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The possibility of using the Digital Image Correlation (DIC) technique, along with Lamb wave analysis, was investigated in this study for damage detection and characterization of polymer carbon fiber (CFRP) composites with the help of numerical modeling. The finite element model (FEM) of the composite specimen with artificial damage was developed in ANSYS and validated by the results of full-field DIC strain measurements. A quantitative analysis of the damage detection capabilities of DIC structure surface strain measurements in the context of different defect sizes, depths, and orientation angles relative to the loading direction was conducted. For Lamb wave analysis, a 2D spatial-temporal spectrum analysis and FEM using ABAQUS software were conducted to investigate the interaction of Lamb waves with the different defects. It was demonstrated that the FEM updating procedure could be used to characterize damage shape and size from the composite structure surface strain field from DIC. DIC defect detection capabilities for different loadings are demonstrated for the CFRP composite. For the identification of any composite defect, its characterization, and possible further monitoring, a methodology based on initial Lamb wave analysis followed by DIC testing is proposed. [ABSTRACT FROM AUTHOR]

Plate-like structures, including composite material plates, are extensively employed in major engineering structures such as aerospace, high-speed rail, and civil engineering. The growing utilization of curved panel structures and the increasing complexity of these structures have emphasized the need for flexible sensors capable of efficient signal front-end reception. Addressing the limitations of conventional sensors made of brittle piezoelectric materials in adhering to the surface of structures for health monitoring, a self-developed flexible dielectric elastomer-interdigital transducer (DE-IDT) array is employed to achieve damage imaging in isotropic aluminum curved panel structures and orthotropic composite material plates using linear frequency modulation (chirp) signal excitation. In addition, a proposed fusion method is utilized to obtain weighted imaging results, and evaluation and analysis are performed using relevant indicators. Experimental findings highlight the improved imaging accuracy achieved through phased array data fusion using the linear chirp signal, attributed to its wideband characteristics and ability to capture more damage signals. Moreover, the effectiveness of the circular DE-IDT array for damage detection and imaging in curved panel structures and composite material plates is successfully verified. [ABSTRACT FROM AUTHOR]

In the detection of metal defects by ultrasonic Lamb waves, the commonly used medical coupling agent is volatile, poor stability, and the coupling effect is easily affected. It is vital to find the coupler materials with better performance. This paper develops sodium alginate hydrogel and acrylamide hydrogel with higher curing degree, and compares them with deionized water and medical coupling agent. At the same time, epoxy glue and organic silicone, which commonly used as adhesives in the aerospace field are selected for performance research. Firstly, analyzing FTIR spectra of the three hydrogels. The results showed that the hydrogel composition are correct and subsequent experiments are reliable. Secondly, the performance characteristics of 6 couplings are studied from 5 aspects of acoustic impedance, ultrasonic energy transfer efficiency, stability, temperature characteristics and oxidation resistance. The results show that the acrylamide gel developed in this paper has higher viscosity and stability than the medical coupling agent, and has a wider application. Based on the five performance studies, it can be concluded that in the measurement environment of long-term fixed sensors in the industrial and aerospace fields, organic silicone can not only play the role of bonding sensors and insulation, but have the advantages of strong stability, high ultrasonic energy transfer efficiency, temperature tolerance, oxidation resistance, etc. It is the optimal coupling material in ultrasonic testing for complex environments. The research in this paper provides a reference for the selection of optimal coupling agent materials for more complex applications, which has important engineering significance.

