling along the joint. Destructive interference conditions were promoted when the adhesive was partially debonded and this was related to the length of the disbond. In the present study numerical simulations based on finite elements were performed and compared with a simplified analytical model describing the Lamb waves propagation through the joint. The simulation results were subsequently compared with the experimental data and a good agreement was found: the proposed method was simple, straightforward and its application on thin single lap joints provided reliable results. [ABSTRACT FROM AUTHOR]

Highlights • Fully noncontact Lamb wave phased arrays based on a pulse laser-scanning laser Doppler vibrometer sensing system. • An improved delay-and-sum (DAS) phased array imaging algorithm with better radial and angular resolutions. • Experimental demonstration of detection and localization of various defects on an aluminum plate. Abstract Lamb wave phased arrays employ sensors placed close to each other in compact distributions and can quickly inspect large plate-like structures through time (or phase) delays in the way analogous to radar. Traditional Lamb wave phased arrays usually adopt large-size ultrasonic transducers bonded on (or placed close to the surface of) a structure, and hence they limit the array configurability and cannot perform inspection from a far distance to the structure. This paper presents Lamb wave phased arrays implemented with a fully noncontact pulsed laser – scanning laser Doppler vibrometer (PL-SLDV) system, which employs a PL for exciting Lamb waves at a single PL spot through the thermoelastic effect and an SLDV for acquiring signals of Lamb waves at multiple scanning points based on the Doppler effect, respectively. The fully noncontact PL-SLDV phased arrays enable inspection from a far distance, as well as easily constructible receiver phased arrays in various configurations due to the use of high-resolution scanning laser. To generate inspection images with the acquired signals of Lamb waves, an improved delay-and-sum (DAS) imaging algorithm for receiver phased arrays is developed, where the exact Lamb wave frequency-wavenumber dispersion relation is considered for both the phase delay and back-propagation phase. This improved DAS imaging algorithm addresses the dispersion effect and results in higher radial imaging resolution, compared to the conventional DAS imaging algorithm. In addition, adaptive weighting factors are implemented to improve the angular imaging resolution. Proof-of-concept experiments demonstrate that multiple simulated defects of different sizes as well as broadside and offside simulated cracks can be successfully detected using a PL-SLDV phased array with a high-dispersion A 0 mode. The experimental study also shows that the improved DAS imaging algorithm can achieve high radial and angular imaging resolution. [ABSTRACT FROM AUTHOR]

Highlights • Develop a model function with five parameters to model the Lamb wave signal. • Adopt EM algorithm to obtain the parameters. • Study the convergence property using synthetic signals. • Experimentally study the effectiveness of mode identification and time locating. Abstract In the structural health monitoring or nondestructive examination system based on the Lamb wave technology, the accurate and valid characteristics of wave packet extracted from the signal are critical factors to evaluate damage. However, the dispersion effect and the multi-mode characteristic in the elastic wave make the data extraction difficult and degrade the resolution, and therefore further prevent the effectiveness of Lamb wave for damage detection. In this study, we proposed a model-based approach for extracting effective characteristics from the noisy signals. By taking the narrow band Gabor pulse as the incident pulse and considering the general non-linear frequency dispersion (quadratic dispersion), we developed a model with five parameters to model the dispersive wave packet and obtained the parameter vector of each wave packet by the expectation-maximization (EM) algorithm. The parameters in the model present the characteristics of signals, which can be further applied to locate and evaluate the structure's damage. To study the convergence property, synthetic signals with different sampling rates and noise intensities were considered. Furthermore the developed approach is also verified by the experimental data from an isotropic aluminum plate. [ABSTRACT FROM AUTHOR]

Highlights • Based on basic characters of trailing pulses, except for amplitude relation. • After the elimination, the trailing pulses will turn into individual reflection. • NDT for thick plates by trailing pulses is better than that of Lamb or bulk waves. Abstract Ultrasonic detection is one of the most commonly used methods in nondestructive testing. Trailing pulses that can be observed in plates and cylindrical rods are a list of longitudinal ultrasonic waves. However, the complexity of trailing pulses brings challenges for ultrasonic–based nondestructive testing. In this paper, a trailing pulses self-focusing method for damage detection in thick plates is proposed by utilizing physical characters of trailing pulses. Firstly, the characteristics of trailing pulses are analyzed by theoretical derivation. Then, three steps of trailing pulses self-focusing method including correlation filtering, energy envelope screening and defect matching are introduced. After that, the trailing pulses of single defect can be focused and the accurate positions of defects can be obtained as well. Finally, a prospective application of trailing pulses, which uses ultrasound phased array (UPA) for nondestructive testing, is discussed. The experiment is carried out on an aluminum plate with a remarkable thickness, which illustrates the effectiveness of the method. [ABSTRACT FROM AUTHOR]

