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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]

[Display omitted] • An advanced ultrasonic technique is employed to detect glass cracks in solar modules. • The Lamb waves scanning method produces map of the defects locations efficiently. • The investigated defects are due to mishandling, representing a real-world scenario. Ensuring the structural integrity of solar photovoltaic modules is crucial to maintain power production efficiency and fulfill the anticipated product lifespan. Hence, implementing quality control procedures and structural health monitoring is necessary throughout the stages of manufacture and operation. The ultrasonic examination has some benefits compared to electroluminescence and infrared approaches, namely in identifying mechanical flaws in the front glass. The Lamb waves (LW) method is an auspicious inspection approach among ultrasonic-based methods. This is primarily attributed to its quicker measurement speed than the standard ultrasonic c-scan. The LW method offers an advantage in terms of its ability to provide long-range coverage. The predominant approach in LW inquiry often centers on using fundamental modes within the low-frequency range. Nevertheless, the findings of this investigation demonstrate that the higher-order mode exhibits superior effectiveness for the specific objective of this research, as it displays a higher level of sensitivity towards cracks in the front glass of the module. The current study ultimately showcases the use of the LW scan. A damage indication threshold is determined by the fraction of energy spectral density (ESD) associated with the crack-sensitive mode. This technology effectively produces a comprehensive map of the designated area by comparing the ESD values at various measurement positions with a predetermined threshold. This map provides precise indications of the existence of areas that are affected by cracks. [ABSTRACT FROM AUTHOR]

• Contact surface boundary conditions between the two surfaces of the vertical crack were considered to study contact acoustic nonlinearity (CAN) from the breathing contact crack in conjunction with the modal decomposition method, Fourier transform, and variational principle-based algorithm. • The clapping effects when the crack closed due to the different ratios of incident wave amplitude to crack width with the contact boundary conditions between the crack surfaces were studied and analyzed to get the quantitative correlation between analytical calculations and FE simulation. • When the crack state was modulated at a low frequency (much lower than the one of the probing ultrasonic waves), the related mode conversion coefficients were modulated accordingly both in the fundamental frequency and harmonic frequency. To characterize fatigue crack, an analytical calculation and finite element (FE) simulation of Lamb wave propagating through the region of a breathing crack in a two-dimensional(2D) isotropic plate were studied. Contact surface boundary conditions between the two surfaces of the vertical crack were considered to study contact acoustic nonlinearity (CAN) from the breathing contact crack in conjunction with the modal decomposition method, Fourier transform, and variational principle-based algorithm. Reflection and transmission coefficients in the fundamental frequency and second harmonic frequency were calculated and analyzed quantitatively. Different ratios of incident wave amplitude to crack width were studied to calculate CAN results related to micro-crack width. In addition, a low-frequency (LF) vibration(10 Hz) excitation was introduced to perturb the free surface vertical crack to close, and an interrogating Lamb wave(1 MHz) was used to study crack-related CAN in different conditions for interpreting the modulation mechanism. The contact boundary conditions between two surfaces of vertical crack were set which were dynamically changed due to the low frequency modulation. The clapping effects when the crack closed due to the modulation of the contact boundary conditions between the crack surfaces were studied and analyzed to get the quantitative correlation between CAN and LF modulation. The results obtained from the analytical model were compared with those from the FE simulation, showing good consistency. Knowledge of these effects is essential to correctly gauge the severity of surface cracks in the plate, which can be spotlighted in its application to quantitative evaluation of micro fatigue cracks in structural health monitoring(SHM). [ABSTRACT FROM AUTHOR]

Air-coupled ultrasonic Lamb wave damage imaging is a method for rapid, non-contact damage detection of plate specimens. However, the current technique based on Lamb waves has limitations regarding accuracy and quality of imaging. To address this issue, an improved bilateral boundary-enhanced reconstruction algorithm for the probabilistic inspection of defects (BBERAPID) is proposed. This algorithm optimizes the probability distribution near the damage boundary to enhance the accuracy of damage imaging. The skewed bilateral boundary distribution algorithm can improve the probability distribution of edges on both sides of the damage and alleviate the problem of abnormal boundary expansion. In addition, a new damage index (DI) calculation method is proposed to accelerate the DI trend near the damage. The new DI can improve the fineness of the damage boundary, thus better distinguish the healthy area from the damage area. Traditional DI has the problem of abnormal distribution at the damage boundary, which BBERAPID solves. The probability distribution near the damage is optimized to improve the quality and accuracy of damage imaging. Experimental studies are performed using composite plates to validate the proposed method further. The effectiveness and accuracy of the proposed BBERAPID algorithm for detecting damages in composite plates are verified by comparing it with existing imaging methods through experimental validation. The proposed non-contact air-coupled Lamb wave damage imaging method is suitable for rapidly and accurately characterizing defects in composite plates. [ABSTRACT FROM AUTHOR]

The air-coupled ultrasonic technique is a reliable method for measuring the stress state of composite plates. This paper aims to address the requirement for efficient and accurate stress measurement of large component composite plates by proposing an air-coupled ultrasonic in-situ measurement method. Ultrasonic Lamb waves can propagate over long distances while maintaining sensitivity to slight changes and are suitable for in-situ stress measurement of CFRP plates. However, due to the serious mismatch in acoustic impedance, the air-coupled ultrasonic Lamb waves have a low signal-to-noise ratio, which can affect the accuracy of stress measurement. Focusing on this problem, the method employs the Lyapunov exponent with a moving window to accurately extract the acoustic-time characteristic instead of just judging the presence or absence of a weak signal compared with the traditional method of using the Lyapunov exponent. To evaluate the effectiveness of the proposed method in different fiber directions of CFRP plates, seven uniaxial tensile specimens are obtained along 0°, 15°, 30°, 45°, 60°, 75°, and 90° fiber directions, respectively. Utilize stress coefficient calibration results K L to realize the uniaxial stress measurement with a spatial resolution of 150 mm. The experimental results demonstrate that the stress measurement errors are within ±7.29 MPa in the range of 0–150 MPa, and the standard deviations of the repeatability measurement experiment are below 4.57 MPa. The proposed method exhibits significant advantages in terms of measurement accuracy and repeatability compared with traditional stress methods based on peak extraction or cross-correlation. It is a suitable technique for health monitoring and evaluation of composite plates. [ABSTRACT FROM AUTHOR]

