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• 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]

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]

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]

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]

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]

• Spectral FEM within the bi-potential framework is developed for modeling CAN. • A semi-explicit algorithm is proposed to facilitate high computational efficiency. • Contact force can be calculated by bi-potential method without defined parameters. • Computational efficiency of the proposed method is much higher than that of FEM. • The proposed method can capture classical contact acoustic nonlinearity features. Nonlinear ultrasonic guided waves are of great importance in structural health monitoring, since they have both the large-area inspection capability of guided waves and the desirable sensitivity to incipient defects from nonlinear ultrasonic features. Efficient numerical methods play a key role in investigating the nonlinear features of guided waves. In this work, a time domain spectral finite element method (TDSFEM) combined with the bi-potential contact theory is developed to capture the nonlinear interactions between guided waves and breathing cracks. The bi-potential approach has better accuracy than the conventional penalty method, since there is no any user-defined parameter for computation in the former method while the latter one requires a user-experience-based penalty factor. In addition, a semi-explicit algorithm is proposed to integrate the wave equation, which can take full advantage of the diagonal mass matrix of TDSFEM while not sacrificing numerical stability. Numerical verifications against ANSYS show that the computational efficiency of the bi-potential method-based TDSFEM is much higher than that of the FEM via the penalty method. Numerical case studies are then performed to investigate the nonlinear interactions between Lamb waves and breathing cracks. Results show that the proposed method can successfully capture the classical nonlinear features, such as higher harmonic generation and zero frequency (DC) response. Moreover, the influence of the length and gap of a breathing crack on the contact acoustic nonlinearity (CAN) is investigated. As an efficient numerical approach, the bi-potential method-based TDSFEM may be a good tool for investigating the CAN. [Display omitted] [ABSTRACT FROM AUTHOR]

In ultrasonic guided wave inspections, incident waves propagate as a finite-bandwidth wave packet. During propagation, the shape of the wave packet is distorted because of dispersion even if there are no viscous or scattering effects. Although the presence of distortion is well known, how the wave packet is distorted has not been sufficiently investigated. We use the method of multiple scales (MMS), a perturbative scheme, to investigate the dispersion of the Lamb waves comprising the finite-bandwidth wave packet. Solutions obtained by the MMS to order O (ε 2) show that the shape of an input wave-packet remains invariant during propagation along the waveguide. The wave packet propagates with a group velocity obtained from a classical analysis. The O (ε 3) -order MMS solutions exhibit a symmetric distortion of the shape, whereas the O (ε 4) -order MMS solutions feature symmetric and anti-symmetric distortion. For the derivation of the O (ε 3) - and O (ε 4) -order MMS solutions, the Fourier transform of the wave-packet shape is introduced. The proposed MMS solutions were verified by comparing them with the numerical time-domain solutions constructed from the classical frequency-domain solutions. The strength of the distortion depends on propagation mode, input frequency, and shape of the input wave packet. Because the wave-packet distortion induces a shift and reduction of the peak, the propagation velocity is different from the group velocity. For long-distance inspections, the proposed method provides the precise propagation velocity and details of the propagation-mode selection. • The method of multiple scales is applied to analysis of the dispersion. • The strength of the distortion depends on propagation mode, input frequency, and shape of the input wave packet. • Using the MMS, we obtained insights into distorting the shape of the wave packet. [ABSTRACT FROM AUTHOR]

• Guided ultrasonic wavefield measurements performed on quasi-isotropic CFRP laminate. • Scattering at artificial, ellipse delamination, located at an asymmetric depth. • Wave trapping on top of delamination and forward scattered wave verified. • Numerical, Finite Element simulations validated from experiments. • Parameter study of guided wave scattering at different delamination shape and depth. Carbon fibre composite laminates are increasingly being used for aerospace structures due to their low weight and improved mechanical performance. Impact damage can cause delaminations below the visible surface of the structure due to limited interlaminar strength. Guided ultrasonic waves can detect and characterize delaminations in composite laminates. The scattering of the A 0 Lamb wave mode at an artificial delamination, located at an asymmetric depth in a quasi-isotropic laminate, was investigated. Full field non-contact laser measurements were used to visualise wave trapping and scattered waves. A three-dimensional finite element model was developed and validated against the experiments. The influence of delamination shape and depth on guided wave scattering were studied. Small variations in delamination shape significantly affected the interference pattern on top of the delamination, but had limited effect on the scattered wave outside the delamination. Delamination depth was found to strongly influence the angular direction and amplitude of scattered waves. Implications for structural health monitoring were discussed. [ABSTRACT FROM AUTHOR]