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

Highlights • We propose TSST for impulsive-like signal whose TF ridge is nearly vertical. • TSST is reassigned in time direction and enables mode reconstruction. • We compare the SST and TSST and present respective application scope. • TSST is applied in lamb wave simulation and rub-impact fault diagnosis. Abstract Synchrosqueezing transform (SST) is an effective post-processing time-frequency analysis (TFA) method in mechanical signal processing. It improves the concentration of the time-frequency (TF) representation of non-stationary signals composed of multiple components with slow varying instantaneous frequency (IF). However, for components whose TF ridge curves are fast varying, or even nearly parallel with frequency axis, the SST still suffers from TF blurs. In this paper, we introduce a TFA method called time-reassigned synchrosqueezing transform (TSST) that achieves highly concentrated TFR for impulsive-like signal whose TF ridge curves is nearly parallel with frequency axis. Moreover, the TSST enables signal reconstruction, compared with the standard TF reassignment methods, such as reassigned short-time Fourier transform and reassigned wavelet transform. In the algorithm of TSST, the group delay estimator is calculated rather than the IF estimator. Furthermore, the TF coefficients are reassigned in the time direction rather than in frequency direction as the SST did. Then we compare the concentration between SST and TSST at different length of Gaussian window and chirp-rate, which is followed by the respective application scope of SST and TSST. Furthermore, we describe an efficient numerical algorithm for practical implementation of TSST. It is found that the SST is suitable for characterizing signal with small chirp-rate while TSST performs better for a large chirp-rate condition. Thus, the TSST is more capable of extracting transient features of impulsive-like signal. Finally, the effectiveness of the TSST and its inverse transform is verified by simulation and experimental studies. [ABSTRACT FROM AUTHOR]

• A rapid localization technique for multiple damages in plate-like structures is proposed. • The prior knowledge of direction-dependent velocity profile is not needed. • The method to remove the artifacts is given. • Multi-damage localizations at different locations are validated by simulation and experiment. When using Lamb wave for damage detection in plate-like structures, factors such as unknown material properties, anisotropy and iterative operation for solving nonlinear equations make it difficult to accurately localize damages. This problem becomes even more difficult when multiple damages exist in the plate. To overcome these difficulties, a rapid technique for localizing multiple damages in plate-like structures without any prior knowledge of direction-dependent velocity profile is proposed in this paper. The time difference of arrival is obtained by performing cross-correlation processing on the damage features extracted from the received signal. Damage artifacts are removed using relative location determination and relative probability density analysis. Multi-damage localization at different locations were simulated. The proposed technique is validated experimentally for steel plates. Simulation and experimental results show that the proposed technique can realize multiple damage localization in plate-like structures with unknown material properties. With the help of L-shaped sensor clusters, damage locations for isotropic and anisotropic materials can be quickly predicted without solving nonlinear equations and without taking into account the dispersion effects. The results suggest that a suitable threshold range is set to 0.5–0.7 to remove damage artifacts. [ABSTRACT FROM AUTHOR]

Full-field imaging of guided ultrasonic waves is an emerging Nondestructive Testing (NDT) technique for the inspection of thin-walled structures. One of the most effective full-field processing techniques is the Local Wavenumber Estimation (LWE) which can be applied to detect various types of damage in both homogeneous and multilayer specimens. In this paper, we discuss the possibilities for optimizing the efficiency of the LWE algorithm to facilitate its practical applications. We propose several different signal processing approaches to optimize the algorithm's computational efficiency without sacrificing the damage imaging quality. We evaluate the influence of these improvements by measuring the computation time necessary to perform particular processing steps of the LWE process. We also provide the implementation guidelines for the proposed optimization techniques. The proposed improvements allow for a considerable reduction in the processing time of the LWE with respect to the baseline implementation of the algorithm, ranging from a factor of 10 to a factor of 24, depending on the input dataset. [ABSTRACT FROM AUTHOR]

