The paper shows that it is possible to obtain reliable information on the dependence of the radial distribution of longitudinal birefringence in glass cylindrical elements with a radial distribution of refractive index (GRIN lenses) on the basis of transverse polarization tomography data on residual stresses. This does not require complicated procedures of sample preparation, as is necessary in the case of longitudinal translucency measurements. The approach developed was verified in the experiments with a set of different GRIN lenses formed with the ion exchange technique, and the closeness of the data obtained from transversal and conventional longitudinal transmission measurements was demonstrated.

For the first time, an algorithm for reconstructing an arbitrary distribution of residual stresses by the polarization tomography method for cylindrical rod structures with a radial distribution of the refractive index has been presented. The reconstruction took into account the ray refraction. The algorithm is based on the expansion of the tensor stress field in angular harmonics (singular value expansion). The case of an axisymmetric tensor field with an arbitrary stress gradient along the cylinder axis was considered. Numerical calculations were carried out for an axially symmetric stress distribution in a grad an for the case of a plane deformation state. The reconstruction was based on the expansion of the stress tensor in eigenfunctions of the boundary value problem. The regularized solution of the resolving equation (of Abelian type) used the expansion in Zernike polynomials. The results of the reconstruction are given with taking into account the additional term due to the deflection of the transmission rays as well as without this doing.

This report presents research of polarization-optical tomography (integral photoelasticity) possibilities for non-destructive diagnostics of mechanical stresses in dielectric cylindrical structures of hexagonal single crystal used in electrooptic and nonlinear optical devices, high-power laser, fiber optics and other photo optic applications. The polarization of light is determined in a plane orthogonal to the optical axis of the object. It is supposed a linear dependence of the stresses are determined within the permittivity tensor on stresses (the Maxwell piezo-optic law) and weak rotation of quasi – principal directions of the stress tensor. The report generalizes the application of polarization-optical tomography with the small curving beam of light in case of radial refractive index gradient.

The residual stresses (RS) in gradient-index (GRIN) rod lens are formed due to the material characteristics gradient. RS can lead to the birefringence effect. If a GRIN lens is birefringent, its applications may be limited, because the anisotropy can cause the image quality degradation. According to this, the urgent task is to develop a non-destructive diagnostic method of RS in the gradient optical structures. This paper presents the relations of the generalized method of integral photoelasticity for reconstructions of RS (optical tomography of the stress field) for the case of the bending probing rays in GRIN optical rods with a radial distribution of the refractive index. The problem is solved with plane strain approximations. RS Distributions are given reconstructed from tomographic data obtained with transverse and longitudinal (coaxial) translucence of GRIN rod lens.

The paper presents the results of the residual stresses reconstruction and their influence on the refractive index profile (RIP) in a number of strengthened preforms for single-mode optical fibers. Strengthening layers were formed by external and internal vapor-phase deposition of titaniumsilicate glass. It was found that an increase in the concentration of TiO 2 in the strengthening layer leads to significant piezooptical distortions of the RIP due to large tensile stresses in the core.

В большинстве случаев цилиндрические структуры с градиентным показателем преломления (граданы) формируются на основе диффузионной технологии.Показатель преломления и остаточные напряжения в них носят аксиально симметрический характер.Дисторсия, обусловленная диффузией, и температурные внутренние напряжения в них моделируются фиктивной температурой остаточных напряжений.Применение интегральной фотоупругости для реконструкции остаточных напряжений в граданах обычно ограничивается

In most cases, cylindrical structures with a gradient refractive index (GRIN) are formed on the basis of diffusion technology. The refractive index and the residual stresses in them are axially symmetrical. The distortion determined by diffusion and the thermal internal stresses in them are simulated by the fictive temperature of residual stresses. The use of the integral photoelasticity for the reconstruction of residual stresses in GRIN lenses is usually limited to the case of a planar deformed state. In this study, this algorithm is extended to the case of the axial change of residual stresses. It is assumed that the illumination procedure is carried out in the sample cross section. The optical problem is solved within the quasi-isotropic approximation. The reconstruction is based on measuring the characteristic parameters of polarized light passed through the sample and on the combined solution of the thermoelasticity problem.

This work presents an application of integrated photoelasticity for non-destructive reconstructions of residual stresses in an axially symmetric gradient index (GRIN) optical structure. Transformation of the polarization of light is measured in a plane orthogonal to the optical axis of the article. Stresses are determined within the framework of the Maxwell piezo-optic law (linear dependence of the permittivity tensor on stresses) and small rotation of principal stress axis. This paper generalizes the method of integrated photoelasticity for the case of the curving translucente rays.

A decrease in the electrical durability, which is defined as an amount of time required for dielectric breakdown at a constant electric field strength, of polyethylene and Lavsan (polyethylene terephthalate) films under tensile loading is registered in a temperature range from 100 to 300 K. It is established that the pulling apart of the axes of neighbor chain molecules in consequence of tensile loading gives rise to a decrease in the energy level of the intermolecular electron traps. In the amorphous region of a polymer, this accelerates the release of electrons from the traps through over-barrier transitions at higher temperatures ranging from about 230 to 350 K and quantum tunneling transitions at lower temperatures in the range from about 80 to 200 K. As a result, the time required for the formation of a critical space charge, i.e., the waiting period of dielectric breakdown, decreases, which means a reduction in the electrical durability of polymers.

