RHEOLOGICAL MODELING OF THE STRESS-STRAIN STATE IN FLAT COMPACTION OF COMPOSITE MATERIALS

Statement of the problem. The article investigates the deformation behavior of a composite material in the process of its flat pressing. To solve this problem, a rheological model is proposed, which is based on the phenomena occurring in a viscous (Newtonian) incompressible fluid, which occupies the volume between two absolutely rigid parallel planes of finite dimensions of rectangular shape approaching at a low speed. Within the framework of mechanics of a continuous medium under conditions of a plane deformed state, the problem is solved in two dimensions about a slow flow in the absence of volume forces and inertial effects. In this case, the solution of the equation of motion with continuity conditions is reduced to the well-known Laplace equation. In addition, on the basis of the model of linear viscoelasticity and uniaxial stress state, an attempt has been made to describe the relaxation phenomena occurring in the solidifying composite at the end of the active pressing process. Results and Conclusions. Analytical dependences of the power parameters of the stress-strain state of the compressed composite are obtained; relations for the kinematic characteristics of the pressing process are obtained; an expression is obtained for the relaxation of stresses during the technological holding of the material under pressure after the end of active pressing. The results of the study make it possible to experimentally determine the numerical values of the dynamic coefficient of viscosity and stress relaxation time, which are important characteristics in controlling the pressing processes.

INFLUENCE OF INCREASED TEMPERATURE ON THE DYNAMIC PROPERTIES OF ASPEN

As a damping material in the structures of containers for the transportation of hazardous materials, along with plastic metals, fiber-claydite concrete and synthetic foams, it is proposed to use wood of different species. Since containers are transported in different climate regimes, there is an urgent need to study the properties of wood at elevated temperatures. The paper presents the results of dynamic tests of aspen under uniaxial compression under conditions of temperature increased to +60°C. The tests were carried out according to the Kolsky method on a Hopkinson split-bar setup. To study the anisotropy of properties, aspen samples were made and tested by cutting samples along and across the direction of the grains. As a result of processing the experimental data, dynamic stress-strain curves were obtained. According to the experimental data, there are determined the stresses at which the integrity of the samples were violated. The mean values of the moduli of deformation in the active loading regions of stress-strain curves are also presented. The highest slope of the load sections and the highest breaking stresses were observed for the specimens when loaded along the grains, and the smallest values of these parameters were noted when loaded across the grains. For specimens loaded along grains at strain rates above 1500 s–1, after reaching the limiting stress values, a decrease (relaxation) of stresses is observed with increasing deformations. For specimens loaded across the grains, an almost horizontal section the diagrams of deforming or even with some strengthening is more typical. The effect of elevated temperature on the strength and deformation properties of aspen is estimated. There is a tendency towards some decrease in the diagrams at a temperature of +60 °C in comparison with the diagrams at room temperature. In this case, both the moduli in the loading and unloading sections and the limiting (breaking) stresses decrease. The obtained features of the behavior of aspen specimens at elevated temperatures should be taken into account when modeling deforming wood.

Mathematical model and numerical calculations of disastrous pressure phenomena in rock mass with weakening cavity

The study focuses on deformation and stability of structurally nonuniform rock masses in the vicinity of a mined-out void. Rock masses can for a long time accumulate energy of external forces as internal stresses. The accumulated energy can release both as aseismic relaxation of stresses and as rock bumps and rock bursts. The numerical analysis uses the rock mass model including self-balancing stresses and strength loss. The paper gives examples of calculations of standard problems in mining and illustrates feasibility of modeling disastrous pressure phenomena.