Abstract: This paper precisely investigated stiffness changes in cross-ply CFRP laminates loaded in tension using Lamb waves. Young’s modulus was obtained as a function of applied strain by in situ measurement of the velocity of the extensional mode during the tensile test. The modulus exhibited a complicated behavior due to stiffening of the 0° plies and transverse cracks. The effect of the stiffening on the modulus was evaluated by the rule of mixtures and the effect of the cracks alone was successfully deduced. The experimental modulus reduction due to the cracks was predicted by a shear-lag analysis including the stiffening effect. Consequently, this study demonstrated that in situ measurement of the wave velocity can quantitatively evaluate damage in the loaded laminates. Furthermore, a new linear location method using the calculated in situ wave velocity was proposed. A dramatic improvement in location accuracy was obtained compared with the conventional method. [Copyright &y& Elsevier]

• Defrosting efficiency with random intermittent phase-stagger is higher than regular. • Optimal defrosting effect can realized under arrayed ultrasonic excited by synergistic random intermittent phase-stagger and multi-frequency. • Average frost layer thickness is approximately 0.282 mm without dead defrosting angle on evaporator surface. Frost dramatically deteriorates the operating performance of an air source heat pump unit and leads to additional energy consumption. Results of our previous study show that intermittent ultrasonic vibration can greatly improve the coefficient of performance (COP) under frosting condition. To meet the ultrasonic defrosting demands of large-scale evaporators and to eliminate the standing wave effect of ultrasonic defrosting, an optimised ultrasonic array method for defrosting is proposed based on synergism of standing wave intermittent phase-stagger and multi-frequency excitation. Firstly, the surface vibration characteristics and defrosting effect difference of copper plate with standing wave intermittent phase-stagger in random mode and regular mode are investigated using a plate model. On this basis, the surface vibration characteristics and defrosting effect of a finned-tube evaporator are analysed in both phase-stagger modes. Finally, to optimise further the defrosting effect of the ultrasonic array, the standing wave intermittent phase-stagger and multi-frequency excitation are integrated to realise chaotic motion of the standing wave on the evaporator surface. Finite element analysis results and experimental results indicate that the standing wave effect of ultrasonic array defrosting can be eliminated using random intermittent phase-stagger, and the defrosting efficiency under random intermittent phase-stagger is superior to the regular method. Moreover, when arrayed ultrasonic transducers are controlled by synergistic random intermittent phase-stagger and multi-frequency excitation, there is no dead defrosting angle on the evaporator surface, the average frost layer thickness is approximately 0.282 mm, and it is 81.1% of mode I (intermittent and multi-frequency excitation). [ABSTRACT FROM AUTHOR]

• First enriched finite element for general wave propagation problems in piezoelastic media. • First element to accurately predict electroelastic, narrowband guided waves to broadband impact waves. • Results show the need of enriching both displacement and electric potential fields. • Accurately captures free edge stress field with steep gradients at piezoelectric actuator-structure interface. • Exhibits much greater computational efficiency, accuracy and faster convergence than conventional FE. A two dimensional finite element (FE) is developed by enriching the conventional Lagrange interpolation functions with local element domain wave functions, for accurate solution of general wave propagation problems in piezoelastic media. The enrichment functions, applied to both displacement and electric potential fields, satisfy the partition of unity condition, and vanish at the nodes. The formulation is developed using the extended Hamilton's principle for piezoelastic solids, considering two-way electromechanical coupling. The enriched FE is shown to perform equally well in terms of computational efficiency and accuracy for broadband impact induced electroelastic wave to narrowband ultrasonic guided wave propagation problems, unlike the other available elements. The solution for impact in a piezoelectric plate is shown to alleviate the spurious undulations in both velocity and electric potential fields, which are encountered in the conventional FE solutions. For the problem of high frequency Lamb wave actuation and sensing in a thin plate bonded with piezoelectric actuator/sensor patches, the element shows significant improvement in the computational efficiency over the conventional FE. Further, the free edge effect of steep gradients in the shear stress distribution at the actuator-plate interface is accurately captured by the proposed element using much fewer degrees of freedom than the conventional FE. [ABSTRACT FROM AUTHOR]

In this study, experimental and numerical approaches based on the nonlinear ultrasonic Lamb wave are used to detect the barely visible impact damages (BVID) in orthotropic CFRP laminates due to low velocity impact (LVI). A drop-weight impact test system and a RITEC nonlinear ultrasonic system are adopted for experimentally measuring the higher harmonics of lamb wave in the damaged CFRP laminates. An integrated finite element (FE) model is proposed to further identify the interaction between Lamb wave and impact damages, in which Hashin criterion and triangular cohesive zone model (TCZM) are used to predict the initiation and evolution of delamination and other damages under LVI. Then, the displacement–time signal and the relative acoustic nonlinear parameters (RANP) of the Lamb wave excited by sinusoidal displacement load are extracted to evaluate the damage in the same FE model. Finally, the relevance between LVI damage and RANP is investigated by numerical and experimental analyses under LVI with different energies, and shows that the amplitudes of harmonics and RANP increase with the raise of the impact energy, as well as the delamination areas. Therefore, the amplitude of harmonics and RANP can be used as indicators to detect BVID. [ABSTRACT FROM AUTHOR]