• A new non-destructive testing technique is proposed for Directed Energy Deposition. • A wave model describes wave propagation in both the substrate and the thin wall. • A laser vibrometer scans the thin wall fabricated by additive manufacturing. • A 0 mode Lamb wave in the thin wall is enhanced by frequency tuning. • A small defect caused by inconsistent powder delivery is detected and localized. Thin-walled metallic structures produced by the Directed Energy Deposition (DED) Additive Manufacturing (AM) process are prone to various fabrication defects, which hinder the wider applications of the technique in practice. In-situ inspection and monitoring methodologies are in high demand for improved quality control of printed parts. This paper presents an ultrasonic guided-wave-based method and a prototype that can potentially be used for in-situ inspection of thin-walled structures produced by DED. Lamb waves are excited by a Lead zirconate titanate (PZT) piezoelectric transducer bonded on the DED substrate remotely from the thin wall. The substrate works as a waveguide to transmit the waves which then propagate along the thin wall. A non-contact laser vibrometer is applied to measure the guide wave signals by scanning the surface of the thin wall. The mechanisms of guided wave generation and propagation along the substrate and printed part are theoretically studied. It allows for choosing proper inspection parameters to enhance the measurement sensitivity of guided waves and help interpret the signals for defect detection. Experiments were conducted with DED-produced stainless steel (316L) thin-walled structure. The new method is demonstrated in one example to detect and localize a small defect caused by inconsistent powder delivery of a fabricated thin wall sample, via analysing the B-scan ultrasonic guided wave signals. The new technique provides strong potential for in-situ online monitoring of the DED process. [ABSTRACT FROM AUTHOR]

• 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]

Rodent models of behavior used in the fields of neuroscience and psychology generate a wealth of multimodal data. For instance, as a rodent moves and behaves in its environment, muscle contractions apply subtle forces to any surface the animal contacts. These forces generate acoustic waves that propagate through the waveguide as Lamb and shear horizontal (SH) waves and contain information about the rodent's physiology, behavior, and underlying psychological state. If the information in these waves were to be tapped, it would provide a novel, non-invasive way to study rodent behavior. This article lays the foundations for using guided ultrasonic waves generated by a mouse's movement on an aluminum plate for detecting behavior and drawing inferences about acoustic startle responses. The experimental setup involves piezoelectric sensors capturing the waves generated by the rodent's movement, which are then stored as discrete acoustic emission (AE) hits using an amplitude threshold-based data acquisition system. This method of data acquisition ensures that data collection occurs only when the animal moves or behaves, and each movement/behavior is represented by values of features within the generated wavepackets (AE hits). Through open field tests with C57BL/6J mice, utilizing piezoelectric sensors and the DAQ system, it was observed that every movement/behavior of the animal generated Lamb wavepackets within the frequency range of 20 kHz to 100 kHz. Furthermore, rearing behavior in the animals also led to the generation of SH wavepackets in the frequency range of 75 kHz to 230 kHz. This criterion was subsequently employed to detect rearing behavior. In the acoustic startle response test, where the animals' responses to intense sound pulse were recorded, AE hits' features proved useful in quantifying the response. These experimental findings validate the proposed technology's practicality and demonstrate its capability to enhance studies of rodent behavior. • An open-field arena is designed to capture the guided ultrasonic waves generated by the movement/behavior of mice. The arena features an aluminum base plate equipped with ultrasonic sensors. • In this setup, we treat the mouse as an acoustic source, and the resulting guided waves are regarded as acoustic emissions (AE). • As the mice freely explore the open-field arena, every movement/behavior gives rise to Lamb waves with frequency content spanning from 20 kHz to 100 kHz. • Notably, specific motions, such as rearing and occasionally sniffing, correlate with the generation of shear horizontal waves, with frequencies ranging from 75 kHz to 230 kHz. • Another novel aspect of this research lies in conducting the acoustic startle response (ASR) in an unconstrained environment, facilitating the quantification of the "intensity" of the response through the absolute energy of the AE hits. Additionally, this study marks the first observation of variations in the physical motions associated with ASR. [ABSTRACT FROM AUTHOR]

The interactions between ultrasonic waves and damage provide crucial information for developing effective damage detection techniques in various fields, including non-destructive testing and structural health monitoring, and they can be investigated through wave propagation. Then, this article presents an approach to investigate the longitudinal wave in thin plates with symmetric damage considering oblique incidence. The approach includes a strategy to determine the plate thickness reduction due to the damage presence by considering an oblique incidence of waves generated by the actuator. Modeling is developed to compute each wave packet of reflected and transmitted waves separately, which allows one to describe the wave scattering. Numerical simulations are carried out and the results show that the sensor size can be adjusted to improve the existence of damage detection and the determination of the damage depth. Results from experimental tests are presented to demonstrate the approach considering a circular actuator. The findings contribute to improving the comprehension of the interaction between guided waves and discontinuities, potentially encouraging the use of waves for damage sizing purposes rather than only for detection and improve the current state of the art in wave propagation by investigating the effects of different excitation frequencies and the influence of the damage parameters and sensor sizing on the resulting waves. • Analysis of the scattering of longitudinal waves in plates with symmetric damage. • A model that describes each wave packet is presented. • Optimal frequencies to detect symmetric damage are presented. • The optimal frequencies depend on the incidence angle. • The sensor size can be adjusted to improve the damage detection process. [ABSTRACT FROM AUTHOR]

As a key part of elliptical vibration cutting (EVC) technology, a two-dimensional ultrasonic elliptical vibration cutting (UEVC) device has become a prevalent area of importance and research interest. This paper dissects through the conventional structural design and creatively proposes the UEVC device with flexible guided wave propagation. The device resolves the problems of poor coupling efficiency of two-way excitation signal, complex structure design, non-orthogonal, and the difficulty in controlling the ellipse trajectories of the tool tip. It has been confirmed that a flexible guided wave exhibits an effective unidirectional propagation and lateral load decoupling ability via establishing its system dynamic response and frequency impedance characteristics. Here, we explain the coupling mechanism of the two-way flexible guided wave in the UEVC device. Results suggested that the experimental measurement of the impedance for the cutting device and the individual transducer shows that the cutting device with the transducer and the tool bar connected by the flexible guided wave has a lower impedance increase ratio compared to the individual transducer, and hardly influence the tool bar mode of the cutting device. At the same time, the tool tip trajectory of the cutting device having a phase difference of 90° showed a satisfactory "elliptical" shape, while the microstructure processed by the tool tip ellipse trajectory with a phase difference of 60° reached a burr-free state. Theoretical and experimental findings reveal that the flexible guided waves exhibit excellent weak coupling characteristics in the non-transmitting direction. This is convenient to the independent, orthogonal and stable propagation of signals in the two directions. Based on flexible guided wave propagation technology, our findings provide theoretical foundation and application value for the design of a two-dimensional UEVC device. • Proposed an innovative method to realize ultrasonic elliptical vibration cutting driven by two-way flexible guided wave. • The frequency impedance model of flexible guided wave ultrasonic elliptical vibration cutting device is established. • The two excitation signals independently drive the tool bar, and the device design becomes simple. • The ultra-precision burr-free machining of microstructures is completed by using the controllable ellipse trajectory. [ABSTRACT FROM AUTHOR]

