Thermofluctuation Theory Research Articles

ФОРМИРОВАНИЕ МАРШРУТА МАГИСТРАЛЬНОЙ ТРЕЩИНЫ РАЗРУШЕНИЯ В МНОГОУРОВНЕВОЙ СТРУКТУРЕ КОНГЛОМЕРАТНЫХ СТРОИТЕЛЬНЫХ КОМПОЗИТОВ

In the publication, conglomerate building composites (concrete) are identified as heterogeneous solids with a hierarchically organized spatial-geometric structure with a characteristic dimension from 10-10 to 10-1 m, with a minimum of 5-6 scale levels and three types of substructure design that differ in scale, genesis and mechanics of properties manifestation. The first type is characteristic of the macro-, meso- and microscale levels and is taken in the form of a two-component “construction” of a spatially continuous matrix and discrete solid and gaseous (macropores) inclusions deterministically and stochastically distributed in it; the second type refers to the submicro-, ultra-micro- and nanoscale levels and is believed to be in the form of a “microscale spatial structure” of new formations of a cementitious substance from consolidated individual crystalline differences; the third type, finally, corresponds to the atomic-molecular structure of new formations of the cementing substance. Characteristics of the distinguished types of substructures are given according to the scale of their components, the peculiarities of formation, the mechanics of manifestation of properties, design criteria and means of synthesis of each substructure. The patterns of formation of the fracture route in substructures of all types and substances of each scale level are analyzed. In this case, the development of the stress-strain state of the conglomerate composite according to the principle of energy dissipation, localization and increase (concentration) of stress is realized in the direction from the macro- to the atomic-molecular level of the structure of the composite, and the destruction itself and, accordingly, the formation of the crack route in time and in space of the composite passes in the direction from the atomic-molecular level to the macrolevel in a cascade through all intermediate structural levels. Within the framework of an integrated mechano-physico-chemical approach, the place of thermofluctuation theory (fracture physics) at the stages of breaking single atomic-molecular bonds and crack mechanics at the stages of development of micro- and macrodamage is shown. The possibilities of using theoretical principles of crack route formation to formulate and solve practical problems of designing and synthesizing optimal structures of conglomerate building composites are discussed.

ТЕРМОФЛУКТУАЦИОННАЯ ТЕОРИЯ РАЗРУШЕНИЯ И ОЦЕНКА ДОЛГОВЕЧНОСТИ ЭЛЕКТРИЧЕСКОЙ ИЗОЛЯЦИИ СПЭ КАБЕЛЕЙ

The article is devoted to research on the creation of a method for assessing the durability ofthe main insulation of power cable lines based on the thermofluctuation theory of destruction of solids.The features of breakdown development in homogeneous and inhomogeneous dielectrics areconsidered. The problematic issues of insulating materials of power cable systems (PCS) are consideredon the basis of the development and development of non-destructive testing methods. The maincomponents that destroy the insulation of the PCS are installed. The thermofluctuation theory ofdestruction and the evaluation of the durability of electrical insulation of power cables are described.An analytical method is proposed for evaluating the processes of cellulose oxidation with the determinationof the most probable. The proposed approach to identifying the main processes of insulationdestruction allows us to present in more detail the prerequisites for the development of partial discharges(PD) in isolation and to determine measures to minimize the processes leading to the destructionof PCS insulation. The results of experiments on the study of the service life of the insulatingmaterial of aged ends of a power cable in a thermal cabinet, to determine the aging criteria, arepresented. The experimental data are interpreted from the standpoint of the thermofluctuation theory.The dependence of the increase in the temperature difference ΔT on the degree of wear of the electricalinsulating properties of the material is revealed and investigated, which makes it possible to predictits resource. The thermofluctuation theory of the destruction of insulating materials caused bythe thermofluctuation rupture of chemical bonds and the evaluation of the durability of electricalinsulation of cables made of cross-linked polyethylene are considered. It is shown that, in order tocalculate the average energy of the monomeric link of various polymer materials, it is necessary toknow: the chemical formula of the monomeric link and the values of the binding energies. In thework, the time before the breakdown of the main insulation of the PCS was determined under theaction of temperature. The model can be used in devices and systems for continuous diagnosis ofpower cables according to temperature conditions.

