Papers by Keyword: Phase Velocity

Abstract: In this paper, the stress influence in the guided wave velocity of the fundamentaltorsional mode is presented. Two analytical models, based on the Acoustoelasticity effect, tocompute the fundamental torsional mode velocity propagating in a specimen subject to anaxial stress are studied. These models are obtained due to the relation between the T(0, 1)guided wave velocity and the bulk shear velocity. The analytical models to calculate the guidedwave velocity are functions of the stress, second and third order elastic constants. A series ofaxial stress levels applied to a cylindrical waveguide is investigated with numerical simulations(Finite Elements) to estimate variations of the T(0, 1) guided wave velocity. This analysisprovides a criterion to evaluate the practical implementation of a stress monitoring schemebased on velocity variations of the fundamental torsional mode.

Abstract: Magneto-Rheological fluid (MRF) is a class of smart material who’s mechanical and rheological properties can be varied rapidly and reversibly by the applied magnetic field. MRF also exhibit fast, strong and reversible changes in the acoustic properties. The goal of this paper is to study the acoustic attenuation and velocity variation of the MRF composite under magnetic field. The model of acoustic wave propagation in MRF was established by Biot-Stoll theory. Some minor changes to the theory had to be applied, modeling both fluid-like and solid-like state of an MRF. The calculated results show that the attenuation coefficient of acoustic wave is improved obviously under the application of magnetic field in wide range frequency.

Abstract: In the simulation of the rotational seismic ground motion, the apparent velocity of Love wave is always assumed to be equal to the S wave velocity of top layer of the site approximately, with the dispersion of surface wave not being fully considered. In this paper, the effect of the velocity structure to the Love wave dispersion is discussed based on the stiffness matrix theory. It shows that to assume the velocity to be equal to the S wave velocity of the top layer may greatly overestimate the low frequency rotational seismic motion. A simplified dispersion curve, is suggested for rotational seismic ground motion simulation. The shape of the bilinear curve is shaped by 3 parameters. They are the corner frequency, the minimum phase velocity and the velocity ratio. The parameters are affected by the velocity structure of the site.

Abstract: This paper investigates the dispersion characteristics of shear horizontal waves propagating in a piezoelectric cylinder covered by orthotropic layer. The surface of the orthotropic layer is assumed to be mechanically free; The stress and displacement at the interface is continuous. The solution of the equation is expressed by Bessel function, and the dispersion equation is derived by using the boundary conditions and the interface conditions, the numerical examples are provided to show the influences of the orthotropic degree and the properties of piezoelectric materials on the dispersion characteristics of SH waves.

Abstract: The viscoelasticity and orthotropic of composite materials has been investigated. Based on the essence of dynamic wave equation, a numerical modeling of guided waves propagating in anisotropic viscoelastic plates is developed by employing the spectral finite element method (SFEM). The phase velocity, energy velocity and attenuation curves can be obtained and analyzed by simulating carbon-epoxy film and carbon-epoxy film on the different substrates. The results illustrate that the mode coupling phenomenon will occur due to consider viscoelastic of composite materials which can use for choice of frequency. In the viscoelastic film/substrate system, the more modes and the lower attenuation amplitude appear which explains the better for monitoring long distance. Knowledge of these properties is important for evaluating material damage and designing material structure .

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: The geometry structure, band structure and density of states of KH2PO4 (KDP) and NH4H2PO4 (ADP) single crystals were investigated by using density functional theory (DFT) method. Parameters including the phase velocity, electromechanical coupling factor and power flow angle (PFA) were calculated for KDP and ADP at X, Y, Z cuts, respectively. Our calculated data are in good agreement with the experimental results. Compared with quartz, KDP and ADP crystals are of lower phase velocities. Considering PFA, electromechanical coupling coefficients and phase velocity, it was concluded that the propagation direction of 60 of X-cut and Y-cut possess superior SAW performance for both KDP and ADP crystals.

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: As the requirement for the low loss phase shifter increases, so does the development of RF MEMS as a solution. This paper presents the design & simulation of Switched line MEMS phase shifter for Ku band using GaAs substrate. The phase shift can be achieved by varying the lengths in delay path to the reference path for the same phase velocity. The electromagnetic & electromechanical simulations were carried out with various structural parameters to optimize the design. The novelties like low insertion loss, low actuation voltage with distributed actuation pads for DC and RF are used to make the design unique. The EM simulations are carried out using 3D simulator HFSS and a phase shift of 172.6 deg./dB for a total Phase shift of 348.75deg was achieved with return loss of 15.5dB over a frequency band from 16-18 GHz and a phase shift error less than ±2 degree in the 32 states. The electromechanical simulations are carried to achieve the low actuation voltage of 15.3V. These parameters make these suitable for the Phased array applications [1, 2].

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.