Elastic behavior of a screw dislocation in the wall of a hollow nanotube

The elastic behavior of a screw dislocation lying in the wall of a hollow cylindrical nanotube is studied theoretically. The corresponding boundary-value problem of the classical theory of elasticity is formulated and solved (for stresses) for a linear elastically isotropic or transversely isotropic body. The elastic energy of the nanotube with the dislocation and the image force exerted on this dislocation by the inner and outer nanotube surfaces are calculated. The internal space of the nanotube is shown to cause the following qualitative differences in the dislocation stress-field distribution: a change in the sign of stress-tensor components near the inner nanotube surface, a high stress concentration at this surface, and a strong stress gradient at this surface. The dislocation has only one position of unstable equilibrium in the nanotube wall, which is always shifted toward the inner nanotube surface. As the internal-space radius increases, the dislocation energy decreases and the position of its equilibrium shifts toward the outer nanotube surface; in the limiting case of a flat plate, it reaches the center of the plate. Near the nanotube free surfaces, the image force on the dislocation is large and can substantially affect the dislocation behavior.     

Spectrum of transverse vibrations of a pipeline element under longitudinal load

Transverse eigenvibrations of a pipeline element fixed between two supports and transporting an ideal fluid are investigated. The global properties of the spectrum in dependence on the value of the transported-fluid velocity, the relative linear densities of the pipe material and the fluid, the longitudinal force, and its direction are established.     

Specific features in low-frequency vibrations of nanocrystals in fluorophosphate glassy matrices

A Raman scattering study of low-frequency vibrational spectra of mixed cadmium sulfoselenide nanocrystals in a fluorophosphate glassy matrix is reported. The measurements have revealed a low-frequency feature in the nanocrystal density of states, whose frequency is related to the glass-matrix annealing time and, hence, to the size of the nanocrystals prepared, which is interpreted as a manifestation of the size quantization effect. Besides the line associated with the constraints on the longitudinal acoustic wave in a nanocrystal, a lower-frequency feature assigned to torsional vibrations of a nonspherical nanocrystal has been found. Fiz. Tverd. Tela (St. Petersburg) 41, 1306–1309 (July 1999)     

Forced transverse vibrations of a submerged tensioned string

For a submerged cable, the classical equation of a vibrating string must be modified to account for added mass and damping effects. This can be done with the aid of Morison's formulae. The resulting equation is non-linear. It is solved for the case when one end of the string is forced into sinusoidal transverse motion. The first harmonic approximations for both resonant and non-resonant responses are obtained by the method of matched asymptotic expansions under the assumption that the non-dimensional damping coefficient is small.     

He-Zn laser with a transverse high-frequency excitation

Laser oscillation on ZnII ion transitions in a HF transverse helium-zinc discharge is reported. The optimal conditions for laser action are found. Unsaturated gain coefficients of 40% and 45% per meter on the most intensive lines 492.4 nm and 758.8 nm respectively are measured.     

Specific features of the excitation spectrum of helium II in a plasma initiated by an electron beam

The emission of a low-pressure helium plasma (P≤2 Torr) initiated by a monochromatic electron beam is investigated. It is found that an increase in the current leads to a drastic increase in the rate of charge exchange of doubly charged helium ions. The assumption is made that inelastic collisions of He⁺⁺ ions with metastable helium atoms provide the main channel of charge exchange of these ions due to the reaction He⁺⁺+Hem→ He⁺*+He ₀ ⁺ .     

Surface effect on free transverse vibrations and dynamic instability of current-carrying nanowires in the presence of a longitudinal magnetic field

Free transverse vibration and instability of current-carrying nanowires immersed in a longitudinal magnetic field are of concern. On the basis of the surface elasticity theory, a model is developed to investigate the problem. The analytical expressions of dynamic transverse displacements as well as natural frequencies of the magnetically affected nanowire for carrying electric current are obtained. The influences of the surface effect, initial tensile force within the nanowire, strength of the longitudinal magnetic field, and electric current on the natural frequencies as well as dynamic displacements are examined. The obtained results reveal that the transverse stiffness of the nanostructure is enhanced by the surface effect and the initial tensile force, while electric current or longitudinal magnetic field reduces the nanowire's stiffness. The condition which leads to the dynamic instability of the nanostructure is obtained. Further, the roles of the influential parameters on its stability are inclusively discussed.     

Experimental study of propagation of instability waves in a submerged jet under transverse acoustic excitation

An experimental study was conducted on the specific features of instability wave propagation in the mixing layer of a turbulent jet when the jet is excited by an external acoustic wave. We used the technique of conditional phase averaging of data obtained by particle image velocimetry using the reference signal of a microphone placed near the jet. The influence of the excitation frequency on the characteristics of large-scale structures in the mixing layer was investigated. It is shown that the propagation patterns of the instability waves agree well with previously obtained data on the localization of acoustic sources in turbulent jets.     

Stress field around a high speed screw dislocation

Using a simple lattice model, the stress field around a high speed screw dislocation is calculated. On the basis of the results, a possible pattern of “breakdown” which was discovered by Earmme and Weiner in a one-dimensional dislocation model is discussed and a new model of twinning is proposed. Mid-ribs in b.c.c, twins can be explained on the basis of this model.     

The free transverse vibrations of anisotropic beams

A uni-directional composite has high Young's modulus in the fibre direction but low shear modulus. This allows large shear strains and so invalidates some of the assumptions of engineering beam theory; in particular, the sectional loadings cannot be assumed to be related to a linear stress distribution. A theory for anisotropic materials which relates the deflection of a beam to the stress pattern is considered. This allows the calculation of corrections to the natural frequencies derived by standard beam theory. It appears that the Euler-Bernoulli theory with a shear correction similar to that of Timoshenko theory gives adequately accurate results.     

Local vibrations of one-dimensional model of dislocation

Changes in the frequency spectrum of the vibrations of a one-dimensional model of a crystal, which is perturbed by dislocations, are determined by the method described in paper [10]. The influence of an external homogeneous stress on the frequency of the local vibrations is qualitatively evaluated.The author thanks assist. prof. O. Litzman for valuable discussions and help in solving the problem.