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Data-driven machine-learning models offer considerable promise for acoustic source localization. However, many existing models rely on training data that correlates time-of-flight (TOF) measurements with source locations, yet they struggle to handle the complexities arising from nonlinear wave propagation in materials with varying properties. Furthermore, these models overlook the noise and uncertainties inherent in real-world experiments when predicting outputs. This paper aims to bridge a gap in impact localization for such structures, particularly focusing on scenarios involving noisy field measurements. This study proposes a framework based on probabilistic machine learning to identify impact locations, utilizing wavelet scattering transform (WST) and Multi-Output Gaussian Process Regression (moGPR). WST extracts informative features from Lamb waves, capturing relevant signatures for training the probabilistic machine learning model, while moGPR estimates correlated impact location coordinates (x, y) while accounting for inherent uncertainties in the data. To assess the proposed method's performance in handling measurement uncertainties, an experiment was conducted using a CFRP composite panel instrumented with a sparse array of piezoelectric transducers. The results demonstrate that the probabilistic framework effectively addresses measurement uncertainties, enabling reliable source location estimation with confidence intervals and providing valuable insights for decision-making. • The research proposes a probabilistic ML framework for impact localization. • Methodology involves WST and moGPR for impact localization with confidence interval. • Results demonstrate improved accuracy of WST over TOF-based approaches • Experimental validation demonstrates its ability to handle measurement uncertainties [ABSTRACT FROM AUTHOR]

• Multi-domain features characterize different stages of damage. • VAE for dimensionality reduction and fusion of high-dimensional features. • The proposed semi-supervised model accurately predicts and quantifies damage. • Hyperparameter value functions improve the accuracy and robustness of the model. Accurate monitoring of the damage state of composite structures is crucial. However, uncertainties such as complicated dispersion processes make it difficult to monitor the damage state. In this paper, a semi-supervised model integrating variational auto-encoder (VAE) and Bayesian neural network (BNN) is proposed to predict the damage state. A high-dimensional damage feature set is constructed by extracting different features from Lamb wave signals, which are then fused and downscaled by the VAE model's hidden layer. To avoid overfitting, noise signals are added into the VAE hidden layer to generate diversity samples. Finally, the VAE hidden layer is fed into the BNN network to predict the damage state of the composite plate, and the model's uncertainty is measured with 95% confidence intervals. Experimental validation results on composite plate demonstrate that all experimental measurements fall within the 95% confidence interval, achieving accurate prediction of the damage state of the composite plate. [ABSTRACT FROM AUTHOR]

Guided Lamb waves are highly valued in structural health monitoring for identifying critical damage. The essence of Lamb wave-based damage detection is finding and processing damage-sensitive features (DSFs) that can reliably reflect and interpret structural damage. The primary emphasis of this work is the first time a comprehensive evaluation and comparison of the fifteen most commonly used DSFs in the time, frequency and time–frequency domain for Lamb waves, along with their statistical and wave interactive features based on the proposed five feature evaluation criteria. Five feature evaluation criteria are proposed concerning diagnostic capability, stability against environmental variability, computational complexity, sensor framework and structural complexity. These criteria aim to facilitate the evaluation of the robustness and reliability of each feature category. These evaluations highlight the strengths and limitations of each feature and qualitatively quantify its performance at four levels. Additionally, the paper provides explicit guidance on selecting appropriate Lamb waves' features, including when to use specific features for damage description and determination. The Lamb wave-based data pre-processing and damage characterization approaches, as well as the influence of environmental and operational factors on damage features, are also discussed in this work. Conclusively, this work outlines four prospective research avenues concerning DSFs. • Proposed are 5 feature evaluation criteria for damage-sensitive features (DSFs) evaluation. • Evaluated and compared are the 15 commonly used Lamb waves' DSFs for the first time. • Offered are 7 recommendations on the selection of appropriate DSFs. • Presented are 5 methods of excitation and reception of lamb waves. • Discussed in detail are the effects of 6 different environmental and operating conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel two-stage non-destructive procedure for determination of elastic constants was proposed. • Ultrasonic waves were successfully used to characterize elastic behaviour of AM samples with different raster angles. • Integrated use of bulk and guided waves was proved to be a simple but robust approach. • The method allowed characterization of samples represented by transversely isotropic and orthotropic material models. The subject of the current work was a simple but robust novel two-stage procedure for the non-destructive determination of effective elastic constants using ultrasonic wave propagation. First, ultrasonic bulk wave velocities measured on cubic samples were used to calculate most of the elements of the stiffness matrix. Secondly, the remaining elements were determined using the dispersion curves of elastic guided waves measured on plate samples. Based on the complete stiffness matrix it was possible to calculate the complete set of effective elastic constants. The algorithm was verified for AM elements produced from PLA filament satisfying the conditions of transversely isotropic and orthotropic material models. For the transversely isotropic samples, Young's moduli (E) varied from 2.6 to 2.9 GPa, shear moduli (G) equalled between 0.9 and 1.2 GPa, whereas Poisson's ratios (ν) ranged between 0.20 and 0.32. In orthotropic sample the corresponding values were: E = 1.3–2.8 GPa, G = 0.6–1.2 GPa, and ν = 0.07–0.59. The results of the current study have been compared with references from the literature, giving satisfactory agreement. [ABSTRACT FROM AUTHOR]

• Broadband signal is used for excitation and the full wavefield is measured using SLDV. • A multi-stage vibrometric NDT approach is introduced for defect detection in plates. • The proposed broadband entropy-based two-stage damage imaging algorithm provides high contrast damage map, revealing both shallow and deep defects. • The Method is successfully tested on the experimental data of different test specimens containing various defect types. This paper presents a new algorithm called broadband entropy-based two-stage damage imaging (BE2DI) for the detection and imaging of defects in multi-layer plates. The method differentiates defects from either local scattering phenomena of propagating Lamb waves or locally increased vibrational activity for stimulated global plate resonances by exploiting the higher vibrational activity of full-field broadband vibration signals at defective regions and the distinct spatial behavior of defects and artifacts. Using these features, first, a candidate damage map is constructed by means of iterative thresholding based on descriptive statistics of broadband data. Afterwards, an adaptive regional damage index based on local entropy is introduced, which utilizes operational deflection shapes to process previously identified candidate regions and subsequently reveal both shallow and deep defects. BE2DI can be seen as an automated local defect resonance identification approach in the case of shallow defects. Multiple plates having various defect types with different sizes and depths are investigated. Broadband chirp signals are used to excite the plates using low-power piezoelectric actuators, and vibrational data is registered by 3D scanning laser Doppler vibrometry. The obtained results quantitatively and qualitatively demonstrate that the proposed method can effectively detect different kinds of damage, regardless of being shallow or deep. [ABSTRACT FROM AUTHOR]