Thermal destruction of polymeric fibers in the theory of temporary dependence of strength

Objectives. This study mathematically describes the mutual influence of micro- and macrostages of the process of destruction of polymer materials and determines its main parameters and limiting characteristics. In addition, a relationship is established between molecular constants characterizing the structure of a material and those characterizing its macroscopic characteristics of strength. Finally, theoretical representations of the thermokinetics of the process of thermal destruction of polymer fibers from the standpoint of the kinetic thermofluctuation concept are developed, which makes it possible to predict the thermal durability of a sample under thermal loading.Methods. The structural–kinetic thermofluctuation theory was used to describe the initial stages of the fracture process and to derive a generalized formula for the rate of crack growth. The mathematical theory of cracks is used to describe the thermally stressed state of a material in the vicinity of an internal circular crack under mechanical and thermal loadings of the sample.Results. A theoretical formula for the full isotherm of durability in the range of mechanical stresses from safe to critical, as well as a theoretical relationship for the time dependence of the strength of polymer fibers under purely thermal loading in the full range of heat loads from safe to critical and at the stage of nonthermal crack growth, is given. The main parameters and limiting characteristics of durability under thermal loading are also indicated.Conclusions. A generalized structural–kinetic theory of the fracture of polymer fibers under purely thermal action on cracked specimens is presented. The developed theory combines three independent approaches: structural–kinetic (thermofluctuation theory), mechanical, and thermodynamic. The obtained theoretical relations are of practical interest for the development of methods for localization, intensification, and control of the crack growth kinetics.

Creation of theoretical bases of tribotechnologies of running-in and restoration as means of effective increase of operational wear resistance of motor transport and mobile agricultural machinery

The bases for creation of theoretical bases of tribotechnologies of running-in and restoration of conjugations of details of systems and units of motor transport and mobile agricultural machinery are defined. The specifics of selective transfer on the surface of contacting parts of machines and creation of servito films, formation of coatings during running-in and restoration from antifriction materials, which are a part of additives in motor and transmission oil, are considered. A number of tribophysicochemical processes that occur in the conjugations of parts materials and how they affect the creation of tribotechnologies of running-in and restoration are clarified. The creation of tribotechnologies using geomodifiers is considered. It is proposed to build a single theory of tribotechnologies of running-in and recovery on the basis of the mechanism of triboplasm formation in the conjugations of parts of systems and units of machines. It is also proposed to add to this the thermofluctuation theory of S.M. Zhurkova taking into account the change of dilaton and compression bonds of atoms of materials of tribocouples of details with creation of local areas of deformations of compression and tension and zones of thermoplastic deformation.

Modeling of Crack Self-Healing Kinetics

A model based on the concept of interface crack bridging and thermofluctuation theory is proposed for analyzing the efficiency of crack self-healing in composite materials in view of three main stages: (1) crack initiation and growth under external load, (2) self-healing activation, and (3) self-healing with partial or complete bond recovery between crack faces. A system of singular integro-differential equations is derived for numerical estimations of bond tractions in bridging zones during their formation and stress intensity factors (major characteristics of crack self-healing efficiency) with account of external loads and bond tractions. Kinetic estimates are presented for the time of bond recovery during the formation of a crack bridging zone. The model is applied to an interface crack between two dissimilar materials with a self-healing adhesive.

Method and results of assessment of the performance of friction pairs with solid lubricating coatings under various operating conditions

An algorithm for the assessment of the durability of friction pairs with solid lubricating coatings under normal atmospheric and vacuum conditions is proposed. It is based on Zhurkov’s formula, which explains the destruction of the binding components from the viewpoint of the thermofluctuation theory. The dependences of the activation energy of the destruction of solid lubricating coatings with different compositions on the frictional temperature and the frictional temperature on the load and velocity parameters of the friction pair are determined to implement the proposed method. Design of experiments is used to obtain the dependences of the friction coefficient of the pairs with a VNII NP 212 solid lubricating coating on the pressure and sliding velocity under normal atmospheric and vacuum conditions. Verification of the models confirms their adequacy.