The application of non-stationary Lamb wave signals is a promising tool in various industrial applications where information about changes inside a structure is required. Phase velocity is one of the Lamb wave parameters that can be used for inhomogeneities detection. The possibility of reconstructing the segment of the phase velocity in a strong dispersion range using only two signals is proposed. The theoretical study is performed using signals of the A0 mode propagating in an aluminium plate at a frequency of 150 kHz, 300 kHz, 500 kHz and 900 kHz. The experiment was carried out at a value of 300 kHz. The studies conducted indicated that the maximum distance between two signals, at which the time-of-flight can be measured between the same phase points, is the main parameter for the two signals technique application. Theoretical and experimental studies were performed, and the mean relative error was calculated by comparing the obtained results with those calculated via the SAFE method. In the theoretical study, the mean relative error of 0.33% was obtained at 150 kHz, 0.22% at 300 kHz, 0.23% at 500 kHz and 0.11% at 900 kHz. The calculated mean relative errors δ c p h = 0.91 % and δ c p h = 1.36 % were obtained at different distances in the experimental study. The results obtained show that the estimation of the phase velocity in dispersion ranges using only two received signals was a useful tool that saved time and effort. [ABSTRACT FROM AUTHOR]

This article proposes an electromagnetic acoustic transducer (EMAT) for selectively improving the purity and amplitude of ultrasonic Lamb waves in non-ferromagnetic plates. The developed EMAT consists of a racetrack coil and a group of periodic permanent magnets (PPMs). Two-dimensional finite element simulations and experiments are implemented to analyze the working mechanism and performance of the PPM EMAT. Thanks to the specific design, the eddy currents increase with increasing wire density and the directions of the magnetic fields and Lorentz forces alternate according to the polarities of the magnet units. Wires laid uniformly beneath the magnets, and the gaps between adjacent magnets generate tangential and normal Lorentz forces, resulting in-plane (IP) and out-of-plane (OP) displacements, respectively. The constructive interference occurs when the wavelength of the generated Lamb wave is twice the spacing of the magnets, leading to large amplitudes of the targeted ultrasonic Lamb waves. Therefore, the PPM EMAT is capable of generating pure symmetric or antisymmetric mode Lamb waves at respective frequencies. The results prove that the developed PPM EMAT can generate pure either S0 or A0 mode Lamb waves at respective frequencies. The increase in wire width and wire density further increases the signal amplitudes. Compared with the case of conventional meander-line-coil (MLC) EMAT, the amplitudes of the A0 and S0 mode Lamb waves of our PPM EMAT are increased to 880% and 328%, respectively. [ABSTRACT FROM AUTHOR]

• A Lamb waves solution to monitor µm-sized corrosion pit on steel is proposed. • Pit growth is controlled by imposing potential and injecting locally NaCl. • Pit growth is monitored in-situ by a sparse array of PZTs bonded to the plate. • Resulting damage indexes correlate with electrochemically estimated pit radius. • Pit size from 20 µm can be detected, located, and its upcoming size extrapolated. Corrosion is a major threat in the aeronautic industry, both in terms of safety and cost. Efficient, versatile, and cost affordable solutions for corrosion monitoring are thus needed. Ultrasonic Lamb Waves (LW) appear to be very efficient for corrosion monitoring and can be made cost effective and versatile if emitted and received by a sparse array of piezoelectric elements (PZT). A LW solution relying on a sparse PZT array and allowing to monitor µm-sized corrosion pit growth on stainless 316L grade steel plate is here evaluated. Experimentally, the corrosion pit size is electrochemically controlled by both the imposed electrical potential and the injection of a corrosive NaCl solution through a capillary located at the desired pit location. In parallel, the corrosion pit growth is monitored in-situ every 10 s by sending and measuring LW using a sparse array of 4 PZTs bonded to the back of the steel plate enduring corrosion. As a ground truth information, the corrosion pit volume is estimated as the dissolved volume balancing the electronic charges exchanged during corrosion. The corrosion pit radius is additionally checked post-experiment precisely with an optical measurement. Measured LW signals are then post-processed in order to compute a collection of synthetic damage indexes (DIs). After dimension reduction steps, obtained DI values correlates extremely well with the corrosion pit radius. Using a linear model relating those DI values to corrosion pit radius, it is demonstrated that corrosion pit from 30 µm to 150 µm can be reliably detected, located, and their upcoming size extrapolated. Two independent experiments were achieved in order to ensure the repeatability of the proposed approach. LW managed by a sparse PZT array thus appears to be reliable and efficient to monitor growth of µm-sized corrosion pits on 316L steel plates. If embedded in aeronautical structure, such an approach could be a versatile and cost-effective alternative to actual non-destructive maintenance procedures that are time and manpower consuming. [ABSTRACT FROM AUTHOR]

