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• A method for liquid level measurement based on leaky lamb waves was presented to improve the sensitivity and accuracy of liquid level detection. • Liquid level detection with lamb waves was simulated using the finite element method. • The field experiment of liquid level detection was performed, and the comparison tests of the detection of lamb waves and longitudinal waves were conducted. The accurate detection of liquid-level plays an important role in industrial process. This paper presents a method for liquid-level measurement based on Lamb waves. The dispersion equation of Lamb waves under unilateral liquid load is deduced to obtain the dispersion curves. The structure and attenuation curves are analyzed under a certain frequency and thickness, and the modes with more energy loss are selected. The ratio of the signal energy of the liquid-level at the upper and lower positions is defined as the discrimination to indicate the change of the liquid-level. The models of the liquid storage tank are built and the Lamb waves of different modes are simulated using the finite element method. The experimental results reveal that it is easy to detect the difference in ultrasonic signals when the liquid-level changes. Compared with the longitudinal wave detection method, the proposed method improves the discrimination of liquid-level detection. The experimental results are consistent with the theoretical calculations. [ABSTRACT FROM AUTHOR]

• The displacement field of the 2D theory is designed according to the wave structure. • A higher accuracy in high frequency simulation can be achieved compared with FSDT. • The modeling error of A0 mode can be controlled within 1.43 % up to 2500 kHz·2 mm. • Combined with FEM, the method can simulate the 2D wavefield of A0 and A1 accurately. • More computational resources can be saved compared with the 3D FEM. For the modeling of Lamb wave propagation, the accuracy of some two-dimensional (2D) plate theories like the first order shear deformation theory (FSDT) may decline at a high frequency, since their displacement fields cannot match those complicated Lamb wave structures. To address this issue, this paper proposes an improved 2D thin plate theory which has a more accurate high frequency modeling ability of Lamb wave than the FSDT. This is done by designing the displacement field according to the high frequency Lamb wave structure. Since the shear stress curves of the Lamb wave are complicated and would vary with the frequency, the initial displacement field takes a combination of polynomial and trigonometric forms, which has two coefficients that need to be determined by fitting the shear stress curve at a certain frequency. Then, a strategy is applied to determine the eventual displacement field by searching the high frequency range for the optimum coefficients that enable a best modeling accuracy. The relative error of its modeling result of A0 mode can be controlled within 0.03 % below 800 kHz·2 mm, and keeps the highest error of 1.43 % up to 2500 kHz·2 mm. To verify its high frequency modeling performance, a finite element method program is finally built. Results show that both the wavefields of A0 and A1 can be accurately simulated with the improved two-dimensional thin plate theory. [ABSTRACT FROM AUTHOR]

• A semi-analytic method to explain the interaction between lamb waves and bolt joint. • The power transmission coefficients change periodically with contact area. • The first S0 wave packet in the received signal is used to monitor bolt torque. • The semi-analytic method is helpful for frequency selection in bolt monitoring. Bolt joints are extensively used in aerospace and civil engineering. Online monitoring of bolt looseness is crucial to ensure the function and safety of these structures. Understanding the interaction between guided waves and the joint interfaces can aid in the design and optimization of such monitoring systems, and hence becomes important. However, the influence of contact area on guided wave energy passing through these jointed plates is still not clear. In this paper, a semi-analytic method, which combines finite element models with a wave superposition method, is proposed to predict the transmission of guided waves in joint interfaces. The wave mode conversions in the contact area are included. The model was validated by comparison with independent finite element models. In addition, the influence of the contact length on transmission coefficients of guided waves passing through the joint interface is investigated. The results were experimentally validated by wire electrical discharge machined specimens with various contact areas. Then, the transmitted guided wave and the contact area in a bolt jointed plate under different torques were measured experimentally and compared with the present method. The results of the two different experiments agree well with those predicted using the semi-analytic method, respectively. In particular, the experimental results of the bolt jointed plate show that the direct S0 wave packet energy decreases with the bolt preload, which can be explained by the proposed model. Thus, the proposed semi-analytic method demonstrates efficiency in the prediction of the power transmission coefficient and has potential applications in the selection of excitation frequencies in the online monitoring of jointed structures. On this basis, a normalized power transmission coefficient can be directly calculated by the energy of the first S0 wave packet in the received signal to monitor bolt torque. [ABSTRACT FROM AUTHOR]

