Search

• Zero-group velocity (ZGV) combined harmonics is firstly generated experimentally by Lamb wave mixing. • A hybrid experimental system is proposed to detect the ZGV combined harmonic. • Controllability of the generated combined harmonics induced by the mixing of Lamb waves is proved. In this paper, we present an experimental observation of the phenomenon known as zero-group velocity (ZGV) combined harmonic generation, which is induced by the mixing of counter-directional Lamb waves. We utilize internal resonant conditions to selectively choose the primary mode pair at specific frequencies for the purpose of combined harmonic generation. To detect the ZGV combined harmonic component, we propose a hybrid system that incorporates dual wedge-transducers for generation and a laser interferometric system for receiving. The appearance of the predicted S 1 -ZGV combined harmonic at a specific mixing frequency is clearly observed in our experiments. Furthermore, we experimentally verify the controllability of the generated combined harmonics induced by the mixing of Lamb waves. [ABSTRACT FROM AUTHOR]

In this work, we analyze theoretically and observe experimentally the frequency mixing response induced by the collinear interaction of two primary Lamb waves with different frequencies in an isotropic and homogenous plate. The internal resonance conditions for generating the combined harmonics are analyzed by using a perturbation approach and a normal-mode-expansion technique for waveguide excitation. The second- and third-order combined harmonics generated by the collinear wave mixing of two primary Lamb waves propagating in a specimen are analyzed, and the existence of the combined harmonics at specific frequencies is predicted. An experimental procedure is proposed to measure the combined harmonics induced by the collinear cross-interaction of the two specific primary Lamb waves at given frequencies. The theoretical prediction of the appearance of the combined harmonics at the specific mixing frequencies is consistent with experiment. The theoretical analysis and experimental observation provide a clear physical insight into the frequency-mixing response induced by the collinear cross-interaction of two primary ultrasonic Lamb waves. [ABSTRACT FROM AUTHOR]

The generation of second- and third-order combined harmonics induced by the interaction of two primary Lamb waves with different frequencies has been analyzed theoretically and observed experimentally in our previous research [W. Li et al., J. Appl. Phys. 124, 044901 (2018)]. However, only third-order combined harmonics were observed experimentally because in the experimental investigations, the conditions for both synchronism and the symmetric feature were not satisfied simultaneously for the generation of the second-order combined harmonics. In this paper, first, the numerical perspective of the second-order combined harmonic generation by the collinear interaction of two primary Lamb waves with different frequencies in an isotropic and homogeneous plate is investigated. The results indicate that the synchronism and symmetric feature of two primary Lamb waves selected significantly affect the generation of second-order combined harmonics at the mixing frequencies. Next, an experimental scheme is proposed to measure the second-order combined harmonics predicted theoretically and numerically, through which it is found that the experimental results agree well with that of the theoretical and numerical predictions. The numerical and experimental results in this paper provide a clear physical insight, not previously available, into the second-order combined harmonic generation induced by the collinear interaction of two primary ultrasonic Lamb waves. [ABSTRACT FROM AUTHOR]

• Numerical perspectives of the generation, propagation, and mode conversion of QSPs of GWs in composites are firstly obtained. • Unique features of QSPs generated by GWs propagation along principle and non-principle material directions in composites are provided. • Both cumulative and divergence effects of generated QSPs are analyzed. • Factors affecting the generation efficiency of QSPs are analyzed and given. In this paper, the generation of quasistatic pulses (QSPs) of guided waves propagation in fiber reinforced plastic (FRP) thin plates is investigated. Numerical perspective of the generation, propagation, and mode conversion of QSPs of GWs in the composites are obtained. It is indicated that the displacement direction and generation efficiency of QSPs depend on the wave propagation direction, the coefficients of the strain energy function, and the primary GW field. Group velocity, wave structure, and wavenumber domain analyses show that the generated QSPs are either S0 mode, or partly coupled S0 and SH0 modes with only in-plane displacements with different direction, regardless of the excitation of primary waves. For GWs propagation in principal material directions, comparative studies show that the generation efficiency of QSPs are depended on both the mode types and primary GW propagation direction. With the considerations of both cumulative effect and the divergence effect, comparative study of generation efficiency of QSP and second harmonics shows that, although the divergence loss is not negligible, appropriate excitation of primary GW can achieve a higher efficiency of cumulative QSP generation than that of the phase matched second harmonics. [ABSTRACT FROM AUTHOR]

