Long-Term Strength Prediction of Wood Based Composites Using the Kinetic Equations

The existing behaviour models of the structures under constant load (creep) have a fairly wide forecast horizon and low accuracy. As a rule, they consider the transition from an undestroyed state of an element to a destroyed one, in one stage. The purpose of this study is to substantiate and develop a new approach to predicting long-term strength based on kinetic equations, which, in turn, should consider the multistage nature of the process of gradual destruction of structure elements. To achieve this purpose, the study solves the tasks of creating a multistage kinetic transition of individual structure elements from an initially elastic state to a viscoelastic state, and then to a fractured state. When describing this process, the authors employed the methods of formal kinetics and the theory of continuum damage mechanics, including the method of basic diagrams. Wood-based composites were used as the materials under study. Based on the results of the conducted full-scale and computational experiments, the study discovers that a mathematical model based on kinetic equations adequately describes the behaviour of the materials under study for long-term strength; the proposed two-stage model determines the forecast horizon much more accurately than the available one-stage models. The kinetic parameters that determine the rate of transition of a structural element from an elastic state to a viscoelastic state, and then to a destroyed state, were determined based on experimental base chart. The time to fracture was determined at three-point bending at a load equal to 70% of the flexural strength at temperatures of 20°C and 60°C, constant humidity RH 65% and moisture content MC 8%. When building control charts, the load increased by another 15%. The method allows narrowing the forecast horizon and determining the moment of transition of a structure from a stationary state to a blow-up regime with a higher accuracy

Predicting the Healthy Operation of Heavy Oil Well Casings in Permafrost Regions

This study explores the distribution of stress and deformation on casings in heavy oil recovery wells and the distribution of stress in the thaw bulb in permafrost areas. Considering the expansion of the thaw bulb, the simulation analysis method is used to explore the internal mechanisms of vertical settlement displacement development and stress redistribution within thawed soil and casing. Calculation results show the following: (a) The maximum settlement of the thawed soil and the casing was positively correlated with the expansion of the thaw bulb. Although the settlement of the thawed soil was greater than that of the casing, the initial increase in maximum settlement difference between the thawed soil and the casing eventually tended to be constant due to stabilization of the thaw bulb’s expansion. (b) The size of the thaw bulb directly affects the redistribution of internal stress in thawed soil, leading to different distribution rules for the vertical displacement of thawed soil and casing with depth. (c) Beyond a certain formation depth, the vertical stress of thawed soil gradually transits from a tensile stress state to a compressive stress state. The depth of a soil layer whose horizontal stress value is initially greater than its vertical stress value will gradually deepen with an increase in thaw bulb radius. (d) There is no significant negative friction on the lateral wall of casing in yield state, but significant negative friction exists on the lateral wall of casing in elastic state. The vertical stress of casing in elastic state increased gradually with the increase of casing depth, due to the existence of continuous negative friction and dead weight.

Physicochemical modification of heat-shrinkable epoxy polymers

An approach to the physicochemical modification of heat-shrinkable epoxy-diane polymers was considered, these polymers being used as couplings for the repair of polymer pipelines for various functional purposes. The purpose of the modification is to stabilize and improve the performance of the end couplings that are heat-shrinkable. We assessed the prospects of preparation of the products of various profiles by forming cross-linked polymers in a highly elastic state by plunger extrusion via creating favorable conditions for the orientation of interstitial fragments in epoxy-diane polymers. The starting epoxy-diane composition contained rigid and elastic components. The polymers fabricated by hardening of these compositions have both a glass transition temperature, which is convenient for operation, and high deformability in glassy and highly elastic states. We investigated the tensile strength, the elastic modulus, the failure deformation and the flaring deformation of the inner diameter of the preform of epoxy-diane polymers. Physical modification of a liquid filled epoxy-diane composition before mixing with a hardener was performed by using low-frequency ultrasonic treatment. We analyzed the results associated with the effect of combined ultrasonic treatment on the physical-mechanical and service properties of heat-shrinkable epoxy-diane polymers filled with short glass fibers.