• Diffuse Lamb-wave field is utilised for fatigue-crack detection in an aircraft panel. • Extensive experimental programme of fatigue testing is undertaken. • Ultrasonic Lamb-wave responses are captured using a network of piezoceramic transducers. • Phase/frequency characteristics of Lamb waves are used to detect and monitor fatigue cracks. • Outlier statistical analysis is performed to support the analysis. A diffuse Lamb-wave field is utilised for fatigue-crack detection in a multi-riveted strap-joint aircraft panel. The work involves an extensive experimental programme of fatigue testing. Ultrasonic Lamb-wave responses are captured using a network of low-profile, surface-bonded piezoceramic transducers. Various phase/frequency characteristics of Lamb waves are used to detect and monitor fatigue cracks. An outlier statistical analysis – based on these characteristics - is also performed for damage detection. The results show that phase analysis can be used successfully for damage detection in such structures. In contrast, frequency analysis does not offer reliable damage detection. The results demonstrate the potential and limitations of the method for reliable fatigue-crack detection in complex aircraft structures. [ABSTRACT FROM AUTHOR]

• The MC1-DCGRU was proposed to solve the problem that 1-DCNN cannot capture time-varying information in Multi-channel Lamb waves. • Proposed an "end-to-end" damage localization model combining wavelet threshold denoising and MC1-DCGRU. • High-precision and high-resolution damage localization were realized with only single-point Lamb wave excitation and seven sensors. • Multi-point cumulative damage localization was realized by the MC1-DCGRU generalization capabilities, which could also be applied to self-healing CFRP. Lamb wave has become one of the most promising methods for carbon fiber-reinforced plastics (CFRP) damage detection. However, the wave velocity difference caused by CFRP anisotropy makes the time information-based localization methods lack accuracy. Obtaining high resolution and accuracy with limited excitations and sensors is difficult, which leads to difficulties in localizing small-size impact damage. Therefore, this paper proposed multi-channel one-dimensional convolutional gated recurrent unit (MC1-DCGRU) for surface dent deformation and inter-laminated damage caused by low-velocity impact behavior localization combining the feature extraction capability of the multi-channel one-dimensional convolutional neural network (MC1-DCNN) and the temporal information capturing ability of gated recurrent unit (GRU). The localization mean Euclidean distance (MED) of single-point and two-point cumulative damage is 4.29 mm and 5.78 mm, respectively. The proposed method overcomes high-resolution requirements for damage localization. It achieves higher accuracy without increasing excitations and sensors, which also achieves multi-point cumulative damage localization. [ABSTRACT FROM AUTHOR]

• A non-contact ultrasonic method of gas flow metering in a duct is described. • Wave propagation was visualized by COMSOL Multiphysics®. • Multipath signals with different time shift were successively received by a receiver in this method. • Receiver could collect the signal in a wide region (from 50 mm to 250 mm in the x direction in this work). • Simulations and experiments show good agreement with the analytical model (RMSE are 0.0708 and 0.0767 respectively). This work investigated the signal reception from multipath including direct transmission and reflection paths in a non-contact gas flow measurement system based on air-coupled leaky Lamb waves. Analytical model was established to provide the multiple reflective signal propagation time differences between upstream and downstream paths at different flow velocities. A multiphysics model based on finite element methods was established to facilitated the visualisation of the leaky Lamb waves reflected between the plates and the Lamb waves excited by multiple reflective paths waves. The propagation time differences were evaluated by performing simulations and experiments, which are in good agreement with the analytical values (RMSE are 0.0708 and 0.0767 respectively). The receiver can collect multipath signals at a large distance separated from the transmitter. Time resolution is enhanced by using received signals from reflection paths, as the transit time increased with the ultrasound path between the transducers. [ABSTRACT FROM AUTHOR]

• Simultaneous excitation of multiple transmitters is efficient for data acquisition. • Sparse reconstruction imaging for Lamb wave simultaneous excitation is proposed. • The proposed method is applied for high-resolution damage imaging in CFRP. Most Lamb wave methods utilize transducer array for damage detection and localization. Simultaneous excitation of multiple transmitters is a strategy for efficient data acquisition, in which matched filtering is generally applied for signal separation from different sources. However, the signal-to-noise ratio (SNR) and subsequent damage imaging performance is limited due to transmitter interference. To obtain both efficient data acquisition and high imaging performance, sparse reconstruction imaging for Lamb wave simultaneous excitation system is proposed in this paper that exploits the sparsity of structural damage. The key of success is to construct accurate damage-reflection Lamb wave model based on the linear characteristic of simultaneous excitation system. In addition, the model performance is further improved by excitation waveform design for each transmitter. The effectiveness of the proposed method is experimentally demonstrated with array imaging of simulated delamination in composite laminates. [ABSTRACT FROM AUTHOR]

• Fast phase shift technique for rapid inspection of samples is presented. • Results were compared with those extracted by three-step phase shift. • In term of accuracy, the obtained results are not bad. • This technique can be applied for fast inspection of optoelectronic samples. This paper presents a two-step phase-shifting interferometry technique for fast inspection of uniformity of optoelectronic samples. The in-line configuration was applied to make full use of the space-bandwidth product (SBP) of the camera and hence can resolve finer spatial details of the sample. Two-step phase-shifting (TSPS) algorithm was used to reconstruct the object with only two in-line interferograms generated when a quarter wave plate (QWP) is placed in the light path of one arm of the Mach-Zehnder interferometer. Experimental results show that the extracted phases calculated by the TSPS technique match with the phases extracted by the three-step phase-shifting (THSPS) technique. [ABSTRACT FROM AUTHOR]

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

Delamination is one of the most common types of damage in carbon fiber reinforced polymer composites. This paper analyzes the interaction of fundamental symmetric (S0) and anti-symmetric (A0) mode Lamb waves with the start and the end of a delamination using finite element simulations. Propagations of Lamb waves in a 4-layer [0/90]s laminate were compared with propagations obtained in a 1-layer [0] sub-laminate and in a 3-layer [90/90/0] sub-laminate, which are the two sub-laminates separated by the delamination. Results show that the Lamb waves travel at different velocities in the delaminated region and in the healthy region. Comparing the signals obtained in the delaminated region with the excitation, it was found that mode conversions happen at the edges of a delamination for both S0 and A0 waves. The mode conversion of S0 wave at the end of a delamination generates large A0 waves and causes most significant changes to the detection signal. Based on this feature, a reference-free localization method is proposed in this paper. By measuring the difference between the arrival times of two modes, the distance between pick-up sensor and the end of delamination can be estimated. This method was verified by signals obtained in finite element simulation. [ABSTRACT FROM AUTHOR]