Data transmission through solid metallic channels is recommended in certain industries where no other options are proposed, such as nuclear, aerospace, and smart vehicles. In addition to the Faraday shielding effect of electromagnetic waves, another issue related to damage presence due to mechanical loads exists. Severe damage in the transmission channel leads to signal loss at the receiver. For this sake, ultrasonic guided waves, such as Lamb waves, maybe a good substitute since they can propagate through long distances in solid metallic structures. The scope of this work is to build a reliable, reproducible, and high data-rate wireless communication experimental platform, using ultrasonic guided waves, through healthy and damaged plates for industrial usage. The target is to compensate at first for the effect of dispersion, reverberation, scattering, and boundary reflections for the healthy plate. The novelty of this work falls within the performance analysis of the demodulation algorithm based on cross-correlation combined with binary phase-shift keying (BPSK), using a finite-element simulation through healthy and damaged plates with different depths of symmetrical and asymmetrical notches (SN and AN) and steps based on the bit error percentage (BEP). Furthermore, another contribution related to the impact of multiple reflections and mode conversions caused by symmetrical and asymmetrical steps and notches is taken into account. After this, numerical results are validated using an ultrasonic guided wave experimental platform. Results based on BEP analysis prove that the algorithm has successfully compensated for the effect of dispersion and boundary reflections for the healthy plate and multiple reflections and mode conversions for the damaged ones. A highly effective data rate of up to 350 kb/s can be reached even in the presence of severe damage in the transmission channel. [ABSTRACT FROM AUTHOR]

The ultrasonic Lamb wave detection principle can realize the noncontact measurement of liquid level in closed containers. When designing an ultrasonic Lamb wave sensor, it is vital to thoroughly study and select the optimal wedge size at the front of the sensor. In this paper, firstly, we select the best working mode of Lamb waves according to their propagation dispersion curve in aluminum alloy, and we obtain the best angle of wedge through experiments. Secondly, we study the impact of the size of the wedge block on the results, and we obtain the selection method of wedge block parameters. The evaluations show that, when the frequency–thickness product is 3 MHz·mm, the Lamb waves work in the A1 mode, and the experimental effect is the best. At this time, the incident angle of the ultrasonic wave is 27.39°. The wedge thickness should be designed to avoid the near-field area of the ultrasonic field, and we should choose the length as odd multiples of 1/4 wavelength. The rules obtained from the experiment can effectively select the best working mode for ultrasonic Lamb waves, while also providing a basis for the design of the wedge block size in a Lamb wave sensor. [ABSTRACT FROM AUTHOR]

The Lamb waves are named one of the promising solutions for future wind turbine blade (WTB) failure prevention. The compliance with safety assurance of WTBs by detecting structural changes during the manufacture and performing their monitoring during the service life are effective tools for environmental sustainability. This work presents the basic characteristics of Lamb waves and highlights two main unusual limitations—the dispersion and an infinite number of modes—as a great challenge that complicates the application of such waves. This requires the investigation and development of new signal processing methods (SPMs) for conducting accuracy assessments according to the requirements of ISO 17025. The general principles for the accuracy assessment of the signal processing methods applied to evaluate the dispersion of Lamb waves are presented here, and a suitable procedure for estimation of errors and uncertainties is proposed. These should facilitate the verification analysis of any signal processing method used for the dispersion evaluation of Lamb waves. This information allows determining parameters that define the measurement reliability and facilitates the application and utilisation of the proposed methods and their choice. Moreover, it is a necessary prerequisite for setting reliable testing, inspecting, and monitoring standards for WTBs certification. [ABSTRACT FROM AUTHOR]