Resistance spot welding can instantaneously join two or more plates by the resistance heating of the metal and is in high demand owing to its high productivity and high-work efficiency. For the quality control of resistance spot welding, the size and quality of the welded part, called the nugget, are important. Therefore, this study aims to establish a highly efficient and high-speed resistance spot-weld inspection method that can be applied to whole-lot inspection, which is currently a difficult process. The objective of this study is to measure the nugget diameter using laser ultrasonic technique that enable remote, non-contact ultrasonic inspection. An investigation of the available ultrasonic waves using simulated test specimens demonstrated the feasibility of estimating the distance from the generation/detection point to the joint using the diffraction of the Lamb wave, which can propagate long distances in a thin plate. By measuring the actual resistance spot welding specimens, it was determined that differences in the nugget diameter of approximately 0.5 mm could be clearly distinguished from the arrival time of the diffraction waves. It was also inferred that the nugget diameter could be calculated by determining the propagation velocity of the diffraction wave with a similar accuracy to that of the measurement using a contact-type probe. [ABSTRACT FROM AUTHOR]

• Determination of the thickness of each layer of a thin metal bilayer by applying multiple zero-group velocity Lamb modes. • Deducing the relationship between the zero-group velocity frequency of Lamb waves and the thickness ratio of the bi-layer metal sheets. • Thickness measured by zero-group velocity Lamb wave method agreed well with the metallographic microscope results. A zero-group velocity (ZGV) resonance method based on laser ultrasonic Lamb waves is proposed for measuring each layer thickness of a thin bilayer with uneven thickness at the interface. The relationship between the multiple ZGV frequency of Lamb waves and the thickness ratio of the bi-layer metal sheets was theoretically deduced, and the results showed that S 1 S 2 -ZGV frequency variation with the thickness ratio is non-monotonic, and S 1 S 2 -ZGV Lamb mode vanishes at some thickness ratios. Then a high-order S 3 S 6 -ZGV frequency is utilized to determine the desired thickness ratio. Experimentally, the two ZGV modes in a thin Al/Cu bilayer excited by laser and detected by laser Doppler vibrometer (LDV) were used to evaluate the thickness of each layer according to the theoretical curves. [ABSTRACT FROM AUTHOR]

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

• A new multi-GPU-based spectral element formulation for ultrasonic wave propagation. • Novel direct GPU-to-GPU message exchange strategies enabled by CUDA-aware MPI. • Significant accelerations over a multi-CPU core-based counterpart formulation. • Achievable accelerations and model sizes scalable with the number of GPUs. • Practicality shown through simulating real-life ultrasonic inspection scenarios. In this paper, we introduce a new multi-GPU-based spectral element (SE) formulation for simulating ultrasonic wave propagation in solids. To maximize communication efficiency, we purposely developed, based on CUDA-aware MPI, two novel message exchange strategies which allow the common nodal forces of different subdomains to be shared between different GPUs in a direct manner, as opposed to via CPU hosts, during central difference-based time integration steps. The new multi-GPU and CUDA-aware MPI-based formulation is benchmarked against a multi-CPU core and classical MPI-based counterpart, demonstrating a remarkable acceleration in each and every stage of the computation of ultrasonic wave propagation, namely matrix assembly, time integration and message exchange. More importantly, both the computational efficiency and the degree-of-freedom limit of the new formulation are actually scalable with the number of GPUs used, potentially allowing larger structures to be computed and higher computational speeds to be realized. Finally, the new formulation was used to simulate the interaction between Lamb waves and randomly shaped thickness loss defects on plates, showing its potential to become an efficient, accurate and robust technique for addressing the propagation of ultrasonic waves in realistic engineering structures. [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]

Lamb waves have the characteristics of long propagation distance, and they can be used to realize rapid and wide-scale nondestructive testing of plates. The detection sensitivity can be improved by using a high-order Lamb wave mode with a short wavelength. However, Lamb waves have multimode characteristics. The number of Lamb wave modes greatly increases at high frequencies, and it is difficult to obtain a pure single high-order Lamb wave mode with electromagnetic acoustic transducers (EMATs); this increases the difficulty of signal processing and analysis. In this study, an obliquely incident EMAT based on the inclined static magnetic field was designed to solve this problem. The mode selectivity of the proposed obliquely incident EMAT is superior to that of the traditional EMAT. A new parameter is proposed to improve the mode selectivity of EMATs: the polarization angle of the permanent magnet. The incident angle of incident ultrasonic waves is controlled by selecting the polarization angle of permanent magnet. A rectangular permanent magnet is obliquely polarized to provide a biased inclined static magnetic field. The obliquely incident Lorentz force is formed through the coupling between the eddy current and the inclined static magnetic field by an obliquely polarized permanent magnet. The obliquely incident Lorentz force is the source of the obliquely incident ultrasonic wave. Simulation and experimental results show that the developed obliquely incident EMAT can reduce the amplitude of the unwanted modes and enhance the mode selectivity. It was also verified that the developed obliquely incident EMAT had better frequency-response characteristics and sound-field directivity. • The polarization angle of permanent magnet was added to select Lamb wave mode generated by proposed EMAT. • Obliquely incident Lorentz force was formed by coupling between eddy current and inclined static magnetic field. • The amplitude of unwanted modes was reduced and mode selectivity was enhanced by obliquely incident EMAT. • Obliquely incident EMAT has better frequency-response characteristics and sound-field directivity. [ABSTRACT FROM AUTHOR]

• The aim of the present work is to develop a multi-mode imaging technique that will allow for identification of damage size and location using ultrasonic guided waves. • The proposed technique combines a reverse-time migration (RTM) imaging algorithm with a 3D wave propagation simulator using different wave modes. • This combination enables the separation of multiple modes using the wavefield filtering techniques, potentially providing more information to damage types. • Without the limitation of generating single dominant mode waves, wide frequency ranges are accessible, enabling optimal frequencies for a variety of ultrasonic data acquisition systems. The sensitivity of Lamb wave modes to a particular defect or instance of damage is dependent on various factors (e.g., the local strain energy density due to that wave mode). As a result, different modes will be more useful than others for damage detection and quantification, dependent on damage type and location. For example, prior work in the field has shown that out-of-plane modes may have a higher sensitivity than in-plane modes to surface defects in plates. The excitability of a certain data acquisition system and the corresponding resolution for damage imaging also varies with frequency. The aim of the present work was to develop a multi-mode damage imaging technique that enables characterization of damage type and size, general sensitivity to unknown damage types, higher resolution imaging, and detectability regardless of the data acquisition system used. A reverse-time migration (RTM) imaging algorithm was combined with a numerical simulator—the three-dimensional (3D) elastodynamic finite integration technique (EFIT)—to provide multi-mode damage imaging. The approach was applied to two simulated case studies featuring damaged isotropic plates. Sensitivities of damage type to wave mode were investigated by separating the A 0 and S 0 Lamb wave modes obtained from the resultant RTM wavefields. [ABSTRACT FROM AUTHOR]