Wave propagation signals are commonly used as information carrier in structural health monitoring. To facilitate decision making, wave signals are often decomposed into multiple components to reveal its frequency or time-frequency content. In this research we investigate the use of time-frequency decomposition techniques for feature extraction. The method based on the adaptive harmonic wavelet transform (AHWT) possesses high computational efficiency and at the same time inherently avoids certain issues in some other time-frequency feature extraction methods, e.g., the empirical mode decomposition (EMD). EMD entertains the adaptivity with respect to signal features, but that adaptive nature affects the comparability of the resulting intrinsic mode functions (IMFs). In contrast, the AHWT based method has similar feature extraction capabilities in the time-frequency domain while using a deterministic basis. Therefore, wavelet features can become cross-comparable when common wavelet basis is used. Our case study shows that the AHWT based approach can identify the critical features in Lamb wave signals to realize effective and robust decision making in structural damage detection. [ABSTRACT FROM AUTHOR]

• An improved probabilistic damage imaging algorithm for tracking hole-edge crack growth is proposed. • Unifying the relationship between the damage index and the relative ellipse size control factors from the crack to the transmitting-receiving paths. • Using the unified relationship to define the new dynamic ellipse control parameters. • The crack imaging error of the improved PDI algorithm is within 1 mm, which can clearly show the position and length of the crack. Probabilistic damage imaging (PDI) algorithm based on Lamb wave is one of the most promising structural health monitoring technologies for quantifying the hole-edge crack. However, it is still a challenge how to use a limited number of sensors to quantify the hole-edge crack. This paper proposed an improved probabilistic damage imaging algorithm (IPDI) to track the hole-edge crack growth. A simulation has been conducted to deeply study the interaction of cracks and Lamb waves. A damage index calculated by the frequency-domain energy has been found to have a good capacity for monotonically characterizing crack propagation. The further study derives a unifying relationship between damage index and the ellipse size control factor from the crack to transmitter–receiver paths with different lengths and angles. The unifying relation is used to define a new dynamic elliptical control parameter in PDI algorithm to improve the capacity of crack quantification. The improved PDI algorithm is verified by both of simulation and experiments with different exciting frequencies, whose results show that the crack imaging error of the IPDI algorithm is below 1 mm, and the accuracy is much higher than that of the traditional PDI algorithm. The proposed IPDI algorithm can effectively quantify the hole-edge crack growth. [ABSTRACT FROM AUTHOR]

The characterization of the full set of elastic parameters for an orthotropic material is a complex non-linear inversion problem that requires sophisticated optimization algorithms and forward models with thousands of iterations. The intricacy of this type of inversion procedure limits the possibility of using these algorithms for large-scale automation and real-time structural health monitoring. At this point, the introduction of machine learning-based inversion strategies might become helpful to overcome the existing limitations of conventional inversion algorithms. In the present study, a multilayer perceptron algorithm is used to identify elastic stiffness parameters of orthotropic plates using guided wavefield data. A large and diverse training dataset is created by using a semi-analytical finite element model, and the effect of both the training dataset size and the signal-to-noise ratio on the inference outcome are examined. The performance of the multilayer perceptron-based inversion method is first validated on a numerical dataset, and the method is then further applied on experimental data obtained from a multilayered glass-fiber reinforced polyamide 6 composite plate. Finally, the multilayer perceptron-based inference results are compared with the outcome of a traditional inversion algorithm, showing a difference of less than 0.5%. • A multi-layer perception method is used to determine the elastic orthotropic parameters of materials. • SAFE is embedded as a forward model to create training dataset. • COMSOL simulations of ultrasonic wave propagation are conducted to generate virtual data and to verify the inversion. • 3D IR-SLDV is employed to measure full field guided wave data and to validate the inversion for wood and GFRP. [ABSTRACT FROM AUTHOR]

• A solid-liquid-solid coupling interface model was designed. • Mechanism of AE signal propagating in the model was expounded. • The effect of oil film thickness and center frequency on AE signal was analyzed. • Passing coefficient was proposed to characterize the propagation law of AE signal. The propagation path of acoustic emission (AE) signal contains several structures with solid-liquid-solid coupling interfaces. In this paper, the effect of the coupling interface on the AE signal propagation is investigated. Based on the theory of Lamb wave, a solid-liquid-solid coupling interface model is designed. The propagation characteristics of the AE signals with different frequencies are predicted by simulation in the coupling models with different oil film thicknesses and contact areas. Furthermore, experimental verification is carried out. The results of the experiments are consistent with the numerical simulations. The results indicate that the oil film thickness and the center frequency will have an effect on the propagation of AE signal. Two passing coefficients are proposed to quantify the propagation characteristics. This paper provides guidance for the study of the AE signal propagation characteristics in the complex structure with multi-media interfaces. [ABSTRACT FROM AUTHOR]

• How cointegration theory can be applied to SHM is clearly explained for the first time. • Common trend removal and anomaly detection using cointegration are clarified. • Two case studies using time series data and Lamb wave data are presented. • Providing a clear explanation of the technique for readers and potential users. Cointegration theory has been effectively applied to SHM. However, there is no work that has focused on explaining specifically how cointegration can be applied to SHM problems. Consequently, the technique is not fully clear for readers and one may experience technical challenges when approaching it. This work aims to mitigate the drawback through providing a clear explanation and illustration for three concerns: (1) how can common stochastic trends, induced by varying operational and environmental conditions, be removed from the analysed data? (2) how can a fault or damage be detected using cointegration residuals? (3) how are the relations between the number of cointegrated variables, common stochastic trends, and cointegration residuals? First, these concerns are explained using simulation data generated by four time series processes. Then, a case study using Lamb wave data in the form of time series is presented. Three different damage conditions are considered. Lamb wave series in the presence of an increasing temperature trend and a simulated nonlinear trend are analysed using cointegration. The results show that cointegration can remove both natural and artificial common trends from Lamb wave series, and the peak-to-peak and variance values can be used to detect and classify the damage. [ABSTRACT FROM AUTHOR]

• The quantitative classification of composite structures is challenging. • The sparse sensing method has become one of the application trends of structural health monitoring. • A sparse sensor arrays and guided waves based damage detection and evaluation method is proposed to achieve multi-area quantitative damage location, classification, and identification with a limited number of sensors. Damage detection techniques using Lamb waves have shown excellent capabilities in the diagnosis of composite structures. However, structural health monitoring of composite structures is challenging, especially for damage classification. This study proposes a machine learning-based method with a sparse sensor array to achieve quantitative classification of the damage location and severity on a composite plate. First, multi features extraction is used to construct a support vector machine (SVM) damage localization model. Second, optimal path extraction combined with principal component analysis (PCA) is used to construct an SVM model for classification. To reduce the operational burden of structures, the sparse array is employed. To improve the damage classification accuracy, Fisher clustering is proposed to extract the optimal detection path. Then, PCA is used to achieve data fusion. Experimental results on a glass fiber-reinforced epoxy composite laminate plate demonstrate that the proposed technique can accurately locate and classify the quantitative artificial damage. [ABSTRACT FROM AUTHOR]