Influence of high strain rates on stress–strain relationship, strength and elastic modulus of concrete

Abstract Earlier papers showed a method for strength evaluation at high strain rate from the results of static tests, and a method for evaluating modulus of elasticity at static loading from tests based on stress-wave propagation. This paper presents new results on the influence of high strain rates on the stress–strain relationship, strength and modulus of elasticity using the thermofluctuation theory, principle of accumulation and development damages as well as nonlinear behavior of concrete under loading. Obtained equations explain and unify the influence of high rates on the mechanical properties of concrete for different kinds of uniaxial stress. Comparative study showed again substantial agreement with existing experimental results and general equations by the Comite Euro-International du Beton (CEB) Model Code. The developed approach diminishes the labor expenses for tests of concrete under high rates of loading.

Comparison of optical properties and mechanical/electrical degradation of some low density polyethylene-and polypropylene-based polymer blends

In this study, the optical properties as well as mechanical and electrical degradation of low-density polyethylene (LDPE)/polypropylene fiber (PP fiber) (10–50% PP fiber), polypropylene (PP)/PP fiber (10–50% PP fiber), and LDPE/diamond (0.1–3% diamond) blends, which are prepared by hot pressing method, with changing thicknesses ranging from 30 to 225 µm, are compared. The spectra, in the wavelength range 200–2500 nm, are examined. Based on optical absorption spectra obtained, Tauc graphs are plotted. Determined values of the direct optical energy gap (Edopt), the indirect optical energy gap (Eiopt), the width of the band (ΔE), and ultraviolet transmittance (TUV) are listed. The direct Edopt and indirect Eiopt values for organic blends are in the range of 3.10–3.17 eV and 1.52–2.99 eV; for inorganic blends they are 1.80–4.13 eV and 1.55–4.7 eV respectively. The electrical strength (e) and the mechanical tension (σ) have been investigated, and graphs (the dependence of the electrical life time log τe on e) are given. The experimental results are analyzed from the viewpoint of the validity of the thermofluctuation theory. LDPE and LDPE/0.5% diamond composite parameters consecutively changed: σ from 68 to 82 MPa, e from 60·106 to 85·106 V/m, mechanical lifetime τσ from 10 to 1.5·105 sec, electrical lifetime τe from 2· 103 to 2·105 sec, and structure-sensitive parameters γ and χ — from 1.48 to 1.18 (J)MPa/mole and from 0.97 to 0.70 (J)Vm−1/mole respectively. The values of mechanical and electrical durability were observed to increase by 20 and 41%, respectively, for LDPE/0.5% diamond composite.

Influence of network topology on mechanical properties of network polymers

We studied theoretically how the network topology influences the mechanical properties of polymers. We used conclusions of thermofluctuation theory of fracture and graph theory. The long-term strengths of monofunctional and polyfunctional networks were compared. The cross-link functionality distribution of the polyfunctional networks is a power function. All other conditions being equal, the long-term strengths of the polyfunctional polymer networks are some three to four times the long-term strengths of the monofunctional networks.

Temperature and time dependence of electrical and mechanical durability of LDPE/diamond composites

In this study, the dependence of the electrical ( τ E), mechanical ( τ σ ) life time on electrical strength ( E) and mechanical tension ( σ) of low density polyethylene composites with commercial diamond additive (LDPE/diamond) at different temperatures are investigated and compared with those of pure LDPE. 50–100 μm thick samples with the additive amount varying between 0.1–3% were prepared by using hot pressing method. log τ E− f( E, T), E versus diamond additive percentage are drawn and the determined values of mechanical degradation parameters at constant temperature are listed. Since τ E and τ σ both depend on temperature, electrical degradation mechanism is similar to the mechanical one proving the validity of the thermofluctuation theory. The results indicate that E and σ values reach maximum for 0.5% diamond and are equal to 8.5×10 7 V/m (6.0×10 7 V/m for pure LDPE) and 82 MPa (68 MPa for pure LDPE), respectively which complies with our other studies.

Effects of mechanical and thermal stresses on electric degradation of polyolefins and related materials

AbstractDegradation under the simultaneous effects of mechanical stress and temperature in polyolefins (PE, PP), composites on their basis (PE+PP fibre, PP+PP fibre, PP+glass fibre) and radiation low‐density polyethylene (X‐LDPE) used in high‐voltage cables obeys the thermofluctuation theory of Zhourkov (in certain σ and τ0 intervals) based on the theory of Arrhenius is presented in the following form:τσ = τ0 exp[(U0 − γσ)/ RT] (1)where τ is durability. τ0 is a constant (10−12‐10−13s) equal to period of vibrations of atoms around equilibrium position, U0 is the activation energy of the mechanical destruction process (at σ = 0), γ is a structure‐sensitive parameter, T is absolute temperature and R is universal gas constant. Electric degradation under the effects of electric field and temperature in the materials mentioned above obeys the equation:τE = τ0 exp[(W0 − χE)/ RT] (2)Here, τE, W0 and χ are analogous to τσ, U0 and γ, respectively.It is assumed that the following equation is valid under the simultaneous effects of E, σ and T:τσ,E = τ0 exp[(U0 − (γσ + χE))/ RT]. (3)electric degradation