In this research, a thorough experimental and numerical study is conducted to investigate a novel application of the ultrasonic Lamb waves which is its capability to detect the coating disbond in a double-layer waveguide (aluminium-adhesive) and a triple-layer waveguide (aluminium-adhesive-coating). The experimental tests are set up and implemented to evaluate how the attenuation of Lamb waves is affected by the adhesive bonding beside the contribution of the coating layer to the behaviour of Lamb waves. In the presence of additive white Gaussian noise (AWGN), a range of important indices including energy index (EI), amplitude index (AI), pulse width index (PWI), and the time of flight index (ToFI) are worked out in association with the disbond length. It is inferred that a) even in the very noisy environment of 0 dB SNR, the Lamb waves are effectively able to identify debonding defects particularly using the EI, giving sensitivity rates 105% and 30% for triple-layer and double-layer, respectively; b) the contribution of adhesive to the absorption of the guided wave energy is 30%, 40%, and 50% in EI, AI, and PWI, respectively, corroborating the value of the features to distinguish the debonding between each layer of bonded structures. [ABSTRACT FROM AUTHOR]

The Lamb waves are named one of the promising solutions for future wind turbine blade (WTB) failure prevention. The compliance with safety assurance of WTBs by detecting structural changes during the manufacture and performing their monitoring during the service life are effective tools for environmental sustainability. This work presents the basic characteristics of Lamb waves and highlights two main unusual limitations—the dispersion and an infinite number of modes—as a great challenge that complicates the application of such waves. This requires the investigation and development of new signal processing methods (SPMs) for conducting accuracy assessments according to the requirements of ISO 17025. The general principles for the accuracy assessment of the signal processing methods applied to evaluate the dispersion of Lamb waves are presented here, and a suitable procedure for estimation of errors and uncertainties is proposed. These should facilitate the verification analysis of any signal processing method used for the dispersion evaluation of Lamb waves. This information allows determining parameters that define the measurement reliability and facilitates the application and utilisation of the proposed methods and their choice. Moreover, it is a necessary prerequisite for setting reliable testing, inspecting, and monitoring standards for WTBs certification.

The aviation industry faces the challenge of offering aircraft that are lighter, more economical, and safer. One of the solutions is to increase the use of composites. For these materials, adhesive bonding has proven to be the appropriate joining technology. To check these adhesive joints, costly and time-consuming maintenance measures are carried out. An intelligent Structural Health Monitoring (SHM) system can extend these intervals and allow the use of a predictive maintenance system. This paper describes the method of Ultrasonic Lamb Waves for monitoring a adhesively bonded Carbon Fiber Reinforced Polymer (CFRP) aircraft fuselage. Prior to this, the production of a segment of a fuselage and the characterization of the materials (CFRP and adhesive) is shown. Afterwards the method of Ultrasonic Lamb Waves with the use of piezoelectric transducers and signal processing based on the Reconstruction Algorithm for Probabilistic Inspection of Damage (RAPID) algorithm are explained. At the end, the experimental evaluation of an undamaged and a damaged fuselage structure is done. The results have shown the possibility of RAPID algorithm for damage detection on adhesive. An outlook on future work is given. [ABSTRACT FROM AUTHOR]

