Creep Theory Research Articles

Rolling contact between a rubber ring and rigid cylinders: Mechanics of rubber belts

We revisit the classical creep theory for a belt drive system as a rolling contact problem between a rubber ring and rigid cylinders to evaluate the effects of linear viscoelasticity and large strain of the belt on the system in a steady state. We first construct the general theory of belt mechanics for a multi-pulley system by dimensional reduction from 3D extensible, unshearable rod. We then develop the belt mechanics for a linear viscoelastic belt and show that the effect of retardation time can be involved in the effective friction coefficient: the classical Euler's belt formula can be recovered. Next, we assume that the belt is a neo-Hookean material and examine the effect of large strain of the belt on the contact pressure distribution. We compare the theory with the results by finite element analysis and show that the theory provides good quality.

Study of Material’s Creep Relaxation Capability

Creep relaxation is an important form of system failure, how to understand and analyze the effect of creep relaxation, to solve system failures and improve economic efficiency are of great significance. The paper describe researches of creep relaxation , mathematical models and theories at home and abroad in detail, and introduce the finite element method applied in this field. By comparing with the mathematical models, finite element Methods with test results, improving current mathematical models and utilizing finite element method is an important trend.

Universal Scaling Analysis on Vortex-Glass State of High-Temperature Superconductor ${\rm HgBa}_{2}{\rm Ca}_{2}{\rm Cu}_{3}{\rm O}_{8+\delta}$

The collective creep theory indicates that a vortex-glass behavior in a system of superconductors shows the similar I-V curves, which suggests that if there is a second-order phase transition to this vortex-glass, which is similar to spin glasses in magnetism, both the characteristic correlation length ξg and the relaxation time τg of the fluctuations of the glassy order parameter diverge at the glass-transition temperature Tg. In this study we deduced based on Π theorem a scaling function about the temperature dependence of resistance in region around Tg. A simple method to describe the ρ-T relationship is therefore obtained. This scaling function applies successfully to the HgBa2Ca2Cu3O8+δ (Hg-1223) and the iron-based NdFeAsO0.7F0.3 superconductors.

Strong pinning and elastic to plastic vortex crossover in Na-doped CaFe2As2single crystals

We study the vortex dynamics of NaxCa1-xFe2As2 single crystals with x = 0.5 (underdoped) and x = 0.75 (optimally doped), having T-c approximate to 19.4 and 33.4 K, respectively, by performing magnetization measurements of the critical current density J(c) and flux creep rate S. We find that the J(c) versus temperature, T, dependence is consistent with delta T-c pinning, indicating strong pinning associated with randomly distributed defects larger than the coherence length xi. The temperature dependence of S shows a crossover between glassy (elastic) and plastic creep regimes. The boundary T-cr(H) between both creep regimes coincides with the upper limit of the regime of strong pinning by nanoparticles. The glassy exponent mu in the optimally doped crystal is consistent with the thermal collective creep theory previously applied to cuprate superconductors, but in the underdoped sample the plateau in S(T) indicates that mu approximate to 3-3.3, a value larger than the existing theoretical predictions. We discuss the quantum creep contributions in both samples.

Creep deformation and rafting in nickel-based superalloys simulated by the phase-field method using classical flow and creep theories

A phase-field model has been developed to simulate the evolution of both (γ+γ′) microstructure and inelastic strain between γ′ phases (i.e. γ channel) during high-temperature creep in nickel-based superalloys. Inelastic strain is defined as the sum of time-independent and time-dependent components. Previously reported mechanical properties of single-phase γ alloys are considered in the calculation of inelastic strain evolution. A two-dimensional phase-field simulation is performed, and the results of microstructure evolution and the creep rate vs. time curve are fitted to the experimental data of the high-temperature creep of CMSX-4. The slope of the creep rate vs. time curve during the initial stage of transient creep, the plasticity preference in different types of γ channels, and the rafting phenomenon are reproduced well by the simulation. Furthermore, it is demonstrated that the creep rate increases locally at γ/γ′ interfaces when the rafted structure is formed.

Energy-based theory of creep in the pressure treatment of metals

Energy-based theory of creep in the pressure treatment of metals

Edge fixing effect on the life of a vacuum chamber thin spherical cover subjected to creep damage

Within framework of the numerical studies of creep resource of a thin spherical cover of a vacuum chamber, we present mathematical formulation and the calculation method for the solution of the initial boundary value problems of the creep theory of thin shells with account of the damage accumulation process of the material. The effect of edge fixing along the normal and tangential lines to the