• A genetic algorithms-based waveform inversion method is developed for wavenumber extraction of one Lamb wave signal. • The plate properties are identified with the chromosome of searched individual. • The elastic formula of Lamb waves is solved via the bisection method to derive the wavenumber curves. Wavenumber dispersion of ultrasonic Lamb waves associating with the thickness and material property of plates plays an important role in waveguide characterization and non-destructive testing. Classical waveguide characterization with wavenumber estimation methods is performed based on a large amount of Lamb wave signals or sensitive to wave mode overlapping and noise. A genetic algorithms-based waveform inversion method is developed for wavenumber extraction of one Lamb wave signal and characterization of plate in this work. The individual chromosome is encoded in real number by the thickness and material properties of the structure. The elastic formula of Lamb waves is solved by the bisection method to further derive the wavenumber curves; then the wave mode signals are generated based on the propagation function, and are linearly combined to form the individual signal. The signal with the minimum residual ratio to the analysis signal is iteratively searched by the genetic algorithm. Finally, the plate properties and wavenumber of Lamb waves are identified with the chromosome of searched individual. Numerical and experimental studies prove the developed waveform inversion method can realize accurate wavenumber extraction of a Lamb wave signal, even though its time–frequency representation has low sparsity, and characterization of plate. The residual ratio between the inversion wavenumber and theory curves is around 1%; and the estimation deviation of material properties of plate is around 6%. [ABSTRACT FROM AUTHOR]

• Phase difference is generated by a multi-longitudinal mode laser with a wave plate. • Quadrature phase difference is constructed by a phase-shifted quadrature-phase demodulation algorithm. • The proposed quadrature-phase demodulation is unaffected by the inherent phase shift error produced by other optical components in the optical path. • Simplified displacement demodulation process and might be applied in online displacement measurement. • The max error is 37 nm with a relative error of about 0.37 % for a target displacement of 10 μm. We present a novel method by a multi-longitudinal mode laser with a waveplate to generate two orthogonally polarized signals with stable and adjustable phase differences. The phase difference is observed from the output of the Wollaston prism, and is determined by the angle between the optical axes of the Wollaston prism and the waveplate. A phase-shifted quadrature-phase demodulation algorithm is proposed to construct a signal with a 90° phase-shift between one of the two polarized signals. This outcome allows us with low requirements on optical component adjustment accuracy and optical element manufacturing accuracy and also simplifies the processing of phase demodulation for displacement measurement under very weak optical feedback level. Experimental results show that the error is less than 38 nm when the displacement is 10.00 μm. Given its simplicity and efficiency, the proposed system has guiding significance for the design of the self-mixing interferometers and the high-precision online measurement. [ABSTRACT FROM AUTHOR]

• A material multiparameter measurement method is proposed by inversing Lamb wave dispersion curve. • The statistical significance of the influences of various error factors was tested through an orthogonal experimental design. • This method performed better in anti-interference ability, computational efficiency, and computational accuracy. Lamb wave has become an important non-destructive measurement method due to its advantages of small attenuation, easy excitation, and simultaneous inversion of multiple parameters. According to its propagation properties, we proposed an improved particle swarm optimization method for the inversion of the thickness and elastic constants of plates in the study. Numerical simulations and validation experiments further confirmed that the Young's modulus, Poisson's ratio, and thickness of plates could be accurately obtained from the measured zero-order modal Lamb wave dispersion curves. Then, through a series of orthogonal experiments, the influences of the error sources of measurement points on inversion parameters were explored. In addition, the statistical significance of the above influences on each inversion parameter is discussed in detail. This study provides a basis for the accurate and efficient non-destructive measurements based on Lamb waves. [ABSTRACT FROM AUTHOR]

• Interface debonding defects detection method was proposed based on Lamb wave. • The principle of the detection method was depicted. • The detection method was validated numerically and experimentally. • The damage evolution index was defined to determine the initiation and evolution of damage. Grouted connections (GCs) are transitional elements that have been extensively employed in offshore wind turbine support structure to transfer loads from superstructure to foundation. However, interface debonding defects between steel tube and grout layer have an adverse effect on the durability and fatigue resistance of the GC. An effective interface debonding defects detection approach for GC is necessary. In this study, interface debonding defects detection approach based on Lamb wave energy leakage was proposed and validated numerically and experimentally. Numerical studies on Lamb wave energy leakage models and the GC models were performed to study mechanism of detection approach. Lamb wave is excited on the surface, and stress wave fields and time-history response of models were extracted and compared. Experimental investigations were carried out to verify the applicability of the proposed approach in practice. A scaled GC specimen was constructed and Lamb wave was excited by the PZT patches mounted on the surface of GC specimen. The output voltage of surface-mounted PZT sensors under different axial loads were compared. Both numerical and experimental results show that the proposed approach is effective to the detection of interface debonding defects. The damage evolution index (DEI) based on wavelet packet energy is defined and it can be adopted to qualitatively determine the initiation and evolution of the damage. [ABSTRACT FROM AUTHOR]

Effective application of the Lamb waves for structural health monitoring and damage identification intensively relies on the accurate damage-related feature extraction in the received signals. Most of existing signal processing methods extract the damage-related features from the time–frequency joint spectrum which requires a quite amount of effort. In this paper, the soft-thresholding process, based on different signal decomposition methods, is introduced to damage identification so that the damage-related signal features can be manifested more distinctively. By applying two popular signal decomposition methods (i.e., the discrete wavelet transform (DWT) and the empirical mode decomposition (EMD)), the signal of interest can be represented by a series of components with different frequencies. Since most noises exist in the high frequency range, it is feasible to alleviate noise by restricting the energy of high-frequency components. Finally, a denoised signal is synthesized using the corresponding reconstruction method. As an application, the soft-thresholding process is performed to detect a small crack on an isotropic aluminum plate under the white Gaussian noise contamination. The results, from both the numerical finite element simulation and experimental test, indicate that the soft-thresholding process is capable of effectively reducing the effect of noise, convincingly improving the sensitivity of damage identification, and discriminating relatively small damage. [ABSTRACT FROM AUTHOR]