RHEOLOGICAL MODELING OF THE STRESS-STRAIN IN FLAT COMPACTION OF COMPOSITE MATERIALS

Постановка задачи. В статье исследуется деформационное поведение композиционного материала в процессе его плоского прессования. Для решения этой проблемы предложена реологическая модель, в основе которой лежат явления, протекающие в вязкой (ньютоновской) несжимаемой жидкости, занимающей объем между двумя сближающимися с малой скоростью абсолютно жесткими параллельными плоскостями конечных размеров прямоугольной формы. В рамках механики сплошной среды в условиях плоского деформированного состояния решается задача в двух измерениях о медленном течении в отсутствии объемных сил и инерционных эффектов. При этом решение уравнения движения с условиями неразрывности сводится к известному уравнению Лапласа. Кроме того, на основе модели линейной вязкоупругости и одноосного напряженного состояния предпринята попытка описания релаксационных явлений, протекающих в затвердевающем композите по окончании процесса активного прессования. Результаты и выводы. Получены аналитические зависимости силовых параметров напряженно-деформированного состояния прессуемого композита; получены соотношения для кинематических характеристик процесса прессования; получено выражение для релаксации напряжений в процессе технологической выдержки материала под давлением после окончания активного прессования. Результаты исследования позволяют экспериментально определять численные значения динамического коэффициента вязкости и времени релаксации напряжения, которые являются важными характеристиками при управлении процессами прессования. Statement of the problem. The article investigates the deformation behavior of a composite material in the process of its flat pressing. To solve this problem, a rheological model is proposed which is based on the phenomena occurring in a viscous (Newtonian) incompressible fluid that occupies the volume between two absolutely rigid parallel planes of finite dimensions of rectangular shape approaching at a low speed. Within the framework of mechanics of a continuous medium under conditions of a plane deformed state, the problem is addressed in two dimensions about a slow flow in the absence of volume forces and inertial effects. In this case, the solution of the equation of motion with continuity conditions is reduced to the well-known Laplace equation. In addition, based on the model of linear viscoelasticity and uniaxial stress, an attempt has been made to describe the relaxation phenomena occurring in the solidifying composite at the end of the active pressing process. Results and conclusions. Analytical dependences of the power parameters of the stress-strain of the compressed composite are obtained; relations for the kinematic characteristics of the pressing process are identified; an expression is designed for the relaxation of stresses during the technological holding of the material under pressure following the end of active pressing. The results of the study make it possible to experimentally determine the numerical values of the dynamic coefficient of viscosity and stress relaxation time which are important characteristics in controlling the pressing processes.

Relaxation model of dynamic deformation of elastic-plastic media

A variant is considered for the relaxation model of a loaded elastic-plastic medium with dislocation kinetics of plastic shearing. The model is formulated in rates and includes two independent strain rates: total strain rate, which corresponds to the rate of external action, and local rate of plastic response of the material, which represents the ability of the medium to generate strain-induced defects. This makes it possible to describe both local relaxation processes in the elastic-plastic medium and average relaxation of stresses in a loaded specimen. The model being developed amounts to microscopic ones. All model parameters are determined from independent experiments for the evolution of the dislocation continuum during loading of macroscopic specimens. The model provides an adequate description of the dynamic effects of the macroscopic response of materials depending on the strain rate: the upper and lower yield points (yield drop, yield plateau), subsequent strain hardening as well as features of cyclic and alternating loading, ideal and nonideal Bauschinger effect.

Формирование бикристаллических пленок ZnO на ромбоэдрической плоскости сапфира при высоких скоростях роста

A route for the formation of a ZnO bicrystal film using the features of the magnetron sputtering method and the orienting action of the rhombohedral sapphire plane is proposed. It is shown that, when two deposition modes with with growth rates of ~ 2 and ~ 16 nm/s are used consistently, a bicrystalline ZnO film with a bottom (110)-oriented sublayer and a top (002)-oriented one is formed. Recrystallization annealing of the film at 1000°C for 10 h does not affect the top (002)-oriented layer and leads to relaxation of stresses in the bottom (110)-oriented layer.

Evaluation of the rheological properties of photopolymers used in Polymer Jetting technology

The paper presents the results of tests related to relaxation of stresses in models of samples made using the technology of photo-curing of liquid polymer resins polyjet modeling (PJM). The models of the samples were made using two types of materials: VeroWhite and FullCure 720. The tests were performed on samples in compliance with the PN-EN ISO 3384 standard. The technological parameter whose impact was analyzed was the orientation of the models on the virtual building platform, which was defined with three variability levels (0°, 45°, and 90°). The tests demonstrated that the position of the models on the building platform has a significant impact on the relaxation of compressive stresses. Moreover, the most advantageous positions of the models on the building platform from the standpoint of relaxation of stresses were determined.