Abstract In recent years, solar energy becomes one of the most popular renewable energy resources. Among the solar energy systems, solar cells are considered to be one of the most critical components, which are naturally fragile during either fabrication or service. Thus, nondestructive examination of solar cells is required practically. In this research, solar cells made from monocrystalline silicon are selected as the typical samples. The air-coupled ultrasonic nondestructive testing system, based on the generation and reception of the A0 mode Lamb waves, will be adopted for defects detection. Firstly, the dispersion curves of Lamb waves in anisotropic monocrystalline silicon are calculated theoretically, which provides a theoretical foundation for analyzing the behavior of Lamb waves propagating in silicon solar cells. Then, experiments on monocrystalline silicon solar cells are carried out by introducing the air-coupled ultrasonic Lamb waves. The amplitude distributions of Lamb waves propagating along various rotation angles in silicon solar cells are obtained. It shows a strong anisotropic properties of the tested samples. Finally, artificial cracks in solar cells are scanned circularly and laterally. The results of amplitude distributions give the scattering pattern of cracks, which can be used to determine whether the crack breaks through or not. And the extracted amplitude cross-correlation coefficients can help to estimate the length of them. [ABSTRACT FROM AUTHOR]

Abstract The orbital angular momentum of photon is one of the key source of quantum information. Polarized light passing through twisted special wave plates show entangled states of both spin angular momentum (SAM) and orbital angular momentum (OAM). The variable circular and linear retarders (VCR and VLR) are studied here from the view point of quantum processing of entangled states. In association with twist dependent gain of OAM, the geometric phase has been identified here both for VCR and VLR. For the application of VLR in an antenna array, we established a correlation between geometric phase and concurrence to explain the gradual flattened shape of the emergent wave by controlling the external twist of wave plates. [ABSTRACT FROM AUTHOR]

Previous studies carried out for quantifying the material nonlinearity of plate materials have used a nonlinearity parameter β which is derived for longitudinal waves. When Lamb waves are used as the probing waves, a relative nonlinearity parameter β ′ is used. This relative nonlinearity parameter β ′ is a non dimensionless parameter and gives the relative estimate of the material nonlinearity. In spite of these shortcomings, β ′ is widely used for quantifying the material nonlinearity using Lamb waves. Therefore, in the present study, firstly a physics based equation giving the nonlinearity parameter γ phy in terms of higher order elastic constants is derived considering the Lamb wave motion. The parameter γ phy gives an actual estimate of the material nonlinearity. This equation has its elastic component γ e resulting from the lattice anharmonicity and plastic component γ p resulting from the dislocations formed during fatigue or elastoplastic loading of the plate material. Secondly, a novel equation is derived to estimate the material nonlinearity γ amp , which uses the amplitudes of the fundamental harmonic and second harmonic of Lamb waves generated because of the material nonlinearity. With the help of this equation, the actual inherent material nonlinearity can be estimated and it can also be used to quantify the density of dislocations once the amplitudes of the fundamental and second harmonics of Lamb waves in a specimen are made available from the experiments or simulation. Therefore, this equation is practically useful. In order to validate the model, a literature based experimental verification is used here. In the first part, the inherent material nonlinearities of two different materials are estimated and values of β ′ and γ amp are compared qualitatively and quantitatively. In the second part, the density of dislocations is quantified using γ p as well as γ p / γ e . The agreement of results among γ amp and γ phy for virgin as well as plastically loaded specimens, confirms the validity of the proposed models. [ABSTRACT FROM AUTHOR]

• The study presents a method to detect defects at interfaces in a double-casing well. • The relationship between ultrasonic Lamb waves and channels was investigated. • The method of assessing the channel thickness was applicable to all interfaces. In cement-bonding evaluation, channel detection by ultrasonic Lamb waves has been widely studied and applied in single-casing wells. However, its feasibility in double-casing wells remains unknown. The relationship between ultrasonic Lamb waves and channels based on finite-difference time-domain simulations was investigated. According to the Lamb wave propagation, channels near interfaces were determined by the Lamb wave amplitude changes. A channel detection approach in double-casing wells was proposed. The ability of the proposed strategy to accurately detect the spatial position and geometric shape of the channels in double-casing wells was seen. This study provided guidance in double-casing cement-bonding evaluation. [ABSTRACT FROM AUTHOR]

Silicon steel laminations are introduced as the back-plate to an electromagnetic acoustic transducer (EMAT) to increase the efficiency of the EMAT by increasing the magnitude of the EMAT coil's dynamic magnetic field and the eddy current in the sample surface. A two-dimensional, non-linear finite element model is developed to quantify the effectiveness of the back-plate’s different maximum permeability and saturation flux density, on increasing the eddy current density and the dynamic magnetic flux density in the specimen. A three-dimensional FE model is also developed, and confirms the expected result that the laminated structure of silicon steel (SiFe) markedly reduces the eddy current induced in the back-plate, when compared to a continuous slab of the steel. Experimental results show that silicon steel lamination can increase the efficiency of the EMAT in the cases both with and without a biasing magnetic field. [ABSTRACT FROM AUTHOR]

The use of waveguides for ultrasonic transduction is advantageous in numerous applications such as measurement of flow, temperature and material properties under harsh environments. However, typically waveguide transducers do not use any mode control. This paper describes aspects of mode selection for waveguide based transduction of bulk longitudinal ultrasonic waves in a target metallic specimen. It is proposed that specific waveguide modes with suitable modal structure and matched acoustical impedance can significantly enhance the transduction of bulk ultrasonic waves in the specimen sample. Using circular cylindrical waveguides excited by magnetostriction, experimental results guided by finite element simulations and analysis are presented, demonstrating the feasibility of the approach. An example application in the form of a bulk longitudinal wave ultrasonic transducer is also provided illustrating the improvement in the signal quality using the proposed concept. [ABSTRACT FROM AUTHOR]

