Anomalous Reflection of a Longitudinal Ultrasonic Wave from a Strongly Dissipative Medium

Normal reflection of a longitudinal acoustic wave from the plane interface between a solid acoustic line and a strongly dissipative medium (an epoxide resin compound in the process of solidification) has been experimentally investigated. A 14fold change in the reflection coefficient of pulsed signals and a decrease in their duration have been detected and the point of phase transmission has been reliably determined by the minimum of the reflection coefficient. The coefficient of reflection of continuous acoustic waves with frequencies of 1–10 MHz from the interface between a plexiglas (or aluminum) and an epoxide resin compound in the process of solidification has been measured. The influence of the amplitudefrequency characteristic of an ultrasonic piezoelectric transducer on the measurement data obtained has been analyzed. The change in the viscosity coefficient of an epoxide resin compound in the process of its solidification has been calculated by the spectral transform method with the use of computer programs and experimental data on the reflection coefficient dynamics.     

Anomalies of the Boundary Reflection of Ultrasound from the Film of a Dissipative Medium

Theoretical consideration has been given to the reflection of continuous and pulsed longitudinal and transverse acoustic waves from the film of a dissipative medium which is in contact with a solid-state half-space. It has been shown that the reflection coefficient and its phase substantially depend on the coefficient of attenuation of ultrasound in the dissipative-medium film and on its phase thickness. The shape of the reflected acoustic pulsed signal has been calculated using software. The application of the results obtained to investigation of the acoustic properties of viscous fluids is discussed.     

Anomalies of the boundary reflection of ultrasound from a Maxwellian liquid

Reflection of continuous and pulsed longitudinal and transverse acoustic waves from a dissipative medium represented by a model of Maxwellian liquid in contact with a solid halfspace is considered theoretically. The substantial dependence of the modulus and phase of the reflection coefficient on both the viscosity and time of relaxation of stresses in the Maxwellian liquid is shown. Using computer programs, the acoustic pulsed signals reflected from the interface between the media and transmitted through it have been calculated. The calculations were performed for an asymmetric shape of the signal incident on the interface; this shape corresponds to that of a real signal emitted by an ultrasonic piezoceramic transducer.     

Steady propagate crack in a transverse isotropic piezoelectric material considering the permittivity of the medium in the crack gap

This paper derives an exact solution of the steady propagated crack in a transverse isotropic piezoelectric material plane. In order to consider the medium in the crack gap, two cases have been studied. In the first case, the permittivity of the medium in the crack gap _a is far less than that of piezoelectric materials _m. Therefore, the electric induction in the gap (Pak, 1990; Suo et al., 1992) is neglected. In the second case, the permittivity of the medium in the crack gap is comparable with that of piezoelectric material. This electric induction is considered. This result shows that the consideration of the induction has reduced the electric displacement intensity factor k ₄. Due to the influence of the dynamic effect, the elastic constants have become smaller as the dynamic anisotropy case. It must be pointed out that only the small speed of steady propagated crack is considered. Therefore, according to Sosa et al. (1999, 2001), the magnetic effect is neglected.     

Magnetoelastic wave tunneling via a gap between ferroelectric crystals with relative longitudinal displacement

The tunneling of a plane monochromatic acoustic wave via a gap between two ferromagnetic crystals exhibiting relative longitudinal displacement has been studied. It is shown that, at a gap width comparable with the wavelength, the acoustic wave can exhibit complete transmission at the Damon-Eshbach mode frequency. If the gap width is much smaller than the wavelength, the complete transmission takes place at two resonance frequencies. Allowance for the longitudinal displacement of one crystal leads in all cases to violation of the resonance conditions, which results in a significant decrease in the transmission coefficient. The greater the velocity of crystal displacement, the stronger the decrease in the acoustic wave transmission though the gap between ferromagnetic crystals.     

