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[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]

• 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]

• 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]

• 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]

This work assesses the suitability of the two zero-order Lamb wave modes to detect multiple barely-visible impact damage in composite material. Three specimens were subjected to damage at three different low-energy levels and one was left as an undamaged reference sample. Ultrasonic Lamb wave modes were selectively generated by surface-bonded piezoceramic wafer transducers in two tuned configurations. By using an algorithm based on the Akaike Information Criterion the time-of-flight of the Lamb modes was determined, allowing their threshold detection capabilities for the studied application to be successfully benchmarked. The results were consistently validated by digital shearography, ultrasonic C-scan and optical microscopy. A study of the effects on structural integrity was completed with an assessment of the damping ratio and residual bending strength proving to be sensitive parameters to the induced damage. [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]

• 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]

Highlights: [•] Our aim is to analyze the mode transmission in the notched plate within the relatively short propagation distance. [•] The originally-transmitted modes and newly-converted modes were separated. [•] The velocities and energy of converted modes were extracted to locate and evaluate the notches, respectively. [ABSTRACT FROM AUTHOR]