Papers by Keyword: Group Velocity

Abstract: This paper describes the nondestructive evaluation of microstructure using laser-excited Lamb waves to detect the phase transformation in NiTi shape memory alloy sheets. Lamb waves were applied in the NiTi sheet using a pulse laser beam. Piezoelectricity transducers were used to receive the Lamb waves, the group velocities of which were measured using a time-frequency analysis method at different temperatures. Results show that a marked variation in the group velocity occurs during the phase transformation in the NiTi alloy. The dependence of group velocity on temperature provides a effective means of inspecting microstructure transformation in NiTi alloys.

Abstract: A two-dimensional (2D) triangular lattice photonic crystal coupled-cavity waveguide is designed and optimized. The transmission spectrum of the PC waveguide with TE polarization is calculated by using the finite-difference time-domain (FDTD) method, and the group velocity of c/131.18 at the wavelength is obtained. Through optimizing the parameters of photonic crystal waveguide, different resonance length are obtained by changing the number of the continous air holes. The smallest group velocity is obtained to be c/2209 in the coupled-cavity waveguide with 15 air holes. The mechanism of slow light in the coupled-cavity waveguide of photonic crystal is analyzed.

Abstract: It is proved by theory and experiment that the arrival time of elastic scattering wave is determined by group velocity of Lamb wave in plate and the speed of elastic scattering wave in water. The frequency dispersion equation of Lamb wave is derived for submerged elastic plate, and the phase velocity and group velocity dispersion curves are obtained by numerical calculation method. It is found that the phase velocity is greater or less than the group velocity at different frequency-thickness products. The energy propagation speed of wave is group velocity, so the arrival time of elastic scattering wave is determined by group velocity of Lamb wave in plate and the speed of sound in water. Experimental results show that elastic scattering wave is ahead of or behind the edge wave in echoes of elastic steel plate. The experiment results confirm validity of the theoretical analysis results.

Abstract: Factors of affecting BER are determined by its architecture in OCDMA system, the loss of optical devices, encoder / decoder structure, optical fiber transmission channel and so on. Taken together, the main factors are light source chirp, noise, group velocity dispersion, fiber channel attenuation and loss of optical devices. The impact on system BER performance is analyzed in this paper.

Abstract: Applications of long range ultrasonic testing are on cylindrical products(pipes and tubes) and planar products(plates and sheets).Guided wave testing using circumferential and longitudinal modes are getting more and more importance as there are huge requirements for testing pipes and tubes in power generators, Chemical plants, steam generators etc.. There will be infinite number of modes with charectestic propagation and wave structure. This propagation characteristic and wave structure plays a major role in deciding its suitability for use in specific application. It is highly essential to use mode control and to understand the generated modes and its characteristics based on which only testing adequacy can be evaluated. In this paper the characteristic features of L-mode generated using a source design parameter for tube testing are explained. The dispersion characteristic of L-mode are discussed using constructed Dispersion Curves, phase and group velocity, its multiple defect detection capability, pulse shape and amplitude dynamics with propagation range and inclination. The details of controlled experiment conducted and the observations are discussed. The derived inferences are explained based on the applicable particle displacement equations and dispersion characteristic equations.

Abstract: In a trigonal easy-plane -Fe2O3 antiferromagnet magnetic-field-dependent conic refraction due to the renormalization of the coefficients of elasticity effective magnetoelastic interaction is experimentally found in addition to the conventional internal conic refraction of the transverse elastic waves propagating along the trigonal C3 axis. It is shown that the deflection angle () of the energy flow from the C3 axis upon the internal conic refraction does not depend on the value of the magnetic field applied in the basis plane (HC3) and is a constant value determined by the correlation of the C14 and C44 coefficients of elasticity. The deflection angle of the energy flow upon the antiferromagnetic conic refraction () increases with increase in the field and tends to the  value at large H values. The obtained results agree well with the theory of this phenomenon in antiferromagnets and support its conclusions.

