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The mode manipulation of Lamb waves plays an important role in damage detection and identification of damage types, location, and size. In this paper, we propose a composite phononic crystal (PC) plate with antisymmetric and symmetric PCs for realizing mode conversion from A0 to S0 mode of Lamb waves. The theoretical analysis, numerical simulations, and experimental validation are introduced and the mode conversion mechanism of the composite PC plate is systematically investigated. The effect of geometrical parameters on band structures of antisymmetric and symmetric PCs is first discussed. Then multi-physics field simulation models are developed and in-plane displacement fields are obtained in numerical simulations, which shows that the mode conversion is enhanced when the period number of the antisymmetric PC decreases and that of symmetric the PC increases. The composite PC plate specimens are fabricated with precision wire cutting technology for experimental measurements and the self-designed polyvinylidene fluoride (PVDF) comb transducer is used to stimulate the Lamb waves. The experimental results are consistent with the numerical simulations, which demonstrate that the proposed composite PC plate can achieve the mode conversion from A0 to S0 mode of Lamb waves. Our proposed structures have applicable values for the mode manipulation of Lamb waves in damage detection. [ABSTRACT FROM AUTHOR]

Detection and evaluation of micro-damages in the early stages of engineering failure are crucial for various industrial structures to ensure their safety and prevent further catastrophic accidents. The nonlinear ultrasonic technique (NUT) has gained increasing popularity and recognition for breaking through the detection sensitivity limit upon micro-damages that usually are invisible to conventional linear techniques. However, it remains an ongoing challenge to quantitatively characterize micro-damages using NUT due to great difficulties in fully modeling the complicated interaction mechanism between the nonlinear ultrasonic waves and micro-damages. This work presents a data-driven perspective for solving multiparameter underdetermined inverse problems that are at the core of NUT, while allowing by-passing the creation of high-fidelity physics-based models. Nonlinear Lamb wave measurements with group-velocity mismatching are conducted to introduce both size and localization information of damages to the assembled dataset. A nonlinearity-aware discrete wavelet transform-bidirectional long short-term memory network is proposed to directly process nonlinear ultrasonic responses to automatically model latent nonlinear dynamics, thus establishing the complex mapping between the nonlinear ultrasonic signals and the multi-dimensional damage features. In particular, an attempt is made to augment the physical explainability of the proposed deep learning approach through a frequency component importance analysis. The trained network enables accurate and explainable predictions of length and localization of closed cracks and robustness against varying degrees of noise. Our work paves a promising and practical way to promote the transformation of NUT from the qualitative analysis for accurate and efficient quantitative prediction. [ABSTRACT FROM AUTHOR]

Localization of fatigue cracks imposes immense significance to ensure the health of the engineering structures and prevent further catastrophic accidents. The nonlinear ultrasonic waves, especially the nonlinear Lamb waves, have been increasingly studied and employed for identifying micro-damages that are usually invisible to traditional linear ultrasonic waves. However, it remains a challenge to locate the fatigue cracks using nonlinear Lamb waves owing to the enormous difficulties in decoding location information from acoustic nonlinearity. Motivated by this, this work presents a data-driven method for precise location of fatigue crack using nonlinear Lamb waves. A 1D-Attention-convolutional neural network is developed to correlate the fatigue crack location with the wavelet coefficients at the second harmonic frequency of Lamb wave signals. The introduction of the Attention layer enables the models to pay more attention to the desired nonlinear features which dominates locating the fatigue crack. In particular, a convenient dataset creation scheme guided by the relative value label is proposed to generate sufficient data commonly required for deep learning approach. In addition, a lightweight single-excite-multiple-receive signal acquisition method is adopted instead of full-matrix capture method used in the traditional research, which highly improves detection efficiency. Numerical simulation and experimental validation manifest that the trained network can be used to establish the complex mapping between the nonlinear ultrasonic signals and the fatigue crack location features, so as to locate barely visible fatigue cracks. Our work provides a promising and practical way to facilitate nonlinear Lamb waves to accurately locate fatigue cracks in large-scale plate-like structures. [ABSTRACT FROM AUTHOR]