• The optimal Lamb wave mode and frequency were selected through an optimization process to detect wall thinning in the plate efficiently. • The optimization problem is formulated to minimize the number of inspection modes with constraints in wavenumber sensitivity and minimum distance from the reference mode. • The selected optimal frequencies and modes were experimentally validated using an IDT-based SLDV. The continuous excitation-based scanning laser Doppler vibrometer (SLDV) has been gaining prominence as a defect-detecting tool because of its superior performance in discovering faults, such as thinning of structures, compared to conventional non-destructive evaluation methods. To date, the optimal test mode and frequency for interdigital transducers have been heuristically selected. Although this approach is effective for problems involving a small number of modes, heuristic algorithms have limitations with respect to problems involving multiple complex modes. Therefore, in this study, the determination of the inspection mode and frequency is formulated as an optimization problem, and the effective wavenumber and bandwidth of the high-pass and bandpass filters in signal processing are also considered. Numerical optimization examples show that the proposed method is effective and robust. An experiment with a thick plate with shallow defects was performed, and the quantitative images obtained showed the correct location of the defects with the proposed numerical optimal solution. [ABSTRACT FROM AUTHOR]

• The viscoelastic properties of woven composites are obtained based on the mechanical properties using the multi-scale analysis. • The degree of cure and propagation characteristics of Lamb waves during the cure process can be calculated by coupled temp-displacement analysis in ABAQUS. • The degree of cure curves obtained by Lamb waves are compatible with DSC test and ABAQUS simulation, even for complex structure. • The non-invasive and embedded piezoelectric sensors can effectively monitor the cure process of composite structures in-situ. In order to find a feasible on-line cure monitoring method, the simulations and experiments are carried out for woven composites using Lamb waves. The thermo-dynamic analysis is conducted using the parameters of cure kinetic equation obtained by DSC tests. The viscoelastic properties of CFRP are described by the multi-scale analysis. The coupled temp-displacement analysis is carried out in ABAQUS. The amplitudes of response Lamb waves signals increase with the cure process due to the increase of stiffness of CFRP. The same trends can be found with embedded piezoelectric sensors. And for the non-invasive pasted sensors, the amplitudes decrease because the energy leakage of Lamb waves propagating in the mold to the laminate. The degree of cure can be calculated by integrating the absolute values of amplitude changes and the experimental results are compatible with the DSC test and ABAQUS simulation. The curing process of CFRP can be effectively monitored in-situ. [ABSTRACT FROM AUTHOR]

In this paper, a new online uniaxial stress monitoring methodology has been developed. Based on the acoustoelastic effect of Lamb waves, multimodal information at a single frequency, in which the approximate linear fitting between group velocity ratio of two modes and stress is obtained, is fused to characterize stress. The transceiver distance can be eliminated, which is essential knowledge for existing methods, in stress coefficient calibration and practical application by using the ratio of acoustic time. The semi-analytical finite element method(SAFE) is adopted to calibrate the stress coefficient. Next, the time–energy assignment algorithm derived from the smoothed pseudo-Wigner–Ville distribution(SPWVD) is evaluated on the ideal dataset obtained by time-domain finite element(TDFE) simulation. The arrival times of different modes are extracted accurately, and the fitting results are consistent with the theoretical analysis. Finally, the laboratory experiments employing a pair of piezoelectric wafer active sensors(PWAS) are conducted to calibrate unknown coefficients. The same calibration process is carried out at three different distances, and almost the same calibration results are obtained, indicating the distance-dependent nature of the stress coefficients. According to the completed calibration, the actual measurements from the same batch show the maximum absolute error of stress measurement is 4.2MPa within the applied stress range from 20MPa to 100MPa and the standard deviation is less than 3MPa, demonstrating good measurement accuracy and consistency. It shows the great potential of the proposed strategy to measure stress at unknown propagation distances. • A new online stress monitoring based on dual mode Lamb wave is proposed. • This method can improve the measuring ability without reducing the measuring sensitivity. • The method adopted Wigner–Ville time–frequency distribution to obtain the group delay of dispersive Lamb wave signal. [ABSTRACT FROM AUTHOR]

Lamb wave based techniques have been widely used for structural health monitoring (SHM) applications. Most of the studies used either plates or built simple structures similar to airplane fuselage. In this study, the actual wing slats of Boeing 737 aircraft was used for detection of typical damages observed at the normal operating conditions. The cuts and impact damages of the slats were considered. The trailing edge of the slat was made of Al 2024 with 1 mm thickness. This section was supported with a composite honeycomb core. The material of the front section of the slat was Al 7075 with 2.5 mm thickness. Four lead zirconate titanate (PZT) discs were attached to the sections of slats to create surface waves and to monitor their propagation. The envelopes of the monitored signals were calculated by using the Hilbert transform. The cuts generated new boundaries and created well defined reflected waves which may be used to estimate the location of the defects from their arrival time. The impact damage could be detected from the change of the pattern but interpretation of the signal and estimation of the location was much more complicated. The study indicated that the defects of over 3 m long slats can be detected with carefully located PZT discs. [ABSTRACT FROM AUTHOR]

Structures with composite materials in daily use are subjected to impact. Determining if the material is damaged is an important aspect of structural health monitoring of composites. In this paper, a method to determine structural damage as a result of impact and localize the position of the damage based on lamb wave mode is presented. First, the paper analyses the characteristics of lamb wave mode generated by force loading on the plate. These results are validated based on the analysis of experimental results. From the results, it can be concluded that S 0 mode lamb waves were generated by damage of the plate structure and it can be used as the characteristic signal of damage. Then, empirical mode decomposition (EMD) is utilized to decompose to the Intrinsic Mode Functions (IMF) of S 0 mode inside the emission signals of complex acoustics to realize if damage occurs and localize the position of impact damage on the composite material plate. [ABSTRACT FROM AUTHOR]

• The thickness distribution of the stepped plate was determined using time of flight and convex optimization method. • Plate thickness variability was determined using adjacent signals excited and measured by two transducers. • The influence of the size of dictionary matrices on the effectiveness of the CS-based approach has been investigated. Guided waves recently have attracted significant interest as a very promising research area. The signals registered by a specially designed sensor network are processed to assess the state of the tested structure. Despite the constant development of novel damage detection algorithms employing guided waves, the phenomenon of wave propagation still needs detailed recognizing and understanding for the further progress of non-destructive wave-based methods. Special attention is paid to guided waves in plate-like structures, but the majority of considered cases concern plates with constant thickness. However, in the real world, we often deal with specimens with variable thickness. The thickness variability of the specimen is often forced to fulfill the construction requirements and optimize stress distribution or is the result of degradation i.e. corrosion. Thus, the development of NDT methods forces the need of considering specimens with complex geometry and the problem of wave propagation in waveguides with variable thickness is crucial for improving novel as well as so far proposed algorithms. The article presents the results of the analytical, numerical and experimental analysis of wave propagation in plates with variable thickness. The analysis concerns the influence of thickness distribution of plate structure on wave velocity, the time course wave packet and amplitude. Moreover, the novel approach based on constrained convex optimization for determining the plate thickness distribution has been proposed and verified during numerical and experimental campaigns. [ABSTRACT FROM AUTHOR]