An ultrasonic prospecting of shape-memory alloy behaviour under thermal charges

Thermomechanical models may be produced to describe the macroscopic deformations of shape-memory alloys educated to be deformed with special shapes as a function of temperature. To be accurate, these models need to take into account evolution of the microstructure via homogenization theories. So, the aim of this work was to provide all available information about phase transformations occurring in the grain structures from an investigation close to the microscopic scale. In this work, we have visualized grain structures of Cu–Zn–Al duplex alloys using acoustic microscopy. Evolution of phase transformations as a function of temperature has also been followed on these acoustic images with a spatial accuracy up to few micrometres. This observation of sample surface has also enabled estimation of grain baring due to phase transformations. Using the same experimental device, acoustic signatures have been taken on samples in complete austenic or martensitic forms to measure the speed of Rayleigh surface waves. Despite the use of a wide ultrasonic frequency range from 15–600 MHz, it seems that wave attenuation due to viscosity is important and disables velocity measurements by this method. Finally, using an acoustic echographic technique, we have correlated attenuation and velocity of longitudinal waves to the global phase transformation of heated samples. © 1998 Kluwer Academic Publishers     

Anomalies in the ultrasound reflection from a solid-dissipative medium interface

The reflection of longitudinal acoustic waves from the interface between a solid and a strongly dissipative medium was theoretically studied. It is shown that the reflection coefficient and the reflected signal phase significantly depend on the coefficient of ultrasound absorption in the dissipative medium. An algorithm for restoring the time variation of the dissipative medium viscosity during solidification is proposed, which is based on the results of measurements of the amplitude reflection coefficient for a pulsed ultrasonic signal. The reflection and transmission coefficients are determined for the ultrasound wave amplitude, velocity, pressure, and intensity. An expression is derived for the acoustic energy absorbed upon reflection from a dissipative medium. A frequency dependence of the amplitude and generalized phase of the signals reflected from and transmitted through the interface are restored from the results of spectral calculations. The theoretical results are confirmed by experimental data on the reflection of acoustic pulses from an organic glass-epoxy compound resin interface measured during solidification of the latter medium. According to these data, the reflection coefficient and the reflected acoustic pulse duration decreased upon solidification of the epoxy compound.     

Nonlinear response of electron-phonon interaction inn-type nondegenerate piezoelectric semiconductors

The nonlinear response of the electron-phonon interaction in nondegenerate piezoelectric semiconductors such asn-type InSb in the presence of a dc magnetic fieldB directed along the propagation of acoustic waves has been studied by using a quantum mechanical treatment. The effect of electron scattering in solids has been taken into consideration, so the electron relaxation time cannot be neglected. The nonlinear nature of the energy band is corrected for, using the Heisenberg equation of motion. It is found that the nonlinear response is proportional to a nonlinear longitudinal conductivity tensor _zzz when acoustic waves propagate parallel to the [111] direction for both piezoelectric and deformation-potential couplings. Numerical calculations for _zzz of n-type InSb at low temperatures are presented. Results show that the nonlinear response decreases rapidly with the sound frequency, and decreases slowly with the dc magnetic field and temperature. Therefore, the electron relaxation time and the nonparabolicity of energy bands play important roles for the electron-phonon interaction due to the piezoelectric and deformation-potential couplings in the microwave region.     

Monolithic bulk shear-wave acousto-optic tunable filter

We demonstrate a monolithic bulk shear-wave acousto-optic tunable filter combining a piezoelectric transducer array and the acoustic interaction medium in a single crystal. An X-propagating acoustic longitudinal wave is excited in the "crossed-field" scheme by an RF-E/sub y/-field in a chirped acoustic superlattice formed by domain-inversion in lithium niobate (LiNbO/sub 3/). The acoustic longitudinal wave is efficiently (97.5%) converted at a mechanically free boundary into a Y-propagating acoustic slow-shear wave that couples collinearly propagating e- and o-polarized optical waves. A relative conversion efficiency of 80%/W was measured at 980 nm.     