The rapid development of topological physics in recent years has greatly enriched the methods for manipulating classic waves. Fano resonance is a widespread phenomenon featured with asymmetric and ultrasharp transmission line shape, which is induced by the constructive and destructive interference of two resonance modes with different quality factors. The temporal coupling mode theory is introduced to theoretically describe the physical process of Fano phenomena with two topological resonance states. The mechanism of the pass band led by the coupling between local resonance and Bragg scattering is introduced to design the antisymmetric topological resonance state and suppress the interference of the antisymmetric propagation mode. The complex eigenvalue analysis shows that the antisymmetric topological resonance state can be treated as the bound state in continuum (BIC). The periods of the phononic crystal (PnC) can be largely reduced by this scheme. The mechanism of coupling between two topological resonance modes attributed to geometric parameter variation is explained by the perturbing perspective and the temporal coupled mode theory. By theoretical analysis, the antisymmetric topological resonance state can be simultaneously treated as symmetric protected BIC and Friedrich–Wintgen BIC. Then, the design method for PnC with Fano resonance is proposed. The numerical results are in good agreement with the theoretical outcomes. The research results may find potential applications and offer the theoretical directive guideline for design in the fields of non-destructive testing, radio-frequency filters, resonators, and sensors. [ABSTRACT FROM AUTHOR]

Characterization of grain microstructures of metallic materials is crucial to materials science and engineering applications. Unfortunately, the universal electron microscopic methodologies can only capture two-dimensional local observations of the microstructures in a time-consuming destructive way. In this regard, the nonlinear ultrasonic technique shows the potential for efficient and nondestructive microstructure characterization due to its high sensitivity to microstructural features of materials, but is hindered by the ill-posed inverse problem for multiparameter estimation induced by the incomplete understanding of the complicated nonlinear mechanical interaction mechanism. We propose an explainable nonlinearity-aware multilevel wavelet decomposition-multichannel one-dimensional convolutional neural network to hierarchically extracts multilevel time-frequency features of the acoustic nonlinearity and automatically model latent nonlinear dynamics directly from the nonlinear ultrasonic responses. The results demonstrate that the proposed approach establishes the complex mapping between acoustic nonlinearity and microstructural features, thereby determining the lognormal distribution of grain size in metallic materials rather than only average grain size. In the meantime, the integration of the designed nonlinearity-aware network and the quantitative analysis of component importance provides an acceptable physical explainability of the deep learning approach for the nonlinear ultrasonic technique. Our study shows the promise of this technique for real-time in situ evaluation of microstructural evolution in various applications. [ABSTRACT FROM AUTHOR]

Fatigue damage and micro-cracks growth have been proved to be closely related to the nonlinear effect of ultrasonic Lamb waves propagation in materials. In this paper, the relationship among fatigue life, length and width of micro-cracks, and the acoustic nonlinearity parameter was established. The variation of the acoustic nonlinearity parameter caused by fatigue damage was analyzed based on micro-cracks growth. Considering the multimodal nature of nonlinear Lamb waves, we selected the low-frequency mode pairs S0–s0 and S1–s2 to evaluate fatigue damage in 7075 aluminum alloy plates. Both FE simulations and experiments show that the mode pair S1–s2 is more suitable than S0–s0 for the assessment of fatigue damage. It was found that with increasing fatigue cycles, the relative acoustic nonlinearity parameter A2/A12 firstly increases because the micro-cracks length extends while the micro-cracks width does not change much. The relative acoustic nonlinearity parameter A2/A12 reaches its peak at about 60% of the fatigue life. Then the relative acoustic nonlinearity parameter A2/A12 decreases because of the rapid increase in the micro-cracks width. Our findings clarify the mountain-shape curve between the relative acoustic nonlinearity parameter A2/A12 and the fatigue cycles. [ABSTRACT FROM AUTHOR]