The present research focuses on detection and quantification of disbond present beneath the stiffener in a stiffened aluminium panel, using a Lamb wave based nonlinear technique. For this purpose, both experimental and finite element simulation studies are carried out. The contact nonlinearity at the disbond leads to the generation of higher harmonics in the Lamb wave response. This phenomenon is observed in both the experimental and the simulation studies. A Spectral Damage Index (SDI) defined by the authors is extracted from these higher harmonics. This SDI has a decreasing trend with the increase in the size of the breathing damage. The plots of the SDI against the disbond width observed in the cases of experimental and simulation studies are in the close agreement. Thus, the higher harmonics observed in the Lamb wave response and the SDI obtained from these higher harmonics together found to be effective in detection and quantification of the disbond in the stiffened aluminium panel. [ABSTRACT FROM AUTHOR]

• Sinusoidally-shaped phononic beam structures are proposed. • A multi-parameter designing technique is proposed for metasurfaces. • Mass of functional units does not varies with the change of design parameters. In this work, we propose a method to design subwavelength metasurfaces for flexural waves in thin plates based on sinusoidally-shaped phononic beam structures. In the proposed method, the effective bending stiffness of sinusoidally-shaped phononic beam structures is tuned, and the effective phase velocity of the flexural waves is modulated subsequently by adjusting two key geometry parameters, namely, the amplitude of the sinusoidal profiles and lattice constants of the phononic beam structures. Compared with the previous methods, which modulate a single design parameter, the proposed method is a two-parameter modulating method. Thus, it not only maintains the functionality of metasurfaces, but also is much more flexible when obtaining a high wave transmittance. In addition, the mass of functional units of metasurfaces designed by the proposed method is not relevant to the design parameters, which is particularly important in weight-critical applications. To validate the feasibility of the proposed design method, the transmission spectra of sinusoidally-shaped phononic beam structures are modeled using the finite element method, and results are compared with experiments of 3D-printed samples. In addition, two representative metasurfaces, refractive and focusing metasurfaces, are designed with an available minimum transmission coefficient of 0.93 for functional units. Numerical evaluation of these metasurfaces' performances confirms the effectiveness of the proposed method. This work paves a novel way to design elastic metasurfaces to manipulate flexural waves in thin plates and demonstrates a potential new direction to design metasurfaces for other wave types. [ABSTRACT FROM AUTHOR]

Higher Order Modes Cluster (HOMC) guided waves are non-dispersive wave packets consisting of higher order Lamb wave modes. This paper investigates the effect of frequency and dominant mode(s) of an HOMC guided wave on its sensitivity to plate defects. The propagation of HOMC guided waves in a 10-mm thick mild steel plate is simulated by two-dimensional finite element model. The level of contribution of Lamb wave modes to the wave cluster is analyzed by using a two-dimensional Fast Fourier Transform (2D FFT) at various frequency-thickness products ranging from 15 to 35 MHz-mm. The normal stress profiles of the HOMC guided waves are plotted and used to show the changes in normal stress as a function of frequency-thickness product. The HOMC guided waves interactions with notches of 5%, 10%, 30% and 50% depths relative to the plate thickness are examined at various frequency-thickness products ranging from 15 to 35 MHz-mm. It is observed that the reflected energy from these notches decreases with increase in frequency-thickness product. Experimental measurements are also conducted on a similar plate. The experimental results are in line with simulations and confirm the changes observed in the reflection coefficient of notches at different frequency-thickness products. [ABSTRACT FROM AUTHOR]