The use of epoxy adhesives in structural bonding provides lightweight materials, however the assessment of the integrity and quality of the joint is of critical concern. Thus it is essential to know if the adhesive is well crosslinked. This work deals with the nondestructive acoustic characterization of epoxy networks, representative of the adhesive family, of different crosslinking density; i.e. conversion. Different curing cycles were applied and differential scanning calorimetry measurements allowed the determination of their glass transition temperature and epoxy conversion. The acoustic study was performed on epoxy plane plates, all in the glassy state and well beyond the gel point. The methods were based on the propagation of bulk longitudinal and shear waves, as well as on guided Lamb waves. Depending on the conversion of the epoxy, it was shown that the changes in thermo-mechanical properties, resulting from different degrees of cure, greatly influence the acoustic behavior of some Lamb modes if a selection of the sensitive modes has been performed upstream. [ABSTRACT FROM AUTHOR]

This paper presents a multipath exploitation approach for defect imaging in thin-walled structures using Lamb waves under the sparse reconstruction framework. An image of the defects in the region of interest is obtained by inverting a multipath propagation model via group sparse reconstruction. This permits accurate imaging of regions close to the structure boundary without the introduction of ‘false’ defects or ghosts. Real-data measurements of defects close to the edge of an aluminum plate are used to demonstrate the effectiveness of the multipath exploitation scheme. [ABSTRACT FROM AUTHOR]

The Laser/EMAT ultrasonic (LEU) technique has shown the capability to measure weld penetration depths in thick structures based on ray-tracing of laser-generated bulk and surface waves. The ray-tracing method is not applicable to laser-generated Lamb waves when the LEU technique is used to measure weld penetration depths in thin structures. In this work, transmission coefficients of Lamb waves present in the LEU signals are investigated against varying weld penetration depths. An artificial neural network is developed to use transmission coefficients of sensitive Lamb waves and LEU signal energy to predict weld penetration depths accurately. The developed method is very attractive because it allows a quick inspection of weld penetration depths in thin structures. [ABSTRACT FROM AUTHOR]

• The dispersive equation of SH guided waves propagating in a plate subject to the uniaxial stress state has been derived. • The experiment of the LCR wave probe and proposed SH 0 wave transducer has been carried out and compared. • The thermal modulation experiments have been performed to monitor the thermal stress in the aluminum and steel specimen. Thermal stress is one of the major causes of failure of engineering structures and its measurement has attracted more attention in recent years. The ultrasonic wave method is very promising in stress measurement due to its non-destructive nature and easy manipulation. The traditional ultrasonic wave transducers require a coupling medium which would introduce large repeatability errors in travel time measurement and thus in the measured stress. In this work, a methodology based on bonded shear horizontal (SH) guided wave piezoelectric transducers was developed to monitor thermal stress in metal plates. The adhesive bonding between the transducer and the specimen ensures the repeatability in travel time measurements, and the strain gauges are also employed to monitor the wave path length. The dispersive equation of acoustoelastic SH wave propagating in an isotropic medium under the uniaxial stress is derived. Both the uniaxial tension test and thermal modulation test have been performed in aluminum and steel plates. The results show that the acoustoelastic constants of the SH 0 wave are identical to that of the shear bulk wave as predicted by the acoustoelastic theory and the thermal stress measured from −60 °C to 100 °C by the proposed method has a very good repeatability (better than 2 MPa) in both the aluminum and steel plates. Considering the convenience and reliability of the bonded SH 0 wave piezoelectric transducer, the proposed method is very promising for monitoring of thermal stress in engineering structures, such as rails, etc. [ABSTRACT FROM AUTHOR]

As the solar photovoltaics power system sees rapid growth in installed capacity and plays a significant role in the future global energy mix, its reliability becomes a crucial factor in maintaining the stability of the electricity supply. Manufacturing imperfections and harsh operating environments may compromise the module's structural integrity, leading to performance deterioration and power loss. Therefore, nondestructive inspection becomes an indispensable part of the quality assurance or maintenance program to detect defects at any stage of the module's lifecycle. Ultrasound is an essential method for material inspection, and ultrasonic-guided waves have been long explored as a flaw detection technique on plate-like structures, taking advantage of its long-range detection that yields an efficient inspection process superior to the conventional pulse-echo technique. Inspired by the same idea, this work assesses the prospect of harnessing ultrasonic guided waves, particularly Lamb waves, to detect cracks, as they exist in an actual module. However, unlike the commonly investigated plates, solar photovoltaic modules contain stacks of a-few-microns-thick layers of different materials that add complexities to the structure. The investigated specimen is a thin film photovoltaic module with cracks caused during transportation and handling. It, therefore, represents a real-life research case that may occur in situ. Numerical and experimental methods are performed to reveal various Lamb modes that propagate in the structure, where the results of both methods are mutually confirmed. Unlike other works, this investigation is not confined to the utilization of the non-dispersive mode but attempts to find the defect-sensitive mode that can be used to detect cracks. An analysis of the experimental results reveals the mode most sensitive to cracks, while numerical simulations explain the corresponding phenomena. • This work assesses the prospect of harnessing UGW to detect cracks in the front glass of a thin film PV module. • The technique has potentials for QC process in continuous manufacturing lines or in-situ structural health monitoring. • The investigated specimen has actual defects caused by operational activities, hence representing a real-life research case. • This investigation attempts to discover the defect-sensitive mode that can be used to detect cracks. • By analyzing Lamb waves dispersion curves, the particular mode sensitive to cracks in the front glass panel can be identified. [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]

In applications of spot welding, the nugget determines reliability of the material joining, which is characterized by nugget diameter. However, the conventional methods are complex and rough to measure the diameter of the nugget. This paper aims to propose a novel convenient method for measuring the diameter of spot welding nugget. The characteristic curve of laser ultrasonic Lamb waves in nugget area are obtained by laser ultrasonic detection technology under completely non-contact conditions. The variation characteristics of power spectral density in nugget area are analyzed by Fast Fourier Transform, to determine the geometric relationship between the scanning area and the nugget diameter, and the nugget diameter are obtained by further calculation. Experiments show that the accuracy of the proposed method is within 0.2 mm, and the relative errors of experimental data are within 3%. This paper provides an alternative method for the measurement of spot welding nugget diameter, and has great significance for the application of laser ultrasonic detection in spot welding. • A novel convenient method for measuring the diameter of spot welding nugget by laser ultrasonic detection (LUD) is proposed. • The variation characteristics of power spectral density in nugget area are analyzed, and the nugget diameter are obtained by further calculation. • Experiments show that the accuracy of the proposed method is within 0.2 mm, and the relative errors are within 3%. [ABSTRACT FROM AUTHOR]