Within the second-order perturbation treatment, the present work analyzed the complex second-harmonic generation of Lamb waves in a two-layered solid waveguide by combining the modal analysis method and the nonlinear reflection of acoustic waves at interfaces. The formal solution of the second-harmonic field is obtained on the basis of the modal analysis approach, from which a phase matching modification factor can be determined. The solution of the cumulative second-harmonic displacement field derived from the nonlinear reflection of acoustic waves at interfaces can be modified by multiplying the phase matching modification factor, and then be more general and applicable for the total second-harmonic field of Lamb wave propagation. Numerical computation results show that amplitude of the second-harmonic displacement increases clearly with propagation distance when the phase velocity of a double frequency Lamb wave (DFLW) component is exactly or approximately equivalent to that of the primary Lamb wave propagation, and reveal that the amplitudes of the second harmonics exhibit a decreasing tendency when the relative deviation of phase velocity between the primary Lamb wave and the DFLW component increases. Primary experimental measurements have been performed to verify the results of the numerical simulations in a FeCrNi alloy steel specimen. The research provides a further understanding for the physical process of the cumulative second-harmonic generation in a two-layered solid waveguide. [ABSTRACT FROM AUTHOR]

The nonlinear interaction of Lamb waves has attracted considerable attention because of its potential for localizing micro-damage in plate-like structures. Most recent studies have focused on the nonlinear mixing of Lamb waves propagating in the same direction, but this makes it difficult to determine the exact position of micro-damage because of the relatively large mixing zone. To overcome these shortcomings, we consider the nonlinear interaction of counter-propagating Lamb waves in this letter. An obvious resonance phenomenon is first observed in both experiments and simulations, which agrees well with our previous theoretical analysis [Li et al., J. Appl. Phys. 124, 044901 (2018)]. Also, analyzing the time-domain signal shows that the precision of damage localization is a function of the frequencies, the number of cycles, and the phase and group velocities of the primary waves. [ABSTRACT FROM AUTHOR]

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

Symmetry properties of second harmonics generated by antisymmetric primary Lamb waves are systematically studied in this work. In theory, the acoustic field of second harmonic Lamb waves is obtained by using the perturbation approximation and normal modal method, and the energy flux transfer from the primary Lamb waves to second harmonics is mainly explored. Symmetry analyses indicate that either the symmetric or antisymmetric Lamb waves can merely generate the symmetric second harmonics. Finite element simulations are performed on the nonlinear Lamb wave propagation of the antisymmetric A0 mode in the low frequency region. The signals of the second harmonics and the symmetric second harmonic s0 mode are found to be exactly equivalent in the time domain. The relative acoustic nonlinearity parameter A 2 / A 1 2 oscillates with the propagation distance, and the oscillation amplitude and spatial period are well consistent with the theoretical prediction of the A0-s0 mode pair, which means that only the second harmonic s0 mode is generated by the antisymmetric primary A0 mode. Experiments are further conducted to examine the cumulative generation of symmetric second harmonics for the antisymmetric-symmetric mode pair A3-s6. Results show that A 2 / A 1 2 increases linearly with the propagation distance, which means that the symmetric second harmonic s6 mode is generated cumulatively by the antisymmetric primary A3 mode. The present investigation systematically corroborates the proposed theory that only symmetric second harmonics can be generated accompanying the propagation of antisymmetric primary Lamb waves in a plate. [ABSTRACT FROM AUTHOR]

The creep damage in high temperature resistant titanium alloys Ti60 is measured using the nonlinear effect of an ultrasonic Lamb wave. The results show that the normalised acoustic nonlinearity of a Lamb wave exhibits a variation of the “increase-decrease” tendency as a function of the creep damage. The influence of microstructure evolution on the nonlinear Lamb wave propagation has been analyzed based on metallographic studies, which reveal that the normalised acoustic nonlinearity increases due to a rising of the precipitation volume fraction and the dislocation density in the early stage, and it decreases as a combined result of dislocation change and micro-void initiation in the material. The nonlinear Lamb wave exhibits the potential for the assessment of the remaining creep life in metals. [ABSTRACT FROM AUTHOR]