Longitudinal Creep Behaviour of a SiC/Ti-6242 Composite in a Vacuum Atmosphere

This study deals with the longitudinal creep behaviour of a unidirectionally reinforced SM1140+/Ti-6242 composite at 500C in a vacuum atmosphere. The monitoring of acoustic emission and microstructural observations show that filament fracture develops during the creep test. This damage results from the stress relaxation in the matrix, which increases the axial stress in the filament. Time till the creep rupture can be predicted with a model describing the filament damage accumulation.     

Atom positions in the R-phase unit cell in TiNi shape memory alloy

A crystallographic analysis of the atom positions in the unit cell of the R-phase of the TiNi shape memory alloy has been performed. In addition to a homogeneous shape change the atoms are found to shuffle in the [111]_B2 and –211_B2 directions during the B2 to R-phase transformation. The origin of these shuffles is found to be a complex interaction between 1101–10 transverse acoustic and 111 longitudinal acoustic soft phonon modes.     

The elastic behaviour under hydrostatic pressure of poly(methyl methacrylate) and its fully deuterated form

Measurements have been made of the effects of hydrostatic pressure on the velocities of longitudinal and shear ultrasonic waves propagated in a commercial poly(methyl methacrylate) (PMMA) and a fully deuterated PMMA. The experimental results provide a comparison of the bulk and shear moduli and their hydrostatic pressure derivatives for the two polymers. The vibrational anharmonicity of the long wavelength acoustic modes is more pronounced in the deuterated than in the commercial PMMA. B/P is 32% larger in the deuterated form than in the undeuterated polymer, a finding which may be understood by including the isotopic mass difference in a simple vibrational model.     

Mechanical and Thermophysical Properties of Cerium Monopnictides

The ultrasonic attenuation due to phonon–phonon interaction, thermoelastic relaxation and dislocation damping mechanisms has been investigated in cerium monopnictides CeX (X: N, P, As, Sb and Bi) for longitudinal and shear waves along \({\langle }100{\rangle }\), \({\langle }110{\rangle }\) and \({\langle }111{\rangle }\) directions. The second- and third-order elastic constants of CeX have also been computed in the temperature range 0 K to 500 K using Coulomb and Born–Mayer potential upto second nearest neighbours. The computed values of these elastic constants have been applied to find out Young’s moduli, bulk moduli, Breazeale’s non-linearity parameters, Zener anisotropy, ultrasonic velocity, ultrasonic Grüneisen parameter, thermal relaxation time, acoustic coupling constants and ultrasonic attenuation. The fracture/toughness ratio is less than 1.75, which shows that the chosen materials are brittle in nature as found for other monopnictides. The drag coefficient acting on the motion of screw and edge dislocations due to shear and compressional phonon viscosities of the lattice have also been evaluated for both the longitudinal and shear waves. The thermoelastic loss and dislocation damping loss are negligible in comparison to loss due to Akhieser damping (phonon–phonon interaction). The obtained results for CeX are in qualitative agreement with other semi-metallic monopnictides.     

Boundary layer flow of a dusty fluid over a semi-infinite flat plate

Summary Both the drag force due to slip and the transverse force due to slip-shear have been considered in boundary layer equations. The solution has been found in a power series of non-dimensionalx, x being the distance in the down-stream direction. Solutions for high slip region and small slip region characterised byx1 andx1 respectively, have been found separately. In the high slip region the effect of increase in concentration parameter of the dust particles is to increase the magnitude of the longitudinal fluid phase velocityu. Also the magnitude of the longitudinal particle slip velocityu _p -u is becoming maximum on the plate and decreasing along the plate withx. The transverse particle velocityv _p is independent of but it is directly proportional to , the transverse force coefficient. An interesting result is thatv _p is assuming small positive value on the plate. The transverse force has taken an important role in migration of particles away from the plate. In the small slip region the flow of dust particles is mainly governed by the fluid-phase. The effect of on the flow field in this region is to decrease the boundary layer thickness. In this region the particles are having some tendency to accumulate near the plate. Lastly, it has been found that the shearing stress, skinfriction and the dimensionless drag-coefficient on the plate increase with increase of .With 5 Figures     

Alpha-Like Clustering in $$^{}$$Ne from a Quartetting Wave Function Approach

The signal of electric response in superfluid helium was recently observed at the excitation of a second sound standing wave in an acoustic resonator (Rybalko in Low Temp Phys 30:994, 2004). It has been suspected that similar resonance signals may be observed also in a first sound wave; however, earlier experiments showed the absence of any electric response induced by the pressure wave. Here, we report the first experimental evidence of electrical activity that arises at the fundamental frequency of the first sound resonance. It is found that the signal of electric response agrees fairly well with the fundamental frequency of the first sound resonance. The absence of the signal of interest in the original experiment is discussed.     