Abstract: A study on corrosion evaluation by using ultrasonic waves and acoustic emission technique is presented. The experimental equipment was established to improve the corrosion process of carbon steel pipe. The carbon steel pipe was under 473K temperatures and 10Mpa pressure conditions, and ultrasonic wave and acoustic emission techniques were used to inspect the degree of corrosion after a certain period of time. Ultrasonic bulk waves are limited by the poor time resolution when used in the measurement of corrosion depth in thin wall structures because the corroded surfaces cause unclear echo signal edges. Therefore, in this study, the ultrasonic guided waves were generated on the pipe because the thickness of pipe was thin. Various wave modes were subsequently generated on the pipe to evaluate the implications of corrosion thinning on group velocity, transmission and reflection amplitudes. The amplitudes of the transmitted and the reflected waves are influenced by couplent material. In order to reduce the effect of coupling acoustic emission sensor was used. Acoustic emission technique has lots of parameters to evaluate the corrosion besides amplitude parameter. Among parameters energy, count, and frequency were useful parameters to measure the degree of corrosion inside the carbon steel pipe under 473K temperatures.

Abstract: The group velocity of the S0 mode of a Lamb wave under the first cut-off frequency is measured during the tensile loading for the unidirectional composite materials. The group velocity of the S0 mode has the characteristic of increasing with the strain. The propagation velocity of a wave is generally proportional to the square root of the ratio of the modulus to the density. The elastic modulus is considered as a constant as the stress linearly increases with the strain. The increase of the group velocity with the strain is caused by a decrease in the density. It is proposed that the measurement of the group velocity of the S0 mode is useful for making an evaluation of the tensile strain and of any degradation in the unidirectional composite materials.

Abstract: A technique to localize an impact source for an elastic spherical shell is proposed. The conventional source localization techniques when the source is located on a dispersive medium, require both the time-of-arrival differences (TOADs) between the transducer signals and the group velocities. In practice, the material properties or the geometry of the medium are not fully informed, therefore the group velocity is not available. Furthermore, they are only applicable if we have a high signal-to-noise ratio (SNR). In this paper, we propose a method that can be applicable in practice, which does not need to know the group velocity, when we have a relatively small SNR. The scanning procedure over the structure to acquire a minimum variance point of the estimated group velocities is suggested. To reduce the noise effect, an exponential function is asymmetrically weighted in smoothed Wigner-Ville distributions (WVDs). Experiments have been conducted to confirm the validity of this method. As a result, the proposed technique is found to be effective for an impact source localization for a spherical shell without prior information on the group velocity, even in a noisy environment.

Abstract: The elastic waves in the isotropic plate are dispersive waves with the characteristics of Lamb wave, however, S0 symmetric mode is less dispersive in the frequency region less than the first cut-off frequency. In the anisotropic plates such as CFRP plates, the propagation velocities vary with the directions as well as the dispersion of the Lamb wave, and the phase velocity direction does not accord with the group velocity direction. The phase velocity direction is equivalent the wave vector direction, while the group velocity direction is equivalent the energy flow direction. In this work, the group velocity dispersion curves were obtained by the dispersion relation of the Lamb wave in unidirectional CFRP plate with an orthotropic structure. The group velocities of the S0 symmetric mode in the frequency region less than the first cut-off frequency were corrected by applying the slowness surface. The propagation velocities of Lamb wave were decided by measuring the arrival time of the Lamb wave signals received with the two pinducers varying the propagating direction in the laminated unidirectional CFRP plates of 8, 16 and 24 plies having a volume fraction of 67%. The measured velocities are better agreement with corrected group velocity curve, except near the fiber direction at the cusp region. When the propagating direction is not accorded with the principal axis, the direction of the group velocities inclines toward the fiber direction in the unidirectional CFRP plates, suggesting that the energy propagates preferentially toward fiber direction.