Lamb wave-based detection has become a promising technique for structural health monitoring in plate-like structures. However, the dispersion effect of Lamb waves makes the wave packets elongated, which degrades the resolution for damage identification. Hence, it is necessary to develop effective methods for the dispersion removal of Lamb waves. Recently, deep learning has drawn much attention for Lamb wave detection due to its powerful feature extraction capability. Most current deep learning models are developed directly for damage classification or location estimation tasks on the Lamb wave signals, rather than signal processing for further manipulation. Therefore, in this paper, a novel approach based on a convolutional auto-encoder is proposed for the dispersion compensation of Lamb waves. The convolutional auto-encoder model is utilized to construct the mapping relationship between the dispersive signal and the time of flight of the wave packet. The dispersion compensated signal is reconstructed by combining the estimated time of flight from the network with the waveform of the excitation signal. Numerical and experimental validations on aluminum and composite plates are implemented to verify the effectiveness of the proposed method. Compared with conventional methods for dispersion compensation, the results demonstrate that our proposed method not only separates the overlapped wave packets but also enables dispersion compensation of multimodal and multi-packet Lamb wave signals. In addition, this method is still applicable in the case of inaccurate dispersion data due to the generalizability of the data-driven model. [ABSTRACT FROM AUTHOR]

[Display omitted] • Material early-stage local damage can be located simply and efficiently. • The reflected zero-frequency component is a strong phase reversal reflection. • Most of the second harmonic transmits through the damage zone and little is reflected. • The concave-convex property is very sensitive to the relative magnitude of the TOE. Degradation, such as micro-voids, dislocation, persistent bands, and micro-cracks, can give rise to local material nonlinearity and cause early-stage material local damage. The material nonlinearity of a plate can be represented as a distribution function. Depending on whether the nonlinearity distribution function of a plate is smooth or not, early-stage material local damage can be divided into gradual and mutation damage. Gradual damage is usually caused by lattice distortion, and the nonlinearity distribution function is smooth. In contrast, mutation damage is usually induced by a discontinuous interface, and the nonlinearity distribution function is not smooth but stepped. Mutation damage is more harmful than gradual damage. To ensure safety, it is important to locate and evaluate mutation damage. The conventional nonlinear Lamb wave technique has limitations in resolving the location and spatial distribution of mutation damage owing to several reasons. To address these issues, a novel method for locating mutation damage based on the zero-frequency component is proposed in this paper. The theoretical solution of the zero-frequency component generated by the Lamb wave pulse is derived based on certain ideal assumptions. Other complex behaviors of the zero-frequency component can be understood by the contribution of this theory. The region of mutation damage is modeled by enhancing local three-order elastic coefficients, and mutation damage can be located simply and efficiently by using the proposed zero-frequency component-based method. The proposed damage detection based on the zero-frequency component of nonlinear Lamb waves has significant advantages compared to the second harmonic and mixing-wave detection techniques. [ABSTRACT FROM AUTHOR]

In this study, delamination in composite plates was detected by combining a baseline-free detection method based on a time-reversal method with a damage-imaging algorithm based on the reconstruction algorithm for probabilistic inspection of defects. A new adaptive time-reversal method (ATRM) for calculating the damage index values was proposed through the local adaptive analysis of the time-reversal signals of Lamb waves. The time-reversed signals were obtained by determining the maximum correlation between the forward path response and excitation signals. The forward path and time-reversal signals were intercepted using local optimal correlation coefficients to compensate for the peak offset errors that occurred during Lamb wave propagation. Multiple optimal adaptive decompositions were used to eliminate the interference of reflected waves in the forward path. The feasibility of the proposed method was verified numerically using finite element simulations, and was quantitatively compared with that of the conventional time-reversal method. Experimental studies using actual composite plates further validated the proposed method. The influence of the damage location on the performance of the proposed method was also investigated. The results show that the ATRM method significantly improved the temporal reversibility and obtained the optimal global similarity, making it more accurate and reliable for localising the delamination defects in composite plates compared with existing time reversal-based methods. [ABSTRACT FROM AUTHOR]

The waveguides with finite cross-section (such as rail, flat steel) are one of the most crucial configurations in infrastructure. When the shear horizontal (SH) guided waves are excited, the three-dimensional deformation of the elastomer will excite weak Lamb waves. In the finite cross-section waveguides, Lamb waves and SH waves are coupled together with each other. Mode coupling will influence the vibration direction of particles, resulting in wavelength changes of guided waves, which affects precise localization and quantification of defects. In this study, the SH0-like guided wave of flexural-like motion in the rectangular plate are analyzed theoretically. For the rectangular plate, the wavelength difference of SH0-like guided wave decreases with the increase of rectangular plate width and with the decrease of the excitation wavelength, respectively. The wave finite element method (WFEM) was used to calculate the dispersion curves of SH0-like guided wave, and the law of the wavelength changes is consistent with the theoretical analysis. Rectangular plates with different widths are selected and the excitation frequencies also vary according to different wavelengths. The changes of wavelengths and particle vibration direction are obtained by simulation, which are consistent with the theoretical analysis. The periodic-permanent-magnet (PPM) electromagnetic acoustic transducer (EMAT) with different wavelengths are constructed and used in the experiments to excite SH0-like guided wave. The wavelengths of experimental signal are calculated by two-dimensional fast Fourier transform (2DFFT), and the law of the wavelength changes are verified. [ABSTRACT FROM AUTHOR]

