237 research outputs found
Effect of a Magnetic Field on the Dipole Echo in Glasses with Nuclear Quadrupole Moments
The effect of a magnetic field on the dipole echo amplitude in glasses at
temperatures of about 10 mK caused by nonspherical nuclei with electric
quadrupole moments has been studied theoretically. It has been shown that in
this case, the two-level systems (TLS's) that determine the glass properties at
low temperatures are transformed into more complicated multilevel systems.
These systems have new properties as compared to usual TLS's and, in
particular, exhibit oscillations of electric dipole echo amplitude in magnetic
field. A general formula that describes the echo amplitude in an arbitrary
split TLS has been derived with perturbation theory. Detailed analytic and
numerical analysis of the formula has been performed. The theory agrees
qualitatively and quantitatively well with experimental data.Comment: 5 pages, 3 figure
Innovative manufacturing techniques of the profiled drawing tool
In this article, the new technique of fast production of the tool of irregular shape for profiling of variable thickness wall tubes is described. The specific technological sequence is given, recommendations about computer modeling are made, and the explaining illustration for one of characteristic cases of chosen tube type profiling is given. There is an introduction defining relevance of the solved problem, and recommendations about its solution are made. © 2019 Published under licence by IOP Publishing Ltd.The reported study was funded by the state budget themes “Theoretical foundations for the development of new processes and machines for improving the competitiveness of manufactured products”, “Developing the theoretical foundations of technologies and equipment for producing new types of metal products”
Power parameters of the process of hardening of cylindrical parts by a toroidal roller by the method of surface plastic deformation
A theoretical study of the influence of various technological factors on the force parameters of the hardening process of a cylindrical blank by a method of surface plastic deformation was performed. Based on the approximate model of plastic deformation propagation, an engineering technique has been developed that allows one to specify the force regimes of the surface hardening process with a toroidal roller, taking into account the required degree of deformation of the hardened layer, taking into account the mutual influence of the geometric parameters of the workpiece, the deforming roller and the required depth of cold work. The results of the theoretical study are in good agreement with the known experimental data and can be used in the development of technological operations for hardening machine parts by rolling in rollers or balls. © 2019 IOP Publishing Ltd. All rights reserved
Research and improvement of the process of two-stage profiling of tubes for special purposes
Roll-forming process is used in manufacturing multi-beam oil tubes, while reducing their outer profile diameter and forming star-like profile. In this paper a steel tubes are studied with a drawing and pushing processes. This study gets into different issues including elastoplastic behavior, contacts and friction using finite element analysis. In simulations, a plane symmetric steady-state model is used. Numerical results are compared with experimental results. A better understanding and modelling of the behavior of materials lead to a new form of roller tool with significant advantages. © 2019 Published under licence by IOP Publishing Ltd.The reported study was funded by the state budget themes “Theoretical foundations for the development of new processes and machines for improving the competitiveness of manufactured products”, “Developing the theoretical foundations of technologies and equipment for producing new types of metal products”
Studying damage accumulation in martensitic corrosion-resistant steel under cold radial reduction
Cold radial reduction of specimens made of the Kh17N2 corrosion-resistant martensitic steel is studied on a lever-type radial-forging machine (RFM). The mechanical properties of the deformed specimens, the "damage accumulation - strain" relation in the specimens are obtained with the application of hydrostatic and fractographic methods for fractured specimens. The damage of the Kh17N2 corrosion-resistant steel is evaluated as a result of an experimental study considering the data of simulation by a complex finite element model of cold deformation on a lever-type RFM. © 2017 Author(s)
Density of states in random lattices with translational invariance
We propose a random matrix approach to describe vibrational excitations in
disordered systems. The dynamical matrix M is taken in the form M=AA^T where A
is some real (not generally symmetric) random matrix. It guaranties that M is a
positive definite matrix which is necessary for mechanical stability of the
system. We built matrix A on a simple cubic lattice with translational
invariance and interaction between nearest neighbors. We found that for certain
type of disorder phonons cannot propagate through the lattice and the density
of states g(w) is a constant at small w. The reason is a breakdown of affine
assumptions and inapplicability of the elasticity theory. Young modulus goes to
zero in the thermodynamic limit. It strongly reminds of the properties of a
granular matter at the jamming transition point. Most of the vibrations are
delocalized and similar to diffusons introduced by Allen, Feldman et al., Phil.