• Efficiency of second harmonic generation for phase matched Lamb wave modes is theoretically and experimentally investigated. • Nonlinear parameters of Lamb waves are derived for the first time as a function of signal frequency, wave mode and geometric parameters of waveguides. • Nonlinear features of various phase matched Lamb wave modes are studied for the comparison of efficiency of second harmonic generation. • The experimental results are found to be consistent with the theoretical predictions for the efficiency of second harmonic generation. The measurement of second harmonic Lamb waves is investigated as a promising feature extraction technique to characterize material nonlinearity in plate-like structures. In general, the effect of second harmonic generation can be weak, due to the dispersive nature of Lamb waves. The phase matched Lamb modes are explored to generate the second harmonics with a cumulative growth effect. Among all phase matched Lamb modes, it is of particular interest to find the most suitable Lamb modes for improving the efficiency of second harmonic generation and reception. Analytical expressions and experimental observations of second harmonic generation of Lamb waves in an isotropic plate with weak nonlinearity, are presented in this paper. Nonlinear parameters of Lamb waves are derived in this paper for the first time as a function of signal frequency, wave mode and geometric parameters of waveguides. Nonlinear features of various phase matched Lamb modes are discussed for the comparison of efficiency of second harmonic generation. The experimental results are found to be consistent with the theoretical predictions in a qualitative manner. This study shows that physics based feature selection is essential for efficient generation and reception of second harmonic Lamb waves. [ABSTRACT FROM AUTHOR]

It is known that measurement and extraction of the tiny amplitude of second harmonic Lamb waves are the main difficulties for practical applications of the nonlinear Lamb wave technique. In this study, phase-reversal approaches and phase matching technique are combined to build up the second-harmonic generation (SHG) of Lamb waves. A specific Lamb wave mode pair, which satisfied phase matching conditions, is selected to ensure the generation of cumulative second harmonic waves. Lamb wave signals with the same frequency but in reverse phase, propagating in the given specimen, are added together to counteract the fundamental waves, and simultaneously to enhance the signals of the second harmonic generated. The obtained results show that the phase-reversal approach can enhance the signals of second harmonic Lamb waves, and effectively counteract that of the fundamental waves. The approach is applied to assess the thermal-induced material degradation in the stainless steel plates. Distinctions of the acoustic nonlinearity parameters under different degraded levels are clearly shown in an improved repeatable and reliable manner, while those of linear wave velocity in the specimens are neglectable. The experimental investigations performed indicate that the proposed approach can be taken as a promising alternative for assessment of material degradation in its early stages. [ABSTRACT FROM AUTHOR]

• Resonance verification ensures energy concentration within the feature region of the welded joint, preventing leakage. • The 3D time-step FE method allows for unambiguous identification of ZGV propagation modes. • The presence of ZGV-FGWs in the welded joint is confirmed. Extensive research has been conducted on zero-group-velocity (ZGV) Lamb waves in elastic plates, demonstrating significant progress in the field of nondestructive testing. However, there is a scarcity of studies focusing on ZGV modes in complex structures. In this paper, we present our research investigating the presence of ZGV feature guided waves (FGWs) in a welded joint. Our approach follows a similar methodology used to study ZGV Lamb waves in elastic plates. By employing two-dimensional (2D) finite element (FE) modeling, we analyze the response spectra of the welded joint when subjected to a force source, revealing the occurrence of resonance in the response spectra. To investigate resonance modes in the welded joint, we employ the three-dimensional (3D) time-step FE method. By applying spatial 2D and short-time Fourier transforms to the received time-domain signals, we analyze the frequency content and spatial distribution of the signals. This analysis allows us to verify the existence of non-propagation and propagation modes in the welded joint. The non-propagation mode refers to the presence of signals with a zero wavenumber, indicating that they do not propagate or travel along the welded joint. These signals are typically associated with local resonances or vibrations within the welded joint itself. On the other hand, the propagation mode corresponds to signals with nonzero wavenumbers, suggesting that they propagate or travel along the welded joint. Furthermore, by further analyzing the propagation mode in the welded joint, similar to the analysis of ZGV modes in solid plates, we have observed that it also exhibits ZGV characteristics based on the wavenumber-frequency spectra. To further analyze acoustic field distributions at resonance frequencies, we utilize the semi-analytical finite element method in conjunction with the perfectly matched layer method. The results obtained from this analysis are consistent with those obtained from the 2D FE method and 3D time-step FE method, thereby confirming that propagation modes with ZGV characteristics at resonance frequencies correspond to FGWs, which we refer to as ZGV-FGWs. Through this step-by-step analysis, we ultimately establish the existence of ZGV-FGWs in the welded joint. This study introduces fresh ideas and serves as a point of reference for future research on ZGV-FGWs in complex structures. [ABSTRACT FROM AUTHOR]