We propose a novel method for locating and imaging contact delaminations in composite plates, using a network of PZT transducers and the nonlinear Lamb waves. Being immune to noise and sensitive to weak signal, the Duffing chaotic oscillator is utilized to detect the nonlinear harmonics of Lamb waves generated at the contact delamination interfaces. The Lyapunov exponent of the Duffing system output is calculated to quantify the relative intensity of nonlinear second harmonic. Its normalized value is taken as the damage index to locate and image the delaminations by combining with the probabilistic imaging algorithm. The numerical simulation study on the proposed method is performed. Compared with the nonlinearity parameter and the conventional damage index, the Lyapunov damage index (LDI) can image the single delamination or multiple delaminations more effectively, especially under the noise interference. The experiment using the LDI is also conducted on a composite laminated plate with multiple internal delaminations. The results show that the proposed imaging method based on LDI is also valid to the practical delaminations in the composite plate. It has the following advantages: immunity to noise, needlessness of baseline data, high imaging resolution and location accuracy. [ABSTRACT FROM AUTHOR]

Piezoelectric transducer arrays are utilized in Structural Health Monitoring systems as a means for excitation and sensing of elastic waves. Anomalies of propagating waves have enabled to develop damage detection algorithms. Depending on actuation-sensing strategies these algorithms can be classified as pitch-catch and pulse-echo. Despite many signal processing methods such as delay-sum, time-reversal, probability-based diagnostic imaging, etc. the spatial damage information provided by the actuator-sensor paths to reconstruct the damage image is limited. A novel strategy based on Lamb wave focusing is proposed in order to increase damage imaging resolution. In the proposed method all actuators are used at the same time exciting specially designed waveforms so that inspect one specific point of the structure. Damage map is created by applying appropriate signal processing. It uses dispersion curve of selected Lamb wave mode for dispersion compensation. The dispersion curve is acquired by using laser scanning Doppler vibrometer. The damage indicator is calculated based on the energy of compensated signals registered by sensors. It is shown that apart from high energy level at excitation point, energy is concentrated exactly in the damaged region. An example of crack detection and visualization in an aluminum plate is shown confirming the accuracy of the proposed method. Also the proposed method is compared to well-established delay-and-sum algorithm. [ABSTRACT FROM AUTHOR]

The aim of this research is an analysis of full wavefield Lamb wave interaction with impact-induced damage at various impact energies in order to find out the limitation of the wavenumber adaptive image filtering method. In other words, the relation between impact energy and damage detectability will be shown. A numerical model based on the time domain spectral element method is used for modeling of Lamb wave propagation and interaction with barely visible impact damage in a carbon-epoxy laminate. Numerical studies are followed by experimental research on the same material with an impact damage induced by various energy and also a Teflon insert simulating delamination. Wavenumber adaptive image filtering and signal processing are used for damage visualization and assessment for both numerical and experimental full wavefield data. It is shown that it is possible to visualize and assess the impact damage location, size and to some extent severity by using the proposed technique. [ABSTRACT FROM AUTHOR]

Ultrasonic guided waves (GWs), e.g. Lamb waves, have been proven effective in the detection of defects such as corrosion, cracking, delamination, and debonding in both composite and metallic structures. They are a significant tool employed in structural health monitoring. In this study, the ability of ultrasonic GWs to assess the quality of friction stir welding (FSW) was investigated. Four friction stir welded AZ31B magnesium plates processed with different welding parameters and a non-welded plate were used. The fundamental symmetric (S 0 ) Lamb wave mode was excited using piezoelectric wafers (PZTs). Further, the S 0 mode was separated using the “Improved complete ensemble empirical mode decomposition with adaptive noise (Improved CEEMDAN)” technique. A damage index (DI) was defined based on the variation in the amplitude of the captured wave signals in order to detect the presence and asses the severity of damage resulting from the welding process. As well, computed tomography (CT) scanning was used as a non-destructive testing (NDT) technique to assess the actual weld quality and validate predictions based on the GW approach. The findings were further confirmed using finite element analysis (FEA). To model the actual damage profile in the welds, “Mimics” software was used for the 3D reconstruction of the CT scans. The built 3D models were later used for evaluation of damage volume and for FEA. The damage volumes were correlated to the damage indices computed from both experimental and numerical data. The proposed approach showed high sensitivity of the S 0 mode to internal flaws within the friction stir welded joints. This methodology has great potential as a future classification method of FSW quality. [ABSTRACT FROM AUTHOR]