In this paper, the guided wave propagation behavior in damaged 30CrMo steel curved plates was investigated experimentally and numerically. The effects of the notch orientation, depth in the curved plate, as well as its radius, on the wave propagation characteristics were mainly analyzed by the amplitude distribution curves and the directivity diagrams of A0/S0 (zero-th order of the symmetric/antisymmetric Lamb wave) modes. An ellipse-based algorithm was compiled to locate the notches in the curved plates. Results show that the normalized S0 wave amplitude in the circumferential orientation was the largest, and it increases as notch depth increases in the axial orientation. The A0 wave amplitude in axial orientation was the largest, while it decreases with the increasing of notch depth in the other orientations. The normalized A0 wave amplitude in axial orientation increases with the increasing of radius. With the increasing of radius, the other normalized A0/S0 amplitudes linearly decreased for the other paths. The ellipse-based algorithm has high notch localization accuracy, and the notch localization error increase from 0.005% to 1.47% with the notch depth decreasing from 5 mm to 1 mm in the curved plates. For the curved plates with different radius, the maximum notch localization error is 1.20%. These satisfactory results demonstrate the effectiveness of the developed algorithm in locating damages in the researched structure. [ABSTRACT FROM AUTHOR]

Lamb waves provide sensitive and effective approach for nondestructive inspection of large‐scale composite structures. Traditionally in Lamb wave‐based damage detection systems, the piezoelectric (PZT) wafer transducers are attached on the surface of composite components. However, surface‐bonded PZT wafers are fragile, and they may lose serviceability when external load happens. In this paper, numerical and theoretical investigations are firstly carried out to analyze the Lamb wave features generated by PZT wafers that are located in the mid‐thickness of a thin‐walled plate. Subsequently, the authors integrate the PZT wafer arrays in woven glass fiber reinforced epoxy (WGF/epoxy) laminates with stacking sequence of [90°/0°]4. It is found that the approximate single S0 Lamb wave mode is generated by the built‐in PZT wafers. This makes the received signals very simple and easy to be interpreted. Utilizing the signals obtained by the built‐in and surface‐bonded PZT wafer transmitter‐receiver arrays, damage localization algorithms that considered the slowness profiles are compiled to help locating the defects. The compared results demonstrate that the built‐in PZT strategy has great potential for quantitative non‐destructive evaluation of composite structures. This paper presents a built‐in PZT wafer methodology to generate S0 mode dominant Lamb wave in WGF/epoxy composites. Symmetrical dominant waves are generated by the built‐in PZT method. In both the enhanced ellipse and probabilistic imaging algorithms, the damage localization errors are limited and are kept within the range of ±10%. The built‐in PZT method has higher damage detection sensitivity. [ABSTRACT FROM AUTHOR]

Due to the dispersive and multimode natures, only nonlinear Lamb waves with exact phase-velocity matching were generally used in previous studies to evaluate the evenly distributed microstructural evolution in the incipient stage of material degradation, because of the cumulative generation of second harmonics, which was also found within a significant propagation distance for mode pair S0-s0 with quasi phase-velocity matching at low frequency. To explore the feasibility of fatigue damage evaluation by using this mode pair and fully utilize its unique merits, the cumulative second harmonic analysis was performed on aluminum alloy specimens with various material damage produced by the continuous low cycle fatigue tests. Similar to mode pair S1-s2 with exact phase-velocity matching, a mountain shape curve between the normalized acoustic nonlinearity parameter and the fatigue life was also achieved with the peak point at about 0.65 fatigue life for mode pair S0-s0, even though a relatively higher sensitivity to fatigue damage was observed for mode pair S1-s2. The excited frequency selection was further analyzed in a certain frequency range, where the quasi phase-velocity matching condition was satisfied for mode pair S0-s0 owing to the less dispersive property. Results show that the fatigue damage can be effectively detected using the mode pair S0-s0, and a relatively lower excited frequency was preferred due to its higher sensitivity to microstructural evolution. [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]

