• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2005년도 춘계학술대회
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• pp.542-545
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• 2005

This paper is concerned wi th the generation of the Lamb waves in a non­ferromagnetic plate by a recently-developed orientation-adjustable patch-type magnetostrictive transducer (OPMT) and the dispersion analysis from the measured Lamb waves. OPMT is capable of adjusting wave-propagation orientation only with a single installation on a plate. The mechanics behind the wave generation and measurement by the magnetostrictive phenomenon, the working principle of OPMT is explained and the actual generation and measurement of the Lamb waves were conducted in a 3 mm-thick aluminum plate. For the accurate analysis of the dispersion characteristics of the measured Lamb waves, a modified version of the short-time Fourier transform, known as the dispersion-based short-time Fourier transform, was employed. The results presented in this work would serve as the underlying research for an advanced non-destructive evaluation based on ultrasonic waves.

• 한국항만학회지
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• 제10권1호
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• pp.53-61
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• 1996

This paper describes the effect of wave control using submerged flat plate by the numerical calculation and the hydraulic model test. The boundary element method is used to develop a numerical solution for the flow field caused by monochromatic oblique waves incident upon an infinitely long, sumerged flat plate situated in arbitrary water depth. The effect of wave blocking is examined according to the change of length, submerged depth of flat plate and incident angles. Numerical results show that longer length, shallower submergence of flat plate and larger incident angles enhance the effect of wave blocking. To validate numerical analysis method, hydraulic model test was conducted in 2-D wave flume with 60 cm metal sheet. Reflected waves are extracted from water surface elevation in front of the location of a submerged plate by least square method with 3 wave gages. From comparing experimental results with numerical results, efficiency of numerical analysis method by this study could be confirmed well within wide ranges of wave frequencies.

• Smart Structures and Systems
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• 제3권2호
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• pp.201-217
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• 2007

An active damage detection technique is introduced to locate damage in an isotropic plate using Lamb waves. This technique uses a time-domain energy model of Lamb waves in plates that the wave amplitude inversely decays with the propagation distance along a ray direction. Accordingly the damage localization is formulated as a least-squares problem to minimize an error function between the model and the measured data. An active sensing system with integrated actuators/sensors is controlled to excite/receive $A_0$ mode of Lamb waves in the plate. Scattered wave signals from the damage can be obtained by subtracting the baseline signal of the undamaged plate from the recorded signal of the damaged plate. In the experimental study, after collecting the scattered wave signals, a discrete wavelet transform (DWT) is employed to extract the first scattered wave pack from the damage, then an iterative method is derived to solve the least-squares problem for locating the damage. Since this method does not rely on time-of-flight but wave energy measurement, it is more robust, reliable, and noise-tolerant. Both numerical and experimental examples are performed to verify the efficiency and accuracy of the method, and the results demonstrate that the estimated damage position stably converges to the targeted damage.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2007년도 춘계학술대회논문집
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• pp.71-74
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• 2007

The objective of this presentation is to introduce a recent development of a magnetostrictive grating technique using an optimal yoke to efficiently generate and measure SH (Shear-Horizontal) waves in a plate. Gratings are effective means to generate frequency-tuned waves, but the gap between magnetostrictive gratings inevitably obstructs magnetic flow. Because magnetic field is the main physical quantity to actuate and sense ultrasonic waves, the transducer performance is most significantly influenced by the magnetic field distribution in the strips. Thus, wave transduction efficiency can be substantially improved if better magnetic flow is formed in the strips. To improve the efficiency, the topology optimization method was used to determine an optimal yoke configuration. A series of experiments on an aluminum plate were conducted using a grating with and without the designed yoke; when the yoke was used, the signal outputs increased up to 60 %.