The existence of a closed micro-crack in plates can be detected by using the nonlinear harmonic characteristics of the Lamb wave. However, its location identification is difficult. By considering the transient nonlinear Lamb under the noise interference, we proposed a location identification method for the closed crack based on the quantitative measurement of Duffing oscillator transient transfer in the phase space. The sliding short-time window was used to create a window truncation of to-be-detected signal. And then, the periodic extension processing for transient nonlinear Lamb wave was performed to ensure that the Duffing oscillator has adequate response time to reach a steady state. The transient autocorrelation method was used to reduce the occurrence of missed harmonic detection due to the random variable phase of nonlinear Lamb wave. Moreover, to overcome the deficiency in the quantitative analysis of Duffing system state by phase trajectory diagram and eliminate the misjudgment caused by harmonic frequency component contained in broadband noise, logic operation method of oscillator state transition function based on circular zone partition was adopted to establish the mapping relation between the oscillator transition state and the nonlinear harmonic time domain information. Final state transition discriminant function of Duffing oscillator was used as basis for identifying the reflected and transmitted harmonics from the crack. Chirplet time-frequency analysis was conducted to identify the mode of generated harmonics and determine the propagation speed. Through these steps, accurate position identification of the closed crack was achieved. [ABSTRACT FROM AUTHOR]

This paper focuses on the use of a Lamb wave-based methodology for ultrasound imaging of immersed plate structures. In these cases Lamb waves can be strongly attenuated due to leaky waves and viscous losses in the liquid, but there are low attenuation frequency bands that may be used for NDT applications. Experimental measurements were conducted to validate the existence of these low attenuation frequency bands, which were also theoretically predicted for some propagation modes, between the frequencies-thickness products of 0.5 MHz mm and 9.0 MHz mm. Using a 5 MHz linear-array and phased-array techniques, A1 and S1 modes are used to obtain images of an immersed aluminum plate with artificial defects. The signals are post-processed in order to select the desired propagation mode and to obtain an image with dynamic focusing in reception. While the A1 mode is strongly attenuated, the S1 mode, at 3.4 MHz mm, can be used to detect and localize defects in the immersed plate. [ABSTRACT FROM AUTHOR]

Many structural health monitoring systems utilize guided wave transducer arrays for defect detection and localization. Signals are usually acquired using the “pitch-catch” method whereby each transducer is excited in turn and the response is received by the remaining transducers. When extensive signal averaging is performed, the data acquisition process can be quite time-consuming, especially for metallic components that require a low repetition rate to allow signals to die out. Such a long data acquisition time is particularly problematic if environmental and operational conditions are changing while data are being acquired. To reduce the total data acquisition time, proposed here is a methodology whereby multiple transmitters are simultaneously triggered, and each transmitter is driven with a unique excitation. The simultaneously transmitted waves are captured by one or more receivers, and their responses are processed by dispersion-compensated filtering to extract the response from each individual transmitter. The excitation sequences are constructed by concatenating a series of chirps whose start and stop frequencies are randomly selected from a specified range. The process is optimized using a Monte-Carlo approach to select sequences with impulse-like autocorrelations and relatively flat cross-correlations. The efficacy of the proposed methodology is evaluated by several metrics and is experimentally demonstrated with sparse array imaging of simulated damage. [ABSTRACT FROM AUTHOR]