The comparison of relaxation events with spectroscopic properties of polymer composites with fibrillar dopings on the basis of low density polyethylene and polypropylene

AbstractIn this study, the mechanical and electrical degradation properties of the composites of polypropylene (PP)+PP fibers, low‐density polyethylene(LDPE)+PP fiber, and PP+fiber glass have been investigated and the validity of Zhourkov's thermofluctuation theory has been shown.

Formation of an incipient crack in the impact fracture of a brittle body by a wedge

A study is made of the problem of the penetration of a wedge-shaped tool in rock. It follows from the solution of the problem in the elastoplastic formulation that the continuation of the axis of the wedge in the elastic region includes a zone in which the rock is subjected to tension perpendicular to the axis. In accordance with the kinetic thermofluctuation theory of strength, this determines the formation and propagation of microcracks oriented along the wedge axis. A computer model is proposed to describe the process of random accumulation of microdefects in a loaded solid body. The model is used to create an algorithm to calculate the formation and development of an incipient crack before the vertex of the wedge. The length of the crack is determined by the properties of the medium and the parameters of the penetrated tool.

Nature of impact-abrasive wear of steel

The wear mechanism of steel from the standpoint of the thermofluctuation theory of fracture is considered. A model of the intensity of impact-abrasive wear of steel is presented, taking into account the impact self-action and the thermofluctuation properties of the material. Justification is given for the study of tribotechnical problems including the foundations of strength physics.

Thermofluctuation theory of strength and the Griffith criterion

An analysis of the physical significance of the Griffith criterion σG from the viewpoint of the first law of thermodynamics (thermodynamic approach) and the viewpoint of the thermofluctuation-theory of strength (kinetic approach) makes it possible to conclude that the criterion is not the fracture criterion it is usually thought to be. It is actually a safe stress and, thus, is preferably treated as a criterion of the indestructibility of a material. This conclusion is supported by calculations of the quantity σG (Griffith formula) and the safe σG and critical σc stresses (formulas of thermofluctuation theory) for a thin plate containing transverse edge cracks and loaded uniaxially. Organic glass (polymethylmethacrylate), inorganic glass (silicate plate glass), and a crystalline polymer (capron) are used as examples to show that the value of σG is equal or close to the safe stress σ0 but lower than the critical stress σc by one or two orders of magnitude.

WEAK FIELD MAGNETIZATION AND SUSCEPTIBILITY FLUCTUATION IN Bi2Sr2CaCu2Oy SINGLE CRYSTALS

Some magnetization curves of single crystal of Bi 2 Sr 2 CaCu 2 O y in low magnetic field H ‖ c axis are given. The magnetization loop is getting smaller with the temperature increase and then disappears at temperature even higher than T c . The experimental measurements of the susceptibility in zero-field limit show that the results can be treated with two-dimensional thermo-fluctuation theory.

Limiting characteristics of brittle fracture of polymers

The thermofluctuation theory of the brittle fracture of polymers has developed from a kinetic standpoint taking account of the statistics of bond scission and recombination at the apex of a crack, as well as the extent to which this is influenced by the strain energy of stressed samples. A generalized expression for the growth rate of a surface crack is proposed and the critical characteristics and parameters of the fracture process have been calculated. An explanation of the Griffith experiment on the growth of a surface crack is given.

Fracture of polymers in brittle and quasi-brittle states

A study was made of thermofluctuation processes of breakdown of polymer glasses in brittle and quasi-brittle states, characterized by transition from group fracture of chemical bonds to individual fracture of separate polymer chains. An investigation was made of the application of the thermofluctuation theory of strength of solids to quasi-brittle fracture. Corresponding numerical calculations were made.

A thermofluctuation theory of rock fracture and earthquakes

A thermofluctuation theory of rock fracture and earthquakes