This study presents a fast imaging approach with ultrasonic Lamb waves based on time-domain topological energy to identify multiple defects with defect spacing smaller than the threshold of the Rayleigh criterion in terms of imaging resolution. The direct acoustical field and time-domain topological energy of the two fields are used as imaging functions when considering the calculation of direct and adjoint acoustical fields in a non-defective reference medium on the basis of topological theory. The functions are not limited by acoustic diffraction and can achieve super-resolution imaging with multiple defects. First, a 3D finite element model is established. Transient acoustic field diagrams at different moments are used to show the focusing process of direct and adjoint acoustical fields clearly with multiple defects, thereby revealing the physical mechanism of time-domain topological energy imaging. Second, the effectiveness of the proposed approach to characterise multiple defects when the defect spacing is smaller than the imaging resolution threshold is verified through numerical simulation. Finally, the feasibility of super-resolution imaging considering multiple defects is proven by conducting experiments on aluminium plate samples with multiple defects under different defect spacing conditions. Numerical simulation and experimental results show that the proposed approach can overcome the problem of multiple defects with defect spacing smaller than the imaging resolution threshold by breaking the Rayleigh criterion constraint, while the accuracy presented is higher than that of the traditional delay-and-sum method. [ABSTRACT FROM AUTHOR]

Large plate shell structures are key supporting components for aerospace and transportation equipment. Rapid localization of damages in large-scale thin-walled structures is an urgently needed technology for on-site detection and maintenance. This paper proposed a rapid damage localization method based on a laser ultrasonic circular arrays, combining the advantages of the circular arrays and the MUSIC algorithm. It does not need reference signals in advance, the direct waves and scattered waves can be quickly distinguished and extracted according to the time of arrival (TOA). Then, the two-dimensional multiple signal classification method(2D-MUSIC) under near-field conditions is used to quickly locate damage, which is a feature subspace algorithm in array signal processing technology. Compared with other commonly used imaging algorithms, this method has great advantages in detection efficiency and angular resolution. According to the detection method proposed in this paper, the position of single damage and multiple incoherent damages within 0.12 m2 can be quickly found within 1.3 s, and the angle error and distance error are not more than 5° and 5 cm. However, for the damage of coherent sources, there may be artifacts interfering with the imaging results. [Display omitted] • This paper proposed a rapid damage localization method, combining the circular arrays and the MUSIC algorithm. • The direct waves and scattered waves can be quickly distinguished and extracted according to the time of arrival. • The damages within 0.12 m2 can be quickly found within 1.3 s, the angle and distance error are not more than 5° and 5 cm. [ABSTRACT FROM AUTHOR]

The potentials of high frequency Lamb wave modes are investigated in the inspection of plate-like structures. The wave propagation characteristics of higher order wave modes and the corresponding sensitivity and detectability are studied. Finite element simulations are carried out using infinite elements to model the ultrasonic wedge transducer and the inspection system. Experimental pulse-echo measurements are conducted to verify the influence of different modes characteristics predicted from the finite element simulations. The experimental measurements show a good agreement with the obtained numerical results for the fundamental modes, S and A, and the higher order modes, S and A, at 4 MHz mm of frequency-thickness. [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]

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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]

This research investigates a Lamb-wave based structural health monitoring approach matching an out-of-phase actuation of a pair of piezoceramic transducers at low frequency. The target is a typical quasi-isotropic carbon fibre reinforced polymer aeronautical laminate subjected to artificial, via Teflon patches, and natural, via suitable low velocity drop weight impact tests, delaminations. The performance and main influencing factors of such an approach are studied through a Design of Experiment statistical method, considering both Pulse Echo and Pitch Catch configurations of PZT sensors. Results show that some factors and their interactions can effectively influence the detection of a delamination-like damage. [ABSTRACT FROM AUTHOR]

Results of an investigation into the general characteristics of ultrasonic Lamb waves that are excited by a photothermal emitter in metallic plates as a function of its geometric and energy parameters are given. A metal surface area heated during irradiation by nanosecond optical pulses is such an ultrasonic emitter. Excited Lamb waves were recorded using a special wide-band EMA receiver. These findings can be used in the development of devices for the noncontact ultrasonic quality control of thin metalware. [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]

• 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]