Honeycomb core sandwich structures (HCSSs) are used prevalently in the aerospace and marine industries. Use of guided wave-based inspections has attracted much attention in the recent past for facilitating the NDE of such structures. This paper presents the numerical and experimental analyses of the guided wave propagation through a tapered HCSS, with a primary motive to comprehend and further utilize the alteration of local wave properties as a function of the varying thickness. To this end, a low frequency fundamental antisymmetric mode is used as an interrogation signal. The numerical analysis is performed using 3D finite element simulations, while the experiments are performed on the portion of a helicopter blade with GFRP face sheets and aluminium core. The generation of the global guided waves (GGWs) as well as continuous variation of the local wave properties (wavenumber, group velocity, and phase speed) are ascertained both numerically as well as experimentally. Least-square polynomial fits are presented for the variation of group velocity along the propagation direction. A method of predicting the time of flight utilizing a non-uniform group velocity is outlined. Using this method and the polynomial fits, predictions of time-of-flight of the fundamental antisymmetric mode are made and corroborated with the observed values. Further, to circumvent the need of the prior knowledge of the group velocity variation in predicting the time-of-flight, a generalized expression of the variation of the group velocity is developed by employing the adiabatic nature of the GGW. The present analysis and the proposed methods can find their potential use in the wave-based NDE of tapered HCSS. • Low frequency ultrasonic guided wave propagation in helicopter blade structure is investigated. • Effect of smooth variation in core thickness of sandwich on guided wave propagation is analyzed. • Non-uniform variation in local wave properties is demonstrated, experimentally and numerically. • A method for prediction of ToF in light of the non-uniform group speed is presented. [ABSTRACT FROM AUTHOR]

A two-stage inversion scheme is proposed to determine the full set of 9 complex-valued stiffness properties of orthotropic viscoelastic plates using their 3D surface velocity response to a broadband vibrational excitation. A combination of the hybrid TLS-ESPRIT and IWC method is employed to the extract complex-valued wavenumber–frequency pairs corresponding to relevant Lamb wave and shear horizontal plate waves from the data from surface velocity response. Particle swarm optimization is chosen to inversely determine the plate's orthotropic viscoelastic properties using the semi-analytical finite element method as a forward model to compute the real and imaginary parts of the wavenumbers. The inversion procedure first identifies the elastic parameters by matching the real wavenumber values, followed by the characterization of the viscoelastic properties which are merely linked to the imaginary wavenumber values. To validate the accuracy of the proposed method, a series of numerical simulations of 3D wavefield data using a finite element model (COMSOL) is conducted, as well as broadband experimental measurements obtained by means of a 3D Infrared Scanning Laser Doppler Vibrometer. It is shown that the inverted orthotropic viscoelastic properties based on the virtual wavefield data are in close agreement with the target values, showing a mean relative error of less than 2% and 5% error on the elastic and viscous properties, respectively. Additionally, the median absolute deviations are negligible, showing the robust convergence of the inversion procedure to a global minimum, and giving confidence to apply the method on experimental wavefield data in order to reveal the full (unknown) C-tensor for material characterization. The proposed characterization method can be used for various orthotropic viscoelastic materials, including metal sheets, carbon/epoxy laminates, and wooden plates. • A two-stage inversion method is used to recover viscoelastic orthotropic parameters. • COMSOL simulations are conducted to create virtual data and to verify the inversion. • 3D IR-SLDV is employed to measure full field guided wave data. • A hybrid TLS-ESPRIT and IWC method is put forward to estimate complex wavenumbers. • The SAFE method is embedded as a forward model in a particle swarm optimizer. [ABSTRACT FROM AUTHOR]

In the conventional phased array imaging methods based on only single narrowband Lamb waves, as the scattering behavior between Lamb waves and defects is closely related to the wavelength and the defect size, the information within only a single narrowband is limited. Besides, limited by the size of elements in phased arrays, the ratio of the element spacing to the wavelength may not be able to reach a small enough value, as a result, grating lobes in some directions and high sidelobes will inevitably appear. Aimed at utilizing Lamb wave information contained in multiple frequency bands so as to improve the image quality of Lamb wave phased array imaging, a multi-narrowband fusion method is proposed. The information of multi-narrowband Lamb waves with different carrier frequencies is fused using the geometric mean. Because the positions of grating lobes and the sidelobes are related to the carrier frequency once the sensor array is fixed, the mainlobe is retained and grating lobes as well as sidelobes can be suppressed after the fusion of multiple narrowband signals with different carrier frequencies. So imaging results with fewer artifacts are achieved compared with that generated using only single narrowband signals. The results of damage localization on an aluminum plate with two defects verify the effectiveness of the proposed method. • A damage imaging method using multi-narrowband Lamb waves is proposed. • Grating lobes and sidelobes are suppressed while mainlobes are retained. • Imaging results with fewer artifacts are achieved. [ABSTRACT FROM AUTHOR]