The complex design of composite structures makes it very difficult for conventional visual inspection techniques to detect defects in these materials. Therefore, numerous types of nondestructive testing and structural health monitoring techniques have been developed for damage identification in composite structures, among which ultrasonic guided waves, particularly Lamb waves, have gained much popularity. In recent years, apart from piezoelectric pointwise measurements, laser Doppler vibrometry has been used for full wavefield measurements of propagating Lamb waves. Damage imaging can be done with animations of Lamb waves interacting with defects. In this work, two end-to-end deep learning-based models of many-to-one sequence prediction were developed to perform pixel-wise image segmentation. A large dataset of full wavefield images resulting from the interaction with delamination of random size, shape, and location was utilised to train the proposed deep learning models. The main goal of this work is to investigate the feasibility of implementing deep learning methods for delamination identification in composite laminates by only utilising the animations of Lamb waves. The elaborated models worked well on the numerically generated test data and have also shown good performance on the real-world experimental data. These methods can enable the automation of delamination identification and to create damage maps without the intervention of the user. [ABSTRACT FROM AUTHOR]

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

It is found that leaky backward Lamb waves, i.e. waves with negative energy-flux velocity, propagating in a plate submerged in a liquid possess extraordinary energy properties distinguishing them from any other type of waves in isotropic media. Namely, the total time-averaged energy flux along the waveguide axis is equal to zero for these waves due to opposite directions of the longitudinal energy fluxes in the adjacent media. This property gives rise to the fundamental question of how to define and calculate correctly the energy velocity in such an unusual case. The procedure of calculation based on incomplete integration of the energy flux density over the plate thickness alone is applied. The derivative of the angular frequency with respect to the wave vector, usually referred to as the group velocity, happens to be close to the energy velocity defined by this mean in that part of the frequency range where the backward mode exists in the free plate. The existence region of the backward mode is formally increased for the submerged plate in comparison to the free plate as a result of the liquid-induced hybridization of propagating and nonpropagating (evanescent) Lamb modes. It is shown that the Rayleigh's principle (i.e. equipartition of total time-averaged kinetic and potential energies for time-harmonic acoustic fields) is violated due to the leakage of Lamb waves, in spite of considering nondissipative media. [ABSTRACT FROM AUTHOR]

• Study of varying temperature effect on guided wave (GW) propagation in a CFRP panel. • Calculation of a temperature-independent compensation factor for numerical data. • Validation of different FE modelling approaches for GW through dispersion curves. • Numerical and experimental GW S 0 and A 0 modes dispersion curves under thermal loads. • Assessment of S 0 and A 0 modes group velocity sensitivity to temperature variations. Guided wave (GW) structural health monitoring (SHM) systems offer an attractive solution as an in - situ quasi real-time assessment of structural damage, but their sensitivity and efficiency may be impaired under varied environmental and operational conditions. Thus, virtual tests, such as that based on the Finite Element (FE) method, represent a valid approach for simulating and investigating SHM systems, enabling a substantial reduction in experimental campaigns. In this work, GW propagation characteristics in a carbon fibre-reinforced composite plate are studied under a varying temperature condition, representative of the aeronautics application. At first, GW SHM system was physically tested at room temperature (20 ° C), and the results were used to calibrate and assess the proposed FE modelling approaches, characterised by different element types and mesh sizes. A temperature independent averaged time compensation factor is proposed to mitigate the numerical data dependency on excitation frequency and propagation angle. Two temperature variations (from 20 ° C to - 50 ° C , and 20 ° C to 65 ° C) were experimentally and numerically considered to investigate the effect of varying temperature on the GW. For all test cases, the compensated numerical data was compared to the experimental results, and discussed in terms of dispersion curves, focusing on the zero-order symmetric, S 0 , and antisymmetric, A 0 , modes. Results show that both 2D and 3D FE approaches can accurately predict the changes in GW due to varying temperature, with the group velocity of the A 0 mode being less sensitive to temperature variations. [ABSTRACT FROM AUTHOR]

• Conventional ultrasonic fatigue crack detection is unreliable in noisy environments. • Hybrid deep learning model can achieve reliable on-field fatigue crack detection. • Hybrid method comprises LSTM network and trispectrum-based spectral analysis. • Better noise reduction and fatigue crack detection achieved using hybrid approach. Recently, the technique of nonlinear Lamb wave mixing has been developed for the detection of fatigue crack in engineering structures. In this technique, two or three Lamb waves with distinct frequencies are applied to a structure. The cross-mixing between these waves results in nonlinear mixed components depending on the sum and difference of the incident frequencies. However, the amplitude of the mixed components generated by the fatigue crack becomes weak in a noisy environment. Thus, noise elimination is critical for reliable crack detection. To address this gap, a novel hybrid method that incorporates a deep learning (DL) model with higher-order spectral analysis is proposed in this study. First, a nonlinear Lamb wave mixing technique is developed to capture ultrasonic data from the aluminum plates during fatigue testing. Subsequently, the DL model based on long short-term memory (LSTM) accepts an original ultrasonic time signal as input and yields an output of a reconstructed ultrasonic signal after noise reduction. Finally, the random noise in the reconstructed signal is eliminated and the mixed components are extracted by trispectrum (TS)-based higher-order spectral analysis. Furthermore, the proposed and existing methods (e.g., power spectrum) are applied to the ultrasonic data collected from fatigue experiments. The results validated the improved performance of the proposed LSTM-TS method for reliable fatigue crack detection in noisy environments. [ABSTRACT FROM AUTHOR]

Visualizing the propagation of Lamb waves generated by scanning laser injection is a promising and effective method for the nondestructive inspection of carbon fiber reinforced plastic (CFRP) structures. We evaluate the use of two-dimensional time-slowness maps of the acquired data for defect detection in CFRP laminates. The slowness maps can be constructed from the differences in wave propagation characteristics between two adjacent laser injection points. Using a CFRP plate, in which an artificial delamination is located at the center, we show that the delamination can be clearly observed as a hotspot in the two-dimensional slowness map. The visible hotspot is due to the dispersion of the lowest antisymmetric (A 0) mode of the Lamb wave: the A 0 mode slows down at the delaminated region. We also demonstrate that our method can be applied to an L-shaped structure. Two-dimensional slowness images can be useful for developing and applying practical inspection methods based on laser ultrasonic waves. • ・Defects in CFRP laminates are inspected using laser ultrasonic guided waves. • ・A two-dimensional slowness map is made from the data obtained by scanning laser. • ・The slowness map is simple and useful for defect detection. [ABSTRACT FROM AUTHOR]