• We proposed a method to measure dispersion curves base on clustering algorithms. • The method could realize more accurate and reliable measurement results with less measured data. • This method was experimentally verified with an isotropic aluminum plate and an anisotropic CFRP plate. • We obtain A 0 mode phase-velocity dispersion curves for CFRP plate with various braiding angles. Due to the large detection range and high sensitivity to damages, Lamb waves are widely used in the localization and quantification of structural damages in a plate structure. The dispersion curve measurement is significant in the applications of Lamb waves, especially in a material with unknown properties. In the study, a method was proposed to measure dispersion curves based on clustering. Compared with traditional methods, the proposed method could realize more accurate and reliable measurement results with less measured data. This method was experimentally verified with an isotropic aluminum plate and an anisotropic CFRP plate. The relative error between measured and real values in an aluminum plate was less than 1%. With this method, A 0 mode phase-velocity dispersion curves in CFRP plate with various braiding angles were experimentally obtained. This method facilitated Lamb wave defect detection and material parameter inversion. [ABSTRACT FROM AUTHOR]

• A modal decomposition imaging (MDI) algorithm for detecting delamination defects in carbon fiber composite plates using air-coupled Lamb wave is proposed in this paper. • The proposed MDI algorithm contains modal decomposition process and rotating scanning defect probability imaging method. • The delamination defect measurement is realized by single-sided and pitch-catch method and circular scanning. • The proposed MDI algorithm is suitable for achieving accurate characterization of defects in situ testing and large-area rapid scanning of aerospace composite plates after quickly scanning. In this paper, a modal decomposition imaging (MDI) algorithm for detecting delamination defects in carbon fiber composite plates using air-coupled Lamb waves was proposed. Compared with the traditional time-domain imaging method using amplitude difference, the proposed MDI algorithm contained modal decomposition process and rotating scanning defect probability imaging method, which was more suitable for analyzing the nonlinear and non-stationary leakage Lamb waves signal. The cross-correlation coefficient of the instantaneous energy was constructed to be the damage index, which was obtained by the pretreatment of the relatively pure A0 mode Lamb waves. The effectiveness of MDI algorithm proposed in this paper for realizing the delamination defects of carbon fiber composite plates was verified by qualitative imaging and quantitative characterization comparing with the defect probability imaging (DPI) algorithm. It would be suitable for achieving accurate characterization of defects in situ testing and large-area rapid scanning of aerospace composite plates after quickly scanning. [ABSTRACT FROM AUTHOR]

• The phenomenon of the wave mode conversion from A 0 Lamb wave to quasi-Scholte wave (QS) is investigated numerically and experimentally. • At low excitation frequencies, A 0 wave can be mode converted to QS wave rapidly with most of the wave energy conserved in the partially immersed plate. • When the excitation frequency increases, the amplitude of QS wave converted by A 0 wave significantly decreases and the mode conversion process becomes more complicated. • The guided wave energy can shift in the frequency domain if the phase velocity of the incident A 0 wave is larger than the sound speed of water. • Experimental data from empty and partially water-filled tank confirmes the guided wave energy shift in the frequency domain is due to the presence of water. • The mode conversion process is graphically demonstrated by novel semented frequency wavenumber analysis. This paper numerically and experimentally investigates guided wave propagation in a steel plate with one side partly exposed to water. The fundamental anti-symmetric Lamb wave (A 0) is excited on the dry plate section and travels to the water-immersed plate section, where the generated A 0 wave is mode converted to quasi-Scholte (QS) wave. The results demonstrate that the energy of QS wave converted by A 0 wave decreases when the excitation frequency increases. In addition, it is revealed that the guided wave energy can shift in the frequency domain if the phase velocity of the incident A 0 wave is larger than the sound speed of water. The frequency shift phenomenon should be noticed in practical applications because the behaviors of guided waves vary with frequency. Finally, discussions are provided on the frequency selection for exciting guided waves to detect damage on partially immersed structures and assess liquid properties. [ABSTRACT FROM AUTHOR]

[Display omitted] • Construction of the large panel (40-in.) ultrasound touchscreen system. • Implementation with using a few sensors (8 pairs of sensors) • Maximizes the signal contrast by additional inverted phase transmission. • Detecting the single and multi-touch of fingertip contact. In this paper, we propose a transmission method, analysis, and implementation of an ultrasound touchscreen system based on Lamb waves. The basic principle of this system is to use the signals reflected from the fingertip contact. The inverted phase transmission method, which alternately transmits two opposite phases of input signals, was proposed to improve the signal contrast of the reflected signal from the fingertip. Experimental validation of this system was demonstrated on a 40-inch glass plate (800 mm × 600 mm) containing eight pairs of sensors. For imaging and localization, a simple and common method is used: the delay and sum method for imaging and the center of mass for localization. Compared with existing ultrasound technology, the proposed system is implemented in a large panel without using the training or prepared database. The system also provides reliable contact detection, simplicity, and other advantages of the present acoustic techniques. [ABSTRACT FROM AUTHOR]

• The third harmonic phase velocity matching is used to achieve early fatigue damage detection. • The relationship between the third harmonic signal and the length of fatigue cracks is shown. • The third harmonic approximate phase velocity matching also enables early fatigue damage detection. The phase velocity matching Lamb waves are considered as an effective solution to excitation in the detection of nonlinear ultrasound early fatigue damage. In general, it is difficult to meet the phase velocity matching in real-world measurement. In this paper an approach to approximate phase velocity matching early fatigue damage detection is proposed. With the third harmonic approximate phase velocity matching modes treated as excitation frequencies by the Lamb wave dispersion curve, an analysis is conducted with regard to the time–frequency diagrams of the response signals at different frequencies. The change in the third harmonic amplitude with crack growth is analyzed by Fast Fourier Transform (FFT), which leads to the results suggesting that the third harmonic amplitude declines with crack growth. Moreover, a comparison is performed in the normalized relative third-order nonlinear parameters (NRTNP) of phase velocity matching and approximate phase velocity matching, with the relative deviation falling within 0.12. [ABSTRACT FROM AUTHOR]

