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• Multiphysics model visualises wave behaviours and interprets inspection signals. • Collected signals of leaky Lamb waves are sensitive to size and location of droplets. • Lamb wave excitation is impaired by droplets on the test plate surface. • Deformed wavefronts of scattered leaky waves lead to attenuation of received signals. The signals in an air-coupled ultrasonic inspection system using Lamb waves would be affected by liquid droplets on the surface of test plates. To investigate the wave behaviour, a plate-droplet inspection system comprising a thin isotropic plate loaded with water droplets and a pitch–catch transducer configuration was modelled and studied experimentally. The Lamb waves were generated by a non-contact airborne transmitter. Their energy leaking into the adjacent air was collected by a receiver. To obtain a deep understanding of the effects of water droplets, a multiphysics model based on finite element method was established. The model facilitated the visualization of the wave fields scattered by the droplets on the surface of the plate and evaluated the received signals which were then verified by experimental measurements. This entirely modelling method could be a convenient way to interpret the inspection signals for test plate loaded with liquid droplets. [ABSTRACT FROM AUTHOR]

Carbon fibre reinforced polymer (CFRP) has proved to be effective in strengthening ageing steel structures, due to its outstanding properties. However, debonding problems which can occur at the interface severely impair the performance of retrofitted structures. In this paper, a guided wave based structural health monitoring (SHM) method is employed to detect debonding in CFRP-reinforced steel structures. The proposed samples are CFRP-reinforced steel plates with different sizes of debonding. Due to its sensitivity to interfacial damage, the anti-symmetrical Lamb wave mode is chosen as the incident wave and surface-mounted piezoelectric (PZT) wafers are employed to excite and collect the signals. Theoretical derivation is introduced to find the relationship between the time-of-flight (ToF) of the first arrival wave package in the received signals and the extent of debonding damage. A linear relationship is obtained, in that the ToF decreases linearly with the increase in the size of debonding. When corresponding numerical simulation and experimental study are performed to verify the relationship, similar trends are observed. [ABSTRACT FROM AUTHOR]

Guided waves are sensitive to variations in propagation environments. Many recent studies have focused on the uniform thermal effect on Lamb waves. However, there is little research on the thermal effect in a more complex situation, such as a nonuniform thermal effect and wave propagation in an arbitrary cross-section. In this study, a thermo-acoustoelastic theory combined with the semi-analytical finite element (TAE-SAFE) method is proposed to investigate both uniform and nonuniform thermal effects on acoustoelastic guided wave propagation. In the TAE-SAFE method, effective thermo-acoustoelastic constants including third-order elastic constants are employed. Then, an acoustoelastic wave equation of the thermal effect is formulated by Hamilton's principle and computed by the semi-analytical finite element (SAFE) method. The phase velocity, group velocity, velocity thermal sensitivity, and displacement mode shape shift can be extracted by the proposed method. To validate this method, numerical results of Lamb waves in an aluminum plate subjected to a uniform thermal effect are compared with the results of a previous theoretical analysis. The results show computational veracity and validity. Two typical cases are investigated: (1) an isotropic aluminum plate under a linear temperature gradient condition; (2) a uniform temperature case in a rail track with a constant irregular cross-section. This study provides an effective numerical method to analyze thermo-acoustoelastic guided wave propagation. [ABSTRACT FROM AUTHOR]

• Interdigital transducers on piezoelectric substrate are used for excitation of SAW. • Laser Doppler vibrometry yields mechanical displacement amplitude of SAW. • Numerical simulations acceptably reproduce experimental results of SAW amplitude. • Our numerical model does not require experimental displacement data. • Simulation yields best results for low number of interdigital transducer electrodes. The present paper studies experimentally and numerically the surface acoustic wave (SAW) field on a piezoelectric substrate generated by interdigital transducers (IDT). On the one hand, mechanical displacements produced by the SAW are measured with the help of a laser Doppler vibrometer. On the other hand, mechanical displacements are computed by the two-dimensional finite element method in frequency domain followed by the spatial Fourier transform. Combining these two steps of computations results in the intended two-dimensional distribution of mechanical displacements on the substrate surface. The comparison of experimental and numerical data obtained for a series of different IDTs reveals that it is possible to estimate the shape of the SAW beam and the absolute value of mechanical displacement amplitude using only the basic parameters of the IDT and its electrical admittance measured by a network analyzer. [ABSTRACT FROM AUTHOR]

• A model is presented for analyzing the FMR effect of Lamb waves in a layered plate. • An insight into the FMR process of Lamb waves in a layered plate is clearly revealed. • The local interfacial degradation can be located and assessed by the FMR effect. This work investigates modeling of the frequency mixing response (FMR) induced by two counter-propagating Lamb waves with different frequencies in a two-layered plate, and then numerically simulates and analyzes the influences of interfacial properties on the effect of FMR. Based on a perturbation approach and a normal-mode-expansion technique for waveguide excitation, the second-order bulk driving forces and surface/interface stresses at the mixing frequency, originated from the interaction of two counter-propagating Lamb waves within the wave mixing zone, can be regarded as the excitation sources for generation of a series of combined harmonics. It is found that, under the internal resonance condition including the phase matching and nonzero energy flux, the magnitude of the combined harmonic generated increases with increase in the length of mixing zone of the two counter-propagating Lamb waves, and tends to be stable outside the wave mixing zone. Due to the relatively short mixing zone of the two counter-propagating Lamb waves, the effect of FMR has attracted considerable attention because it can enhance the accuracy of location of the local interfacial degradation in the given layered plate. Both the numerical analyses and finite element (FE) simulations performed show that the local interfacial degradation in the two-layered plate may be assessed and located by spatial scanning of wave mixing zone of the two counter-propagating Lamb waves. Through modeling and FE simulations, this paper provides an insight into the physical process of FMR of the two counter-propagating Lamb waves in a two-layered plate, and meanwhile shows a potential for assessment and location of the local interfacial degradation by using the effect of FMR of the two counter-propagating Lamb waves. [ABSTRACT FROM AUTHOR]