Abstract: The cumulative second-harmonic analysis of ultrasonic Lamb wave has been performed to study the precipitation kinetics and microvoid initiation of ferritic Cr–Ni alloy steel during the ageing process. Ageing of ferritic Cr–Ni alloy materials have been done at 1223K and 1173K for different degradation time intervals and air cooled. The results show that the normalized acoustic nonlinearity of Lamb wave increases with the formation of fine precipitates at the early stage of ageing till about 1000h and keeps as a plateau with the precipitates dynamic balance for a long-term ageing, and then decreases gradually at the final holding time with the coarsening of precipitates and initiation of microvoids. The results also show that the variation of nonlinear Lamb wave follows the same trend as that of hardness in materials. Therefore, the cumulative second-harmonic of ultrasonic Lamb waves has been found to be strongly sensitive to the precipitates behavior and microstructure evolution during the thermal ageing of ferritic Cr–Ni alloy steel. [Copyright &y& Elsevier]

Abstract: The thermal degradation in ferritic Cr–Ni alloy steel plates is measured using the nonlinear effect of Lamb wave propagation. Experiments were carried out to introduce controlled levels of thermal damage to determine the nonlinear response of Lamb waves. A “mountain-shape” change in the normalized acoustic nonlinearity of Lamb wave versus the level of thermal degradation in the specimens has been observed. The variation in the measured acoustic nonlinearity reveals, based on metallographic studies, that the normalized acoustic nonlinearity increases due to the second phase precipitates in the early stage and it decreases as a combined result of dislocation change and micro-void initiation in the material. The results show a potential application of the nonlinear Lamb waves for the quantitative assessment of thermal damage in metallic plates or pipes. [Copyright &y& Elsevier]

• Nonlinear Lamb wave imaging of micro-damage is achieved based on wavefield data. • A new approach is proposed for weak nonlinear feature enhancement. • Demonstrated capability of accurately imaging the fatigue crack tip. • Visualize the wave field of the harmonics. The traditional nonlinear ultrasonic technique, as typified by the second-harmonic generation and the frequency mixing response, can be employed to identify and characterize the micro-damage. However, the research on micro-damage characterization using nonlinear Lamb wave imaging technique remains an ongoing challenge and is rarely reported. A method called standardized amplitude difference is proposed for nonlinear feature enhancement, and further for fatigue crack imaging based on the wavefield data. Wavefield data contain abundant information on the spatial and temporal variation of propagating waves in the damaged structure. The nonlinearity index β ′ of the signal difference under the high and low incident wave amplitudes is calculated for fatigue crack imaging. Two scanning methods, including local scanning and global scanning, are introduced to image the fatigue crack tip and visualize the wave field of the harmonics respectively. The experimental validation, based on the imaging results of an aluminum alloy plate specimen with a barely visible fatigue crack and a steel plate with a blind hole, manifests that the proposed method can be used to enhance and extract the nonlinear features and suppress the fundamental frequency, so as to improve the signal-to-noise ratio (SNR) of the micro-damage imaging results. [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]