• 한국해양공학회지
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• 제10권1호
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• pp.81-91
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• 1996

Numerical study is carried out to make clear the wavemaking phenomena for a planning flat plate. Through the mumerical computations, some schematic wave patterns are compared with experimental results, for example, such as the bow divergent waves, triangle waves, stern divergent saves, stern cross waves. The characteristic distance of waves is found to be more inconsistent as the incidence angle increases. The comparison was mode between computations and experiments for incidence angles of 5, 10 and 15 degrees. However, a numerical difficulty is found around and behined the trailing edges for more than 20 degree.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2014년도 춘계학술대회 논문집
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• pp.722-725
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• 2014

The spatial focusing of time reversal Lamb waves on a plate has attracted considerable attention for identifying the location of an input source. This study investigates the spatial focusing performance on a plate with respect to the number of piezoelectric (PZT) sensors for varying locations of input sources. In particular, a small number of PZT sensors produce spatial focusing through the virtual sensor effect due to reflection of Lamb waves at plate edges. The spatial focusing performance with respect to the number of PZT sensors is quantified in terms of signal to noise ratio through numerical simulation and its implication is discussed.

• 비파괴검사학회지
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• 제36권2호
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• pp.149-154
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• 2016

The use of nonlinear ultrasonics wave has been accepted as a promising tool for monitoring material states related to microstructural changes, as it has improved sensitivity compared to conventional non-destructive testing approaches. In this paper, third harmonic generation of shear horizontal guided waves propagating in an isotropic plate is investigated using the perturbation method and modal analysis approach. An experimental procedure is proposed to detect the third harmonics of shear horizontal guided waves by electromagnetic transducers. The strongly nonlinear response of shear horizontal guided waves is measured. The accumulative growth of relative acoustic nonlinear response with an increase of propagation distance is detected in this investigation. The experimental results agree with the theoretical prediction, and thus providing another indication of the feasibility of using higher harmonic generation of electromagnetic shear horizontal guided waves for material characterization.

• Structural Engineering and Mechanics
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• 제34권3호
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• pp.395-398
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• 2010

• Structural Engineering and Mechanics
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• 제81권4호
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• pp.443-459
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• 2022

The paper studies the dispersion and attenuation of propagating waves in the "plate+compressible viscous fluid layer" system in the case where the fluid layer flow is restricted with a rigid wall, and in the case where the fluid layer has a free face. The motion of the plate is described by the exact equations of elastodynamics and the flow of the fluid by the linearized Navier-Stokes equations for compressible barotropic Newtonian viscous fluids. Analytical expressions are obtained for the amplitudes of the sought values, and the dispersion equation is derived using the corresponding boundary and compatibility conditions. To find the complex roots of the dispersion equation, an algorithm based on equating the modulus of the dispersion determinant to zero is developed. Numerical results on the dispersion and attenuation curves for various pairs of plate and fluid materials under different fluid layer face conditions are presented and discussed. Corresponding conclusions on the influence of the problem parameters on the dispersion and attenuation curves are made and, in particular, it is established that the change of the free face boundary condition with the impermeability condition can influence the dispersion and attenuation curves not only in the quantitative, but also in the qualitative sense.

• Smart Structures and Systems
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• 제21권2호
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• pp.195-205
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• 2018

Research on Lamb wave-based damage identification in plate-like structures depends on precise knowledge of dispersive wave velocity. However, boundary reflections with the same frequency of interest and greater amplitude contaminate direct waves and thus compromise measurement of Lamb wave dispersion in different materials. In this study, non-reflecting boundaries were proposed in both numerical and experimental cases to facilitate time-frequency characterization of Lamb wave dispersion. First, the Lamb wave equations in isotropic and laminated materials were analytically solved. Second, the non-reflecting boundaries were used as a series of frames with gradually increased damping coefficients in finite element models to absorb waves at boundaries while avoiding wave reflections due to abrupt property changes of each frame. Third, damping clay was sealed at plate edges to reduce the boundary reflection in experimental test. Finally, the direct waves were subjected to the slant-stack and short-time Fourier transformations to calculate the dispersion curves of phase and group velocities, respectively. Both the numerical and experimental results suggest that the boundary reflections are effectively alleviated, and the dispersion curves generated by the time-frequency analysis are consistent with the analytical solutions, demonstrating that the combination of non-reflecting boundary and time-frequency analysis is a feasible and reliable scheme for characterizing Lamb wave dispersion in plate-like structures.