This study presents a combined experimental and numerical study on the determination of the stiffness matrix of transversely isotropic thin laminated composite structures non-destructively with the use of group velocity measurements. The approach relies on the usage of A0 and S0 modes of Lamb waves, triggered the by pencil-lead break, which propagate in the composite material, and enables the evaluation of various elements of the stiffness matrix. It is proven that the experimental Lamb wave method can be reliably utilized for determining the components of the stiffness matrix, C ij . The results are validated with respect to those of mechanical characterization and finite element analysis. [ABSTRACT FROM AUTHOR]

• Lamb mode amplitude response provides method of source depth characterization. • Mid-plane, quarter-plane, and surface sources are experimentally distinguished. • Approach has applications from geometrically simple to complex plate-like structures. • Algorithm provides fast computation for real-time applications. An inversion algorithm (termed AEDep) is proposed for estimating the depth of acoustic emission (AE) sources in plate-like structural components. The work is motivated by the need for characterizing early-stage fatigue crack growth in such components. The algorithm achieves depth estimation by automatically extracting the depth-dependent amplitude ratio between the fundamental Lamb modes which comprise the AE signals. A finite element model is designed to study the frequency-dependent forward problem of Lamb wave motion due to a given source, from which the relation between source depth and amplitude ratio is established. Elastodynamic theory is used to validate the model in the frequency domain, as well as to derive a sensor tuning factor which may be incorporated into the solution. The proposed algorithm was tested on two plate-like specimens: a 6061-T6 aluminum plate and a 2025-T6 aluminum aircraft fuselage panel. Validation of the algorithm was achieved by generating controlled AE sources at various depths along the edges of the specimens, in the form of Hsu-Nielsen pencil lead breaks. Good agreement was found in the aluminum plate between the true and estimated source depths. A slight decrease in accuracy was found in the fuselage panel between the true values and their estimations. However, both experimental cases demonstrated the ability to distinguish between sources originating near the mid-plane of a plate-like structure from those near the surface. Lastly, the fast computation of the inversion algorithm shows strong potential for real-time monitoring applications. [ABSTRACT FROM AUTHOR]

Nonlinear ultrasonic modulation, a promising fatigue-crack-detection technique, is investigated with various frequency combinations, and an algorithm that can comprehensively analyze the results by comparison with ambient noise is proposed. The magnitudes at the 1st sideband frequencies of the Fourier-transformed result and surrounding noises are measured. Experiments with aluminum specimens before and after fatigue-crack generation show that cracks are detected correctly in both pristine and damaged specimens. When the developed crack-detection method is employed while applying an external force, specimen defects are correctly detected, although the magnitudes of modulated waves decreased. The time and effort required for monitoring the structure's health can be reduced by using ambient noise as a reference. [ABSTRACT FROM AUTHOR]

Composite materials have been widely used in various applications, and guided waves are often used as a non-destructive testing tool to inspect defects or damage in composite laminates which are assemblies of multiple composite layers. These can have anisotropic material properties and arbitrary fibre orientation such that the guided wave properties of Lamb and shear-horizontal modes are direction-dependent. The group velocity of each wave mode is therefore to be considered with a component parallel to the wave propagation direction and a component perpendicular to the wave propagation direction. In this article, a semi-analytical finite element method is developed to model composite laminates with arbitrary fibre orientation and anisotropic material properties in each layer. Galerkin's principle is used to derive the weak forms of the governing equations, and an energy velocity formulation is used to calculate the parallel and perpendicular energy velocities. The finite element solutions are compared with available analytical and numerical solutions in the literature for forward waves, and excellent agreement is demonstrated. On this basis, the guided wave properties of backward waves have also been investigated. It is well understood that in an isotropic plate, the energy velocity of a backward wave is directed opposite to the phase velocity. However, in a composite laminate, the energy velocity of a backward wave is normally not exactly opposed to (180° out of phase with) the phase velocity but exhibits a skew angle. The angular dependences of wave properties of the forward and backward waves are investigated in this article. [ABSTRACT FROM AUTHOR]

Abstract: In order to investigate non-destructively the bonding between rough plates, the problem of Lamb waves propagating on a rough plate is addressed in this paper. Numerical analysis is performed on periodical gratings made of identical triangular grooves. If the surface profile is made up of grooves with one periodicity, then a mode conversion is observed. In the wave-number/frequency space, a phonon relation is written between phonons related to the grating and to the incident and reflected-converted modes. If the grooved surface is made up of several spatial periodicities, then the phonon relation is still verified. Signal processing allows us to give an interpretation of the results in the dual space (wave-number/frequency). An experimental study is also performed to corroborate the numerical predictions. [Copyright &y& Elsevier]