The characteristics of 3D cross-linking, mechanical properties and biocompatibility of new copolymers of polyetherurethane and polysiloxane are presented. New copolymers are shown to be highly durable, strong and stable, thus being prospective for various applications including biomedical implants.

Зарегистрировано снижение электрической прочности (измеряемой как время ожидания пробоя диэлектрика в постоянном по знаку и величине электрическом поле) при растягивающем нагружении пленок полиэтилена и лавсана (полиэтилентерефталата) в области температур 100-300 K. Установлено, что вызываемое растягивающим нагружением полимера раздвижение осей соседствующих цепных молекул приводит к уменьшению энергетической глубины межмолекулярных ловушек электронов. Это ускоряет высвобождение электронов из ловушек посредством надбарьерного перехода при повышенных температурах (~230-350 K) и туннельного перехода при пониженных температурах (~80-200 K) в аморфных областях полимеров. В результате происходит сокращение времени формирования критического объемного заряда --- времени ожидания пробоя, что и означает снижение электрической прочности полимеров. DOI: 10.21883/FTT.2017.01.43972.258

In polymer crystals (in particular, polyethylene), changes in the axial and contour lengths of skeletal interatomic bonds in chain molecules under mechanical loading (stretching) and heat treatment (heating) were measured using X-ray diffractometry and Raman spectrometry. The performed analysis of the measured force and temperature dependences gave theoretical descriptions of the deformation of a polymer crystal (the confirmation of the data available in the literature for mechanical loading and an original development for heating). The components of the potential energy of a deformed polymer crystal were determined both for stretching of skeletal bonds and for bending of chain molecules. It was found that there is a significant difference in the ratios of these components for a deformed polymer crystal under mechanical loading and during heating.

In the paper, the changes in axial and contour lengths of skeletal interatomic bonds in the chain molecules of polyethylene nanocrystals have been measured using X-ray diffractometry and Raman spectrometry. In the course of the measurements the samples were subjected to stretching and heating (mechanical and thermal actions). The measured force and temperature dependences were analyzed and the calculated description of the polymer nanocrystal strain was inferred from them. In so doing the original results were obtained for the thermal action. The potential energy components related to both the skeletal bond stretching and the chain molecule bending were determined for the strained polymer crystal. The sharp distinction between the ratios of these components for the object under mechanical and thermal actions was found.

The kinetics of electrical damage (breakdown) of polymer films 20–50 μm thick in a constant-sign field of 0.5–0.6 GV/m at 77–300 K has been studied. At elevated temperatures (250–300 K), the exponential temperature dependence of the durability and the above-barrier thermal-fluctuation mechanism of electron emission from traps, i.e., space charge accumulation leading to breakdown, take place. At low temperatures (77–200 K), there are separate local decreases in the durability (minima) at the athermal durability level. The identity of the temperatures of durability minima and measured thermoluminescence maxima of polymers was found. A conclusion is made about the mechanism of thermally stimulated tunneling (subbarrier emission) of electrons from traps.

The possibility of optical tomography applying to investigation of a two-dimensional and a three-dimensional stressed state in single cubic crystals has been studied. Stresses are determined within the framework of the Maxwell piezo-optic law (linear dependence of the permittivity tensor on stresses) and weak optical anisotropy. It is shown that a complete reconstruction of stresses in a sample is impossible both by translucence it in the parallel planes system and by using of the elasticity theory equations. For overcoming these difficulties, it is offered to use a method of magnetophotoelasticity.

Structural changes in polymer crystals (polyethylene, polyimide, and others) have been studied using the X-ray diffraction and Raman spectroscopy methods under different influences: tensile loading along the chain molecule axis and heating from 90 to 350 K. An increase in the molecule axial length under loading and a decrease in the molecule axial length upon heating have been identified and measured using X-ray diffraction. A decrease in the skeletal vibration frequency during loading and heating has been identified and measured using Raman spectroscopy, which indicates an increase in the molecule contour length in both cases. A technique for determining the change in the polyethylene molecule contour length in the crystal from the measured change in the skeletal vibration frequency has been justified. The contributions of two components, namely, skeletal (carbon–carbon) bond stretching and the change (an increase during stretching and a decrease during heating) in the angle between skeletal bonds, to the longitudinal deformation of polyethylene crystals, have been quantitatively estimated. It has been shown that the negative thermal expansion (contraction) of the polymer crystal is caused by the dominant contribution of the decrease in the bond angle.

A way to determine residual stresses in cylindrical trigonal single crystals the optical axis of which is directed along the crystal axis is proposed. It is assumed that the residual deformation tensor is of thermal character and is characterized by fictive temperature. The measurements are performed in the middle part of a single crystal the length of which is much larger than its diameter; therefore, the stresses in this part do not vary along the single crystal axis. The reconstruction of stresses is based on determining characteristic parameters of polarized light by use of the tomographic method in the plane orthogonal to the single crystal axis.

Algorithms for determining residual stresses in glass round waveguides by the method of integrated photoelasticity are considered. It is assumed that the residual deformation tensor is of thermal character and is characterized by fictitious temperature varying over the sample. The reconstruction is based on measuring characteristic parameters of polarized light and is carried out by the tomographic method in the plane orthogonal to the waveguide axis.