Mag. B v.79, 1715 (1999).Comment: 4 pages, 5 figure
Sparse random matrices and vibrational spectra of amorphous solids
A random matrix approach is used to analyze the vibrational properties of
amorphous solids. We investigated a dynamical matrix M=AA^T with non-negative
eigenvalues. The matrix A is an arbitrary real NxN sparse random matrix with n
independent non-zero elements in each row. The average values =0 and
dispersion =V^2 for all non-zero elements. The density of vibrational
states g(w) of the matrix M for N,n >> 1 is given by the Wigner quarter circle
law with radius independent of N. We argue that for n^2 << N this model can be
used to describe the interaction of atoms in amorphous solids. The level
statistics of matrix M is well described by the Wigner surmise and corresponds
to repulsion of eigenfrequencies. The participation ratio for the major part of
vibrational modes in three dimensional system is about 0.2 - 0.3 and
independent of N. Together with term repulsion it indicates clearly to the
delocalization of vibrational excitations. We show that these vibrations spread
in space by means of diffusion. In this respect they are similar to diffusons
introduced by Allen, Feldman, et al., Phil. Mag. B 79, 1715 (1999) in amorphous
silicon. Our results are in a qualitative and sometimes in a quantitative
agreement with molecular dynamic simulations of real and model glasses.Comment: 24 pages, 7 figure
Hypersound damping in vitreous silica measured by picosecond acoustics
The attenuation of longitudinal acoustic phonons up to frequencies nearing
250 GHz is measured in vitreous silica with a picosecond optical technique.
Taking advantage of interferences on the probe beam, difficulties encountered
in early pioneering experiments are alleviated. Sound damping at 250 GHz and
room temperature is consistent with relaxation dominated by anharmonic
interactions with the thermal bath, extending optical Brillouin scattering
data. Our result is at variance with claims of a recent deep-UV experiment
which reported a rapid damping increase beyond 100 GHz. A comprehensive picture
of the frequency dependence of sound attenuation in -SiO can be
proposed.Comment: 4 pages, 3 figure
Anharmonicity, vibrational instability and Boson peak in glasses
We show that a {\em vibrational instability} of the spectrum of weakly
interacting quasi-local harmonic modes creates the maximum in the inelastic
scattering intensity in glasses, the Boson peak. The instability, limited by
anharmonicity, causes a complete reconstruction of the vibrational density of
states (DOS) below some frequency , proportional to the strength of
interaction. The DOS of the new {\em harmonic modes} is independent of the
actual value of the anharmonicity. It is a universal function of frequency
depending on a single parameter -- the Boson peak frequency, which
is a function of interaction strength. The excess of the DOS over the Debye
value is at low frequencies and linear in in the
interval . Our results are in an excellent
agreement with recent experimental studies.Comment: LaTeX, 8 pages, 6 figure
Mesoscopic Cooperative Emission From a Disordered System
We study theoretically the cooperative light emission from a system of classical oscillators confined within a volume with spatial scale, , much
smaller than the radiation wavelength, . We assume
that the oscillators frequencies are randomly distributed around a central
frequency, , with some characteristic width, . In
the absence of disorder, that is , the cooperative emission spectrum
is composed of a narrow subradiant peak superimposed on a wide superradiant
band. When , we demonstrate that if is large enough, the
subradiant peak is not simply broadened by the disorder but rather splits into
a system of random narrow peaks. We estimate the spectral width of these peaks
as a function of , and . We also estimate the
amplitude of this mesoscopic structure in the emission spectrum.Comment: 25 pages including 6 figure
- …