• The propagation characteristics of leaky Lamb waves are calculated and verified. • The resin impregnation of in-plane and through-thick directions can be detected. • The dry spots of different sizes could be identified for hand lay-up process. • The measurement methods including noninvasive and integrated piezoelectric sensor network are proposed. • The two- and three-dimensional resin flow front in real-time of VARI process are effectively monitored. The propagation characteristics of leaky Lamb wave (LLW) when the rigid mold is loaded with water or viscous resin by one side are calculated numerically and verified experimentally, and the amplitude of A 0 mode decreases linearly with the increase of liquid layer thickness. Then the valid applications of LLW in two- and three-dimensional resin flow monitoring of hand lay-up and VARI processes are carried out. The amplitude of A 0 mode shows a downward trend in resin impregnation of both in-plane and through-thick directions, and the dry spots of different sizes could be identified. The amplitude and energy of received signals would decrease with the increase of resin coverage and the benchmark model of A 0 mode is established and validated. The new wide sensing range measurement methods are proposed to effectively monitor the two- and three-dimensional resin flow front by the PZT sensor network in both noninvasive and integrated ways. [ABSTRACT FROM AUTHOR]

• A new experimental procedure for damage detection using Lamb wave was developed. • Two new damage indexes which are sensitive to damage depth were proposed. • The anti - noise performance of the proposed damage indexes was analyzed. • The stability of the proposed damage indexes was compared. A new experimental procedure for damage detection based on active acoustic emission was developed which is sensitive to damage depth. Two new damage indexes were proposed by analyzing the amplitudes of low-frequency signal direct wave and high-frequency signal maximum component. The relations between damage depths of a steel plate and damage indexes were studied through numerical simulation considering different signal-to-noise ratios (SNR). It was found that the values of the damage indexes increased with the depth of damage, and the stability of the indexes was further analyzed by the coefficient of variation (Cov). Damage detection experiments for steel plates with different damage depths were carried out, and the results showed good agreement with the numerical simulation which further verifies the feasibility of the proposed method. The present method can provide basis for the engineering application of damage detection with different depths. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents an ultrasonic based Lamb waves propagation method for identifying and measuring the damage location in a material for SHM. The present work determines the experimental and analytical effects of various parameters on the sensitivity of damage detection and a methodology is proposed for estimating and measuring the location of damage in the test specimens. An experimental setup is used for generating Ao Lamb waves by calibrating ultrasonic pulse generation for optimal value of the parameters. The experiment is performed on two carbon fiber reinforced plastic bars in both undamaged and damaged state, where the two damaged states are (1) having a cut partway through the bar, perpendicular to the long axis of the bar and (2) having a circular hole. The Lamb wave propagation parameters are calibrated using the ultrasonic pulse generator test setup and the method was compared with direct measured values of ultrasonic instrument. [Copyright &y& Elsevier]

• Constructing dictionary must consider scattering characteristic of shear wave. • Dictionary with a fusion of shear wave characteristics matches signal better. • The proposed method improves the accuracy of the location. • Guided wave signals contaminated with strong noises can be effectively extracted. Shear wave is suitable for long weld defect detection in offshore platforms with its advantages of long-distance propagation, small attenuation, and high accuracy. Owing to dispersion, multi-mode, and strong background noises, the defect signal can generally be overwhelmed. To solve this problem, a new sparse-based defect detection method is proposed for weld feature guided waves with a fusion of shear wave characteristics. First, an over-complete dictionary is established combining wavenumber and scattering characteristics of shear wave. Then, the split augmented Lagrangian shrinkage algorithm is introduced in the basis pursuit algorithm for the sparse solution. The defect features of the signal can be sparsely extracted. The effectiveness is evaluated via simulation studies and further verified through the practical experiment data. Compared with the wavelet atom method, the location error of the proposed method reduces by about 1.0% to only 0.564% in simulation and about 2.0% to only 0.671% in practical experiment. [ABSTRACT FROM AUTHOR]

• Multiphysics model visualises wave behaviours and interprets inspection signals. • Collected signals of leaky Lamb waves are sensitive to size and location of droplets. • Lamb wave excitation is impaired by droplets on the test plate surface. • Deformed wavefronts of scattered leaky waves lead to attenuation of received signals. The signals in an air-coupled ultrasonic inspection system using Lamb waves would be affected by liquid droplets on the surface of test plates. To investigate the wave behaviour, a plate-droplet inspection system comprising a thin isotropic plate loaded with water droplets and a pitch–catch transducer configuration was modelled and studied experimentally. The Lamb waves were generated by a non-contact airborne transmitter. Their energy leaking into the adjacent air was collected by a receiver. To obtain a deep understanding of the effects of water droplets, a multiphysics model based on finite element method was established. The model facilitated the visualization of the wave fields scattered by the droplets on the surface of the plate and evaluated the received signals which were then verified by experimental measurements. This entirely modelling method could be a convenient way to interpret the inspection signals for test plate loaded with liquid droplets. [ABSTRACT FROM AUTHOR]

• A novel accurate velocity anisotropy probability imaging method is developed. • The anisotropic velocity was obtained by numerical simulation technique. • The probability is determined by CT and DSI, which shows good imaging quality. • This method can accurately show the damage location under different damage state. Due to its excellent performance, composite materials are widely used in engineering structures. However, because of the velocity anisotropy, it is difficult to detect the damage accurately. In this paper, a novel velocity anisotropy probability imaging method is developed. Firstly, the velocity in all directions is obtained through numerical simulation technique. Secondly, the model of velocity anisotropy composite plates is established. The damage probability is determined by time coefficient (C T) and damage sensitive index (DSI). Then, experiments with different damage states are carried out. The results show that the damage information can be displayed accurately, and it's clearer by applying a threshold of 90%. The probability of crack tips is higher than that of the middle, which reflects the influence of crack on the received signal. The sum of damage sensitive index (DSI sum) increases with the damage degree, which indicates the damage severity. This method can successfully detect the damage. [ABSTRACT FROM AUTHOR]

• A nonlinear Lamb wave detection system was established. • The cracks of aluminum plates under different tensile load cycle is investigated. • Influence about defects on nonlinear effects of Lamb waves is obtained. • Crack defects exacerbates the nonlinear effect of the test piece. • Macro-defects have a smaller effect on nonlinearity. In order to study the application of ultrasonic Lamb wave in the detection of defective plates, a nonlinear Lamb wave detection system was established. The system was used to detect aluminum alloy plates with different depth cracks and aluminum alloy plates with different tensile load cycles. The acquired time domain waveforms are analyzed by Fast Fourier Transform (FFT), and the influence of two kinds of defects on the nonlinear effects of Lamb waves is obtained. The test results show that for crack defects, the deepening of the defects will increase the ultrasonic nonlinear effect of the test piece. When the crack depth exceeds 4 mm, the non-linear effect of crack defects does not increase. For fatigue crack defects, the occurrence of crack defects will also exacerbate the nonlinear effect of the test piece, but when the test piece is fatigue fractured, its macro defects have a smaller effect on nonlinearity. [ABSTRACT FROM AUTHOR]