Abstract: It has been shown recently that shear horizontal acoustic waves propagating in piezoelectric plates whose thickness h is much less than the acoustic wavelength λ possess a number of attractive properties for use in sensor and signal processing applications. In order to exploit the potential benefits of these waves, however, one needs to fabricate devices on very thin plates. We have developed a suitable fabrication method which can be used to realize devices on such thin plates. In this method, the device is first fabricated on a plate of normal thickness (approximately 500μm) and the substrate is then lapped from the back side to reduce the thickness. The technique has been utilized to realize devices on plates of thickness less than 70μm. A shear horizontal plate acoustic wave (SH-PAW) delay line of fundamental resonant frequency greater than 25MHz and insertion loss less than 7dB has been realized on a 60μm thick Y – cut, X – propagation lithium niobate substrate. The device also shows strong response near the third harmonic frequency of 75MHz. [Copyright &y& Elsevier]

The analysis of the propagation of Lamb waves in multi- and single-layer systems is presented in this work. The study of this propagation problem is developed from matrix formulations, which describe elastic waves in layered media. The behavior of the different modes that propagate in the overlapped region is characterized. It is found that their relative amplitudes can be estimated based on the properties of the incident wave mode. It was verified that the excitation of these modes is ruled by the degree in which the shapes of both modes match each other. Other result of this method refers to the evaluation of the attenuation of both longitudinal and transversal waves in plates, which is usually difficult, especially in thin plates by using the conventional pulse-echo technique. By using the immersion pitch and catch setup, the total attenuation, the sum of the losses due to the leaking in the fluid and to the material damping can easily be obtained through two measurements at different distances. The leaking losses are calculated by the bulk properties of both the fluid and the plate, and, consequently, the attenuation of the longitudinal and the transversal waves (material damping). [ABSTRACT FROM AUTHOR]

This study is concerned with the detection and characterization of impact damage in a stiffened woven composite structure using high frequency Lamb waves and low frequency modal vibrations. The geometric and material complexities of the structure present practical difficulties in the direct analysis of both wave propagation and modal vibration data using theoretical constructs. Improved ultrasonic and vibration test setups consisting of distributed, high fidelity, and surface mounted sensor arrays are used here to determine changes in the dynamical properties of the composite structural components in the presence of damage. The sensors are assumed to provide both the low frequency global response (i.e., modal frequencies and mode shapes) of the structure to external loads and the (local) high frequency signals due to wave propagation effects in either passive or active mode of the ultrasonic array. A damage index, comparing the measured dynamical response of two successive states of the structure is introduced as a determinant of structural damage. The method relies on the fact that the dynamical properties of a structure change with the initiation or growth of damage. A diagnostic imaging tool is used for the interpretation and graphical representation of the indices to enable automated monitoring of the changes in the indices at a given instant of time. The value of the index at a given sensor increases with the proximity of the damage to the sensor. A sensitivity analysis is carried out in an effort to determine a threshold value of the index below which no reliable information about the state of health of the structure can be estimated. It is shown that the automated procedure is able to identify a defect right from its appearance, with some degree of confidence. The feasibility of developing a practical intelligent structural health monitoring system (ISHMS), based on the concept of `a structure requesting service when needed,' is discussed. [ABSTRACT FROM AUTHOR]

This paper describes a novel, noninvasive method for measurement of liquid level in closed metal tanks that are under high pressure. It is based on the use of ultrasonic Lamb waves propagating along the tank wall. Contact with liquid substantially changes the characteristics of these waves and this can be used as an indicator of liquid presence. Theoretical analysis shows that the symmetric and antisymmetric Lamb wave modes, both fundamental and higher order, are sensitive to presence of the liquid. The optimal wave frequency depends on the thickness of the tank wall and wall material. A prototype level sensor based on this principle has been developed. It uses two pairs of wedge transducers to generate and detect Lamb waves propagating along the circumference of the gas tank. An operating frequency of 100 kHz is found to be optimal for use with tanks having a wall thickness of 30–50 mm. Prototype sensors developed under this program have been used successfully in oil fields in the far northern region of Russia. [Copyright &y& Elsevier]