The formation of dendrite affects the cycling life of a battery and lead to malfunctions such as internal short-circuiting and thermal runaway events. However, existing methods to observe dendrite formation, such as X-ray computed tomography and scanning electron microscopy are either prohibitively complicated or unsuitable for long-term, in-situ monitoring. In this study we present a method which uses the fundamental shear-horizontal mode (SH0*) guided ultrasonic waves to independently monitor the status of the electrodes in a symmetric aqueous zinc-ion battery. Experimental measurements show that the velocity and attenuation of the ultrasonic wave on the opposing electrodes vary in the opposite senses during the cycling. While the velocity and attenuation changes can be partially reversed, a monotonic drift can also be observed with increasing number of cycles. Coupled with optical microscopy, the partially reversible oscillations can be associated with zinc stripping/plating. The irreversible drifting can be associated with the formation of 'dead' zinc dendrite. The technique shows clear sensitivity to the formation of dendrite, especially in the early stages (∼ 10 cycles) of charging and discharging processes. This work should inspire future research to enable quantitative assessment of the technique sensitivity and to improve its resolution. [Display omitted] • Dendrite formation on electrode surfaces is a key degradation process in batteries. • A novel technique for in-situ monitoring of dendrite formation is presented. • Properties of guided ultrasonic waves are affected by the dendrites. • Technique delivers quantitative insights into battery degradation processes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Instantaneous phase characteristics of guided waves contain a rich source of information for defect characterisation. • Gaussian process regression technique is capable of predicting the real defect depth with a large-scale simulated dataset. • Gaussian process regression can successfully estimate the depth of wide defects (≥100% of wavelength) in practice. Ultrasonic Guided Waves (UGW) offer the possibility of inspecting a strip across a structure rather than just the point under a traditional bulk wave transducer. This can increase the rate of inspection and enable inspection under obstructions. This paper investigates the instantaneous phase characteristics of the shear horizontal guided waves for various defect depths and widths. The Gaussian process regression is then evaluated for estimating the minimum remaining wall thickness between a pair of transducers. A Gaussian process regression model is built using the fusion of large-scale simulated and low-scale real experimental data. For this purpose, a more precise model of an electromagnetic acoustic transducer is initially built by integrating both electromagnetic and elastic wave fields. Then the simulated data set is built after having been calibrated using a genetic algorithm. The examination of an unseen simulated evaluation data set shows that 96 % of data has an error during thickness gauging of less than 10 per cent of wall thickness. Finally, an experimental testing data set containing three different defects with depths of 3.7, 5.7 and 9.2 mm was examined, resulting in a good depth prediction of large defects with less than 1 mm error for defects wider than one wavelength. [ABSTRACT FROM AUTHOR]

• Ellipse of uncertainty (EOU) -based algorithm was developed for quantitative evaluation of defect localization using Lamb waves. • Parametric EOU formula was provided. • Correlated ToF data of each inspection pair were fused based on the acoustic reciprocity theorem and the fuzzy control parameter. • Accurate defect localization was realized by searching individuals at the intersection of multiple temporal-spatial mapping trajectories. • Quantitative evaluation of defects localization was realized by analyzing the distribution of individuals and their relative deviation of arrival times to every inspection pairs. Measurement deviation of time of flight (ToF) is inevitable in nondestructive testing based on the sparse array and ultrasonic Lamb waves. It affects the influence zone of temporal-spatial mapping trajectories (TSMTs) of signal parameters in the imaging zone, and further limits the quantitative evaluation of defect localization. In the paper, the ellipse of uncertainty (EOU) of TSMTs was derived from multiple parameters, including the group velocity, ToFs and their measurement deviations, distances between actuators and receivers. Then, an EOU-based algorithm was developed for quantitative evaluation of defect localization. The defects were localized by searching the individual scatterers at the intersection of multiple TSMTs. Based on the eccentricity of the uncertainty ellipse, a fuzzy scaling factor was introduced. It was combined with a fuzzy control parameter to tune the influence zone of TSMTs. Based on the acoustic reciprocity theorem and the fuzzy control parameter, the ToFs of scattering waves were fused to establish the one-to-one relation between individual scatterers and inspection pairs. Experimental results showed that the EOU-based algorithm can reduce the interferences of EOU in the detection; and the quantitative evaluation of defect localization was realized by analyzing the distribution of individuals and their ToF difference to inspection pairs. [ABSTRACT FROM AUTHOR]

In view of the high sensitivity to microstructure evolution of higher harmonics generated by the nonlinear ultrasonic method, the tensile damage in the stainless steel AISI 316L was evaluated by nonlinear ultrasonic Lamb wave. The effect of different degree of tensile deformation on nonlinear ultrasonic response in 316L was analyzed by considering the microstructure evolution and the test results. First, the power-law relation of nonlinear ultrasonic parameter and plastic strain is proposed. Meanwhile, martensitic transformation and the increasing of dislocation density can be observed in the micrographs of specimens with the increase of plastic deformation. Then, the same variation tendency of intergranularly averaged Kernel Average Misorientation (KAM), as well as the relative nonlinear ultrasonic parameter, demonstrated that the nonlinearity due to tensile damage is controlled by the evolution of microstructure that directly affects higher order elastic constant of stainless steel 316L. Furthermore, based on continuous damage mechanics, the method quantitatively characterizes the degradation of mechanical properties was proposed with a functional relation of damage variable and relative nonlinear parameter. The method proposed in this study provides a theoretical basis for online monitoring of the damage status of in-service equipment. It is of great significance for residual life assessment, structure integrity assessment, and service life extension of equipment. • A non-destructive method using nonlinear ultrasonic Lamb wave to detect tensile damage is introduced for AISI 316L. • The power-law relation of nonlinear ultrasonic parameter and plastic strain is observed. • The method quantitatively characterizes the degradation of mechanical properties is proposed with nonlinear parameter. • The effects of different degrees of tensile test on the evolution of the microstructures are examined. [ABSTRACT FROM AUTHOR]

• Cross-shaped sensor cluster is proposed to be used together with nonlinear Lamb waves for microcrack localization. • The determination criteria of appropriate Lamb wave mode pair for microcrack localization are revealed. • Extraction of TDOAs of nonlinear microcrack characteristic signals is investigated. • The investigations verify that the proposed method can realize microcrack localization without prior group velocity. Using the nonlinear interaction effect between ultrasonic Lamb waves and microcracks to detect and locate microcracks has the advantages of fast detection speed and high sensitivity. In this paper, a method for microcrack localization based on cross-shaped sensor clusters in a plate is proposed by combining nonlinear ultrasonic Lamb wave technology and time difference of arrival (TDOA) technology. The antisymmetric (A0) mode at low frequency is chosen as the primary Lamb wave to simplify the complication of the dispersion and multi-mode properties of Lamb waves. The selected mode pair (A0-s0) weakens the influence of the cumulative growth effect of higher harmonics induced by the inherent material nonlinearity on the microcrack characteristic signals. Pulse inversion technique and cross correlation function are used to extract the TDOAs of the nonlinear characteristic signals including microcrack information. The cross-shaped sensor clusters approach proposed for the first time can achieve reliable and fast microcrack localization without being affected by the duration of the excitation signal, and a priori knowledge of group velocities of primary wave modes or generated harmonics. Experimental and numerical results validate the proposed method in isotropic and anisotropic plates. This paper provides a new idea for nonlinear ultrasonic nondestructive evaluation and structural health monitoring of microcracks in plates. [ABSTRACT FROM AUTHOR]