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

• Compared with the traditional nonlinear ultrasound technology. The combination of linear and nonlinear responses in the time–frequency domain of nonlinear ultrasound can improve the characterization accuracy of grain size distribution. • Convolutional neural network (CNN) can establish the implicit relationship between nonlinear ultrasonic Lamb wave and grain size distribution, and construct a grain size distribution prediction model. • As the input of CNN, the Short-Time Fourier diagram of nonlinear Lamb wave can provide multi-dimensional information for the network. • Machine learning can solve the multi parameter inverse problem and ill posed problem in inversion of microstructure with a single nonlinear ultrasonic parameter. • Remove the fundamental frequency and second harmonic respectively to verify the performance of the network, and also confirm that linear and nonlinear ultrasonic information is essential. Nondestructive characterization of grain size distribution is significant for revealing the mechanical properties of metal materials. Traditional linear ultrasonic methods have been applied to estimate mean grain size but are less sensitive to small grain size. Nonlinear ultrasonic technology shows high sensitivity to microstructures, while it unfortunately encounters underdetermined problem in the characterization of microstructures across multiple scales. Combining linear and nonlinear ultrasonic responses would be a quite attractive way to achieve comprehensive observation of microstructural evolution. This work presents a data-driven approach for characterizing grain size distribution using nonlinear Lamb waves. The short-time Fourier transform (STFT) of detected ultrasonic signals were input into a convolutional neural network (CNN) to comprehensively learn the implicit linear and nonlinear dynamics involved with grain size distribution. Multiple convolution kernels slide across the STFT images with multiscale receptive fields to collectively model hierarchical representations and capture local correlation of interesting from time-frequency domain. The trained model achieved 94.7% accuracy in predicting mean grain size, and also achieved 95.4% and 86.3% accuracy in predicting the expectation and standard deviation of the lognormal distribution of grain size, respectively. The visual activation maps demonstrate that local interesting features are successfully captured. In particular, the prediction accuracy was demonstrated to reduce greatly after removing the fundamental frequency and second harmonic from the STFT images, confirming that incorporating linear and nonlinear ultrasonic information is indispensable for accurate characterization of grain size distribution. [ABSTRACT FROM AUTHOR]

In this paper, nonlinear Lamb waves with phase-velocity mismatching are proposed to detect accurately closed cracks by excluding the intrinsic material nonlinearity. Simulations and experimental studies were conducted to analyze double frequency Lamb waves (DFLWs) induced by the closed cracks and the intrinsic material nonlinearity. The results show that DFLWs induced by the material nonlinearity are negligible as compared with that caused by the contact acoustic nonlinearity (CAN) of the closed cracks. The closed cracks can be accurately detected using the proposed method, and the acoustic nonlinearity parameter increases monotonically with the crack evolution. The findings of this study provide a feasible method for detection and characterization of closed cracks. • Closed cracks are effectively detected using phase-velocity mismatching method. • Material nonlinearity is excluded in the detection of closed cracks. • Acoustic nonlinearity parameter increases monotonously with crack length. [ABSTRACT FROM AUTHOR]

• Phase coded excitation and pulse compression were applied to enhance SNR. • Location, size and shape of blind hole was determined by Damage-index-based imaging. • Simulation model of Air-coupled ultrasonic Lamb wave testing was established. The combination of air-coupled ultrasonic testing (ACUT) and ultrasonic Lamb wave is featured with long-distance propagation and high sensitivity to discontinuities, which is a promising method for rapid and accurate inspection of plate-like materials and lightweighted structures. However, dispersive nature of Lamb wave, signal attenuation plus inevitable noises would lead to low signal-to-noise ratio (SNR). To address this problem, phase coded excitation and pulse compression technique are proposed in this paper to achieve higher SNR by over 10 dB in received signals. 13-bit and 1-carrier-period Barker code is employed as both main lobe peak and Peak Side-lobe Level (PSL) are relatively high. It is demonstrated that A0 mode Lamb wave has good localization ability for defects based on these SNR-enhanced signals. Furthermore, Damage Index (DI) and modified Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID) are applied to realize ultrasonic imaging based defect evaluation. Results show that the imaging results agree well with the actual artificial defects in terms of size and shape. Lamb-wave-based air-coupled ultrasonic testing, combined with DI and ultrasonic imaging algorithm, could be a potential way in the NDT of lightweighted structures. [ABSTRACT FROM AUTHOR]