The Lamb wave technique has been widely used for damage detection in plate-like structures. However, for two-dimensional (2D) defects like corrosions or one-dimensional (1D) defects like cracks, conventional Lamb wave-based methodology usually selects the detection algorithm in terms of the damage type, which is restricted if the defect remains unknown. To address the issues, this paper proposes a damage assessment approach based on the Lamb wave factorization method, which is a general method that can adapt to the detection of both 2D and 1D defects. In the process, both the forward-scattered wave across the inspection area and the wave back-scattered from the defect are used for detection, so that the enclosed rich information can be fully exploited. To measure the wave field of defect, a transducer array is firstly set around the inspection area, whose transducers would take turns to excite the Lamb wave while others remain listeners. On the basis, the scattered waves of all directions are then extracted and transformed from the time domain to frequency domain. Under a certain frequency, the inspection area is next visualized by sampling the area and solving the inverse scattering problem using the spectral data at each sampling point. By fusing the results of different frequency together, the final image can be eventually obtained. The method is then numerically investigated and also validated through experiments. The results demonstrate that it can give out the outline description of 2D defect and the size evaluation of 1D defect accurately, thus being an effective tool for damage assessment. [ABSTRACT FROM AUTHOR]

This paper presents a novel method based on modified Paris model with particle filtering (PF) framework to prognosis fatigue multi-cracks in metal structures. Since conventional Paris model is only applicable for the single crack, modified Paris model is proposed by introducing the mutual interaction between multiple cracks into stress intensity factor (SIF) expression and utilized to portray multiple fatigue crack growth process. To accurately monitor multi-cracks lengths, Lamb waves are periodically acquired during the fatigue growth of multiple cracks and fed into a deep autoencoder (DAE) network to automatically track response signals variations. Online monitoring model of multi-cracks lengths is then constructed by fitting the mapping relationship between the deep damage feature extracted by the bottleneck layer of DAE network and crack length. PF framework is further adopted to strategically fusing the prediction results of modified Paris model and real-time measurements of online monitoring model to reduce the uncertainties of material parameters and obtain more reliable prognosis results of multi-cracks lengths. The proposed method is demonstrated on center-hole metal specimens with two fatigue cracks. Experimental results show that the proposed method can accurately prognosis two fatigue cracks lengths, in which modified Paris model could more accurately describe multiple fatigue crack growth than conventional Paris model, and online monitoring model based on deep damage feature could more accurately track the fatigue crack length than artificial damage feature. [ABSTRACT FROM AUTHOR]

Improving the resolution of ultrasound array imaging systems for multiple defects in a measured medium is a popular research topic in nondestructive testing technology. However, because of the diffraction of acoustic waves, the resolution of ultrasound array imaging systems must follow Rayleigh's criterion and hence is limited, and defects with a spacing smaller than the resolution limit cannot be identified using conventional ultrasound imaging methods. Ultrasonic Lamb waves have dispersion and mode conversion characteristics, and the scattering acoustic field after the interaction between Lamb waves and defects is usually complex, making it even more difficult to improve the imaging resolution. In this paper, ultrasonic Lamb wave imaging is investigated with the sign coherence factor-total focusing method (SCF-TFM) when the spacing of the detected defects is smaller than the resolution limit, and the imaging resolution limit of SCF-TFM is analyzed. Experiments on a single blind-hole defect show that the lateral resolution of the SCF-TFM imaging is nearly twice as high as that of the total focusing method (TFM), with a higher signal-to-noise ratio. For experiments on multiple blind-hole defects, the resolution of the SCF-TFM improves by 71.32%, which is higher than that of the TFM when the spacing of the detected defects is larger than the resolution limit. The SCF-TFM enables super-resolution imaging and achieves a higher resolution limit than time-domain topological energy when the spacing of the detected defects is smaller than the resolution limit. [ABSTRACT FROM AUTHOR]