The interaction of ultrasound with contacting asperities: Applications to crack closure and fatigue crack growth

The partial contact of two rough fatigue crack surfaces leads to transmission, reflection, diffraction, and mode conversion of an acoustic signal at those contacts. This paper reviews recent experimental and theoretical efforts to understand and quantify such contact on actual fatigue cracks in greater detail. It is shown that the size and density of individual contacts, or asperities, can be estimated from acoustic measurements. Furthermore, it is shown that this information is useful to provide the static stress across a partially closed crack as well as the effective stress intensity range which activates fatigue crack propagation.     

Nondiffusive hot spot in a confined, narrow domain

The spatial structure and temporal evolution of a nondiffusive hot spot are examined in a confined, planar enclosure. The situation is modelled by the reactive Euler equations and single-step Arrhenius kinetics with a large (scaled) activation temperature 1/ The initial state of the medium is taken to be one of uniform pressure but with a prescribed thermal gradient of order across the domain. The induction equations, which govern order disturbances to the initial state, are analyzed in the limit of small , where measures the width of the domain relative to the acoustic length based on a characteristic reaction time at the initial state. The solution displays a sequence of finite-time singularities as the hot spot grows in strength and shrinks in size in a spatially homogeneous pressure environment. Ultimately, in a region of exponentially small size, significant pressure gradients appear as acoustic and reaction times become comparable and the hot-spot structure is found to obey the full, but previously analyzed, Clarke equations.     

The elastic bahaviour and vibrational anharmonicity of molybdenum phosphate glasses

To examine the effects of vibrational anharmonicity on the long-wavelength phonon dynamics of a series of phosphate glasses, hydrostatic pressure and temperature dependences of ultrasonic wave velocities Ave been measured in molybdenum phosphate glasses (MoO_{3 x} (P₂O₅)_1-x over the composition range 35 to 76 mol % MoO₃. Marked discontinuities occur in the variations of elastic constants with composition, indicating distinct differences in the nature of the structure and bonding in the glasses as a function of composition. The pressure derivatives of the elastic constants are found to be positive and the temperature derivatives negative. Both the longitudinal (_L) and shear (_S) mode Grüneisen parameters are positive, showing that application of hydrostatic pressure produces an increase in the long-wavelength acoustic phonon mode frequencies. The temperature dependences of both longitudinal and shear acoustic phonon velocities are found to be markedly anomalous in that they continue to increase substantially as the temperature is reduced below about 100 K. The low-temperature elastic constant data are compatible with the interaction of the phonons with two-level systems, and provide direct evidence for such systems in phosphate glasses.On sabbatical leave from Department of Physics, University of the Witwatersrand, Johannesburg 2001, South Africa.     

Interaction of the acoustic signal with motionless discretely layered medium containing a layer of bubbly liquid

The dynamics of the pulse perturbation of the low-amplitude pressure in the motionless discretely layered medium containing a layer of liquid with polydisperse gas bubbles has been studied theoretically. Theoretical method basics of the calculation of the acoustic signal distortion during the diagnostics of multilayer samples containing a layer of bubbly liquid are presented. It is shown that specific dispersion and dissipative properties of the layer of bubbly liquid can affect considerably the dynamics of the acoustic signal in the multilayer medium as a function of the main frequency of the signal. The theoretical models of the dynamics of multiphase media can be verified using this method. It was established that it is possible to use this theory for the calculation of the acoustic signal distortion at its interaction with multilayer objects containing the layer of bubbly liquid.