ANALYSIS OF THE MECHANISM OF REINFORCEMENT OF THE ROAD STRUCTURE WITH A UNLOADING SECTION

The problem posed is about the mechanism for reinforcing the road structure with a slab with an unloading cut. The reasons for the appearance of transverse cracks in the upper layer of the road surface in winter have been investigated. Solvable equations are formulated, their finite element analogs are constructed. Cases of loads at different values of elastic module and plate sizes are considered. At low temperatures, the upper layers of the road try to reduce their length, however, due to their almost infinite length, they cannot do this and transverse disordered cracks appear in the upper layer, which, due to the action of transport loads and the forces of frozen water, lead to further delamination and destruction of the road structure. Therefore, the introduction into another layer of a reinforced slab made of a stiffer and more durable material that is capable of absorbing increased stresses allows the upper layer to shorten and lengthen freely and removes the arising longitudinal thermal stress. Due to the fact that this plate is located in the zone of stress concentration, these stresses become less dangerous. KEYWORDS: AUTOMOBILE ROAD, ASPHALT-CONCRETE COATING, LOADING RISK, TRACKED TRACKS, TRANSPORT LOADS, STRESS FIELD, THERMO ELASTIC STATE

Stresses in the experiments with loading tubular specimens by internal pressure

When using different formulas for determination of axial and circumferential stresses in the experiments on loading thin-walled tubular specimens with internal pressure the radial stresses are neglected due to their smallness. We propose a novel procedure for determining stresses in the internal pressure loaded thin-walled tubular specimens. The distribution of stresses in the radial direction of a tubular specimen is studied both for the elastic state and for perfectly plastic state according to the Huber – von Mises criterion of an incompressible material. It is shown that the degree of heterogeneity of the stress state depends on the ratio of the wall thickness to the specimen diameter and on the elastic or plastic state of the material. The circumferential stresses are maximal on the inner surface of the specimen and the axial stresses are constant along the radius of the specimen in the elastic state, whereas in the plastic state circumferential and axial stresses are maximal on the outer- and inner surface of the specimen, respectively. The distributions of radial stresses in the elastic and plastic state of the material are almost identical, i.e., both are maximal on the inner surface and equal to zero on the outer surface of the specimen. The values of circumferential and axial stresses on the middle surface of a thin-walled tubular specimen normalized to the internal pressure almost do not depend on the elastic or plastic state of the specimen material thus providing a basis for determination of the mechanical properties of the material from the stress-strain state of the middle surface of the specimen using the Lame formulas for stress calculations. When determining the stress intensity, it is desirable to take into account the radial stresses, since it increases the accuracy of determining the mechanical properties of the material and reduces the sampling range of the yield point for different types of the stress state.

A METHOD OF BOUNDARY STATES IN A SOLUTION TO THE FIRST FUNDAMENTAL PROBLEM OF THE THEORY OF ANISOTROPIC ELASTICITY WITH MASS FORCES

The purpose of this work is to determine the stress-strain state of the anisotropic bodies of revolution exposed to axisymmetric surface and mass forces. The problem is solved using the method of boundary states. A theory for the construction of space bases of the inner and boundary states conjugated with isomorphism is developed. Determination of the internal state is reduced to a study of isomorphic boundary state. The elastic state components are represented as Fourier series with quadrature coefficients. In the first fundamental problem of mechanics, determination of the elastic state is reduced to the solution of an infinite system of algebraic equations.A particularity of this solution is that the pattern of the determined elastic field satisfies both the conditions specified at the boundary and inside the body. A rigorous solution to a test problem for a circular cylinder, as well as a solution to the problem with inhomogeneous boundary conditions is presented. An elastic field is found in the problem for the non-canonical body of revolution exposed to mass forces and zero boundary conditions. The explicit and indirect indicators of problem solution convergence and a graphical visualization of results are shown.

The Role of Phonons in Stability of Metallic Nanoparticles

A novel model of a metallic nanoparticle is proposed. The model represents the nanoparticle as a hollow sphere. The existence of a nanohollow inside a particle is related to the behavior of phonons. The strain in the walls of a particle when it's heated is calculated utilizing the theory of a thick-walled shell. We show that in the transition to the elastic state, the expansion strains at the surface of a nanoparticle become sufficient for melting. The dependency of melting temperature on the size of a nanoparticle is discussed.

The incremental plasticity condition application for pure bending of a curved bar (Golovin problem)

The limiting elastic state of a bar with large curvature under pure bending is considered on the basis of the elasticity theory exact solution. As a criterion for the limiting state, the gradient plasticity condition is used, which determines the yield onset in an inhomogeneous stress state. Analytical expressions for calculating the corresponding stresses are obtained. The results comparison with a simplified solution is given.