Structural Health Monitoring (SHM) of aeronautic structures by means of Lamb waves opens promising perspectives in terms of maintenance costs reduction and safety increases. Lamb waves interactions with damages are known to be nonlinear, a property still largely underexploited in SHM. Difficulties in this context are (i) to be able to distinguish between nonlinearities due to the waves spatial propagation (i.e. material or geometrical nonlinearities) and those located at the damage position, (ii) to handle computational complexity associated with spatio-temporal nonlinear models, and (iii) to be able to physically link recorded signals with actual damage state. This work proposes to rely on the Green–Volterra formalism to build up a semi-analytical spatio-temporal framework describing longitudinal waves propagation and damage interaction able to physically represent both types of nonlinearities, and computationally simple enough to be tractable in real-time for SHM purposes. This approach is detailed here for longitudinal waves, which corresponds in the low frequency × thickness range to the S 0 Lamb wave mode propagating in a damaged beam. A spatio-temporal semi-analytical model of the nonlinear longitudinal waves propagation is first derived, where the damage is represented by a polynomial stiffness characteristic acting via boundary conditions at a given position in the beam. This model is then used to derive the Green–Volterra series describing the nonlinear input–output relationship of the system. A modal decomposition of the Green–Volterra series is also provided to ease implementation and reduce computational cost. The proposed spatio-temporal semi-analytical approach is then successfully compared to state-of-the-art nonlinear Lamb waves simulation methods based on finite-element models. It is finally shown on a simulated example and discussed in detail how such a nonlinear framework could potentially be relevant for SHM purposes. • Lamb waves interactions with damages are known to be nonlinear, a property still largely underexploited in structural health monitoring. • An analytical spatio-temporal framework describing 1D-longitudinal waves propagation and nonlinear damage interaction relying on the Green–Volterra formalism is proposed here. • The proposed spatio-temporal analytical approach is successfully compared to state-of-the-art nonlinear simulation methods based on finite-element models. • It is finally shown on a simulated example and discussed in detail how such a nonlinear framework could potentially be relevant for SHM purposes. [ABSTRACT FROM AUTHOR]

The authors have developed a high-temperature ultrasonic sensing system with a regenerated fiber Bragg grating (RFBG) sensor, that was fabricated by annealing a fiber-optic Bragg grating (FBG). Different from the common FBG, the RFBG ultrasonic sensor is resistant to temperature as high as 900 °C. In this study, taking advantage of the RFBG sensor, we attempted to establish a laser-ultrasonic visualization method for in-situ damage diagnosis in heat-resistant materials at elevated temperatures. In the first part of this paper, after reporting the fabrication of the RFBG in detail, we validated that the RFBG sensor has reasonable sensitivity to ultrasonic measurement over a broad frequency range at high temperatures. The RFBG was then incorporated into a laser-ultrasonic visualizing inspector to visualize the ultrasonic wave propagating in a SiC plate with an artificial defect at 800 °C. An improved data processing method was employed to verify the reliability of the visualization result. In the processing method, the visualization result was transformed to the spectrum in the wavenumber-frequency domain with the help of three-dimensional fast Fourier transform. Referring to theoretically calculated dispersion curves of the Lamb wave propagating in the SiC plate, we identified S 0 and A 0 modes from the spectral results. In addition, the agreement between the theoretical results and the experimental results also verified the feasibility of our proposed system to visualize the ultrasonic wavefield in high-temperature environment. Moreover, we developed a window function to extract the wave modes from the visualization result based on the dispersion curves. The artificial defect in the SiC plate could be then clearly localized from the visualization results for the separated individual modes. [ABSTRACT FROM AUTHOR]

Interpretation of signals related to Lamb waves propagation and scattering can rise serious difficulties due to the multi-modal nature of these waves. Different modes propagating with different velocities can be mixed up and hinder extraction of damage reflected components. As a feasible solution to this problem we propose a technique for separation of the propagating modes using a new type of polarization filter. The proposed directional polarization filter (DPF) can be applied if two components of particle movement, the in-plane and the out-of-plane, are available, for instance, from the measurement performed using laser vibrometer. The DPF is robust in the sense that it does not need a complete amplitude information of both components. Operation principle of the DPF is presented and illustrated by the simulated results in the form of B-scans obtained for an aluminum plate. The simulated results are verified by the experimental data obtained by processing the signals captured using a 3D laser vibrometer. [ABSTRACT FROM AUTHOR]

The performance of Lamb wave based monitoring systems, both in terms of diagnosis time and data complexity, can be enhanced by increasing the number of transducers used to actuate simultaneously the guided waves in the inspected medium. However, in case of multiple simultaneously-operated actuators the interference among the excited wave modes within the acquired signals has to be considered for the further processing. To this aim, in this work a code division strategy based on the Warped Frequency Transform is presented. At first, the proposed procedure encodes actuation pulses using Gold sequences. Next, for each considered actuator the acquired signals are compensated from dispersion by cross correlating the warped version of the actuated and received signals. Compensated signals form the base for a final wavenumber imaging meant at emphasizing defects and or anomalies by removing incident wavefield and edge reflections. The proposed strategy is tested numerically, and validated through an experiment in which guided waves are actuated in a plate by four piezoelectric transducers operating simultaneously. [ABSTRACT FROM AUTHOR]