A mechanical lap joint is a critical part connecting rare-earth barium copper oxide (REBCO) tapes. Until now, the delamination in joints, which causes severe heating, has not been efficiently evaluated at room temperature. This paper presents Lamb wave method to measure the delamination length in lap joints. The dispersion curves of Lamb waves in REBCO tapes and joints are calculated, and their characteristics indicate the reflected wave is more suitable for testing because of the consistency through a broadband of frequencies. Finite element simulation results show the change in arrival time of the reflected wave is larger than the transmitted wave. In the experiments, an ultrasonic probe is used to test the joints from both sides. The delamination lengths in three samples are determined by the arrival time of the reflected wave, and they are consistent with the results obtained by X-ray microtomography with a maximum difference of 2.0 mm. [ABSTRACT FROM AUTHOR]

• Based on the synchrosqueezing wavelet transform, the strong noise of Lamb wave can be filtered out. • Based on SAE algorithm, damage location and quantitative identification are realized simultaneously. • Incorporates synchrosqueezing wavelet transform and SAE algorithm. • Five other intelligent methods are explored as comparisons. This study proposes a technique for damage location and quantitative identification for composites under strong noise background on the basis of synchro squeezing wavelet transform and stack autoencoder algorithm. Firstly, the simulation of the mechanism of Lamb wave and damage of different degrees were conduct in this stage. Secondly, the simulation of the actual damage was carried out in the selecting composites with strain field of the structure changed by different masses (within the range of 3 grades) and mass blocks at different locations (64 regions). Moreover, the white noise (3 dB) was introduced to the signal detected by the sensor in order to simulate the Lamb Wave signals collected from the sensors under the background of strong noise. Actually, the training sample is composed of 26,880 (3 × 64 × 140) signals. In addition, the synchronized wavelet transform was proposed to denoise the signal through employing strong noise. After that, Fourier Transform was applied to extract damaged frequencies. Additionally, the input of the stack autoencoder was the frequencies, and that output was the result of corresponding damage, and in the meanwhile the damage identification model was established as well. Finally, the test data which had been processed was then input into the damage identification model for further testing and analysis. It was finally concluded that 191(total 192) samples were correctly identified, and the correct recognition rate arrived as high as 99.48%. It can identify different degrees of damage in different locations. It was demonstrated by the results that the method is highly accurate, with extensive application potential in instantaneous locating and quantitative recogniting damages in the composite plate under the condition of strong noise exists. [ABSTRACT FROM AUTHOR]

• PC-MUSIC method is presented for impact localization under temperature conditions. • Phase errors correction matrix is developed to correct the steering vector. • Experiment results in two varying temperature fields has higher accuracy. Lamb waves are commonly used to locate low velocity impacts in composite structures. However, it is readily influenced by varying temperature conditions that the phase drift of uniform linear array signals will quite difference between steering vector and actual signal subspace. Therefore, the traditional multiple signal classification (MUSIC) method might fail to localize the impact sources. Consider the influence of varying temperatures on Lamb waves, a phase correction improved multiple signal classification (PC-MUSIC) method is proposed. The phase errors are calculated by comparing the varying temperature conditions with the normal work condition to obtain a correction matrix, which is employed to correct the steering vector. The impact source location represented by the angle and distance in polar coordinates is finally estimated by scanning the whole monitoring area using the corrected steering vector. The experiment investigations show that the present method has higher location estimation accuracy in composite structures than traditional MUSIC method for the varying temperature conditions. [ABSTRACT FROM AUTHOR]

• A baseline-free time reversal probabilistic imaging algorithm is proposed. • The focusing mechanism of wave source is used to eliminate the dispersion Lamb wave. • The relative distance is used to adjust the weight distribution function. • The proposed algorithm can accurately identify and locate multiple damages. Active ultrasonic guided wave is considered to be the most effective and important on-line damage monitoring method for industrial steel plates structure health monitoring. Aiming at the problem of poor practicability of extracting damage scattering signal from baseline signal, a baseline-free time reversal probabilistic imaging algorithm (BFTRPI algorithm) is proposed in this paper based on ultrasound guided waves time reversal theory and probability statistics principle. Firstly, the time reversal of signal and the adaptive focusing mechanism of wave source are used to eliminate the dispersion effect of Lamb wave by the multi-sensor network. Secondly, the energy characteristic difference coefficient between the reconstructed Lamb wave signal and the original excitation signal, is taken as the damage factor. Finally, the relative distance between the damage and the direct path of the sensor-actuator channel is used to adjust the weight distribution function. The weighted probabilistic values of each sensor path in the sensor network are mapped to the discrete coordinates in the detection area, and the probabilistic imaging of the damage appearing on these discrete coordinates is constructed for multiple damages imaging and location. Experiments on aluminium sheets show that the method can clearly separate the damage scattering signals without baseline signals, achieve the focus of the original excitation signal, and obtain the damage probability images of structure. Compared to traditional probability distribution function method, it has better imaging accuracy and imaging quality. The positioning accuracy of the BFTRPI algorithm is obviously higher than that of traditional algorithm. The estimated position by BFTRPI algorithm is more close to the actual damage position. The location area of damage and artifact for traditional algorithm are both higher than that of BFTRPI algorithm. This method can accurately identify and locate multiple damages in aluminium sheet, which verifies the validity of this method and has certain engineering application value. [ABSTRACT FROM AUTHOR]

• Nonlinearity parameters are derived considering Lamb wave motion in the plates. • The amplitude based parameter ( γ amp s ) measures material nonlinearity in plates. • The physics based parameter ( γ phy ) estimates the global material nonlinearity. • The nonlinearity curve plotted using γ phy helps γ amp s to estimate remaining life. The paper presents development and application of amplitude and physics based parameters for measuring intrinsic and fatigue induced material nonlinearities of plate materials considering Lamb wave motion into them. The amplitude based parameter is derived using Lamb wave equations and expressed in terms of amplitudes of the harmonics. This parameter can be evaluated directly from the results of experiments. The physics based parameter is derived considering the fundamental dislocation theory and it depends on percent fatigue life. It is used to construct a theoretical nonlinearity curve (TNC). Both the parameters are evaluated for Al 7075-T651 at different percents of fatigue life and compared. The close agreement of results show that with reference of TNC, the amplitude based parameter obtained from a field test can be used for estimating the remnant useful life of a plate structure as formation of dislocation substructures during fatigue loading is a prestage of microcrack formation. [ABSTRACT FROM AUTHOR]