• Investigating an innovative approach to determine the damping property of a viscoelastic coating using ultrasonic Lamb wave in a metal-polymer waveguide. • Dropping amplitude signals as well as changing the phase of Lamb wave when travels through the coated waveguide. • Exploiting the obtained damping property in FEM software, ABAQUS. • Reducing computational cost and time by taking advantage of the experiment-based fitting method. • Verification of amplitude ratio as well as group velocity dispersion curves related to the FEM simulation by experimental results with an error of less than 1%. • Confirming the excitability of piezo discs in a range of frequency with tangible precision. This work presents an innovative approach according to an experiment-based fitting method to determine the damping property of a viscoelastic coating layer, in a simple, low-cost, and time-effective manner. In this experiment, symmetric and asymmetric ultrasonic Lamb waves were applied to two coated plates with different thicknesses, and the waves were generated using piezo discs. A viscoelastic coating influences the signal amplitude as well as the wave phase. By comparing the amplitude ratio (AR) of the transmitting signals between the coated and bare plates, the damping property of the viscoelastic coating was experimentally determined. Similar to the experiments employing the finite element method (FEM) software, in this experiment, ABAQUS, was employed to verify the conformity between numerical and experimental AR. By selecting a non-dispersive Rayleigh damping β for the coating layers at all frequencies, the computational cost reduced significantly to one-tenth the original cost. Apart from corroboration by AR matching, the numerical dispersion curves of the group velocity were also validated by experimental curves. The FEM dispersion curves in the frequency range of the tests were found to be highly reliable, with an average error of less than 1% for the first experimental setup and 10% for the second setup. Furthermore, in coated waveguides, the proposed technique could precisely estimate the damping property of the viscoelastic coating layers, where excitability in a wide range of frequencies is required. However, this precision strongly relies on the selected mode, frequency range, PZT quality, and waveguide thickness. [ABSTRACT FROM AUTHOR]

Antisymmetric and symmetric Lamb-type feature guided waves (FGW) have recently been shown to exist in small angle plate bends. This paper reports Semi-Analytical Finite Element (SAFE) method simulations revealing the existence of a new family of Shear Horizontal ( SH B ) type of FGW mode in 90° bends in plate structures. Mode shapes and velocity dispersion curves are extracted, demonstrating the SH -like nature of a bend-confined mode identified in studies of power flow across the bend. The SH B mode is shown to have reduced attenuation in the higher frequency range, making it an ideal choice for high-resolution inspection of such bends. Further modal studies examine the physical basis for mode confinement, and argue that this is strongly related to FGW phenomena reported earlier, and also linked to the curvature at the bend region. Wedge acoustic waves discussed widely in literature are shown as arising from surface-limiting of the SH B mode at higher frequencies. The results are validated by experiments and supported by 3D Finite Element (FE) simulations. [ABSTRACT FROM AUTHOR]

Air-coupled ultrasonic inspection using leaky Lamb waves offers attractive possibilities for non-contact testing of plate materials and structures. A common method uses an air-coupled pitch–catch configuration, which comprises a transmitter and a receiver positioned at oblique angles to a thin plate. It is well known that the angle of incidence of the ultrasonic bulk wave in the air can be used to preferentially generate specific Lamb wave modes in the plate in a non-contact manner, depending on the plate dimensions and material properties. Multiple reflections of the ultrasonic waves in the air gap between the transmitter and the plate can produce additional delayed waves entering the plate at angles of incidence that are different to those of the original bulk wave source. Similarly, multiple reflections of the leaky Lamb waves in the air gap between the plate and an inclined receiver may then have different angles of incidence and propagation delays when arriving at the receiver and hence the signal analysis may become complex, potentially leading to confusion in the identification of the wave modes. To obtain a better understanding of the generation, propagation and detection of leaky Lamb waves and the effects of reflected waves within the air gaps, a multiphysics model using finite element methods was established. This model facilitated the visualisation of the propagation of the reflected waves between the transducers and the plate, the subsequent generation of additional Lamb wave signals within the plate itself, their leakage into the adjacent air, and the reflections of the leaky waves in the air gap between the plate and receiver. Multiple simulations were performed to evaluate the propagation and reflection of signals produced at different transducer incidence angles. Experimental measurements in air were in good agreement with simulation, which verified that the multiphysics model can provide a convenient and accurate way to interpret the signals in air-coupled ultrasonic inspection using leaky Lamb waves. [ABSTRACT FROM AUTHOR]

The interaction of laser-generated Lamb waves propagating in a plate with a sharp-angle conical hole was studied experimentally and numerically. Part of the energy of the incident wave is trapped within the conic area in two ways: the antisymmetric Lamb wave orbiting the center of the hole and the wave localized at the acute edge. Parameters and conditions for optimal conversion of the incident wave into the trapped modes were studied in this work. Experiments were performed using the laser stroboscopic shearography technique, which delivers the time evolution of the acoustic field in the whole area of interest. The effect of trapping can be used for efficient damping, similar to the one-dimensional acoustical black hole effect. [ABSTRACT FROM AUTHOR]

A fully non-contact single-sided air-coupled and laser ultrasonic non-destructive system based on the generation and detection of Lamb waves is implemented for the characterization of A 0 Lamb wave mode dispersion in a composite plate. An air-coupled transducer (ACT) radiates acoustic pressure on the surface of the composite and generates Lamb waves within the structure. The out-of-plane velocity of the propagating wave is measured using a laser Doppler vibrometer (LDV). In this study, the non-contact automated system focuses on measuring A 0 mode frequency–wavenumber, phase velocity dispersion curves using Snell’s law and group velocity dispersion curves using Morlet wavelet transform (MWT) based on time-of-flight along different wave propagation directions. It is theoretically demonstrated that Snell’s law represents a direct link between the phase velocity of the generated Lamb wave mode and the coincidence angle of the ACT. Using Snell’s law and MWT, the former three dispersion curves of the A 0 mode are easily and promptly generated from a set of measurements obtained from a rapid ACT angle scan experiment. In addition, the phase velocity and group velocity polar characteristic wave curves are also computed to analyze experimentally the angular dependency of Lamb wave propagation. In comparison with the results from the theory, it is confirmed that using the ACT/LDV system and implementing simple Snell’s law method is highly sensitive and effective in characterizing the dispersion curves of Lamb waves in composite structures as well as its angular dependency. [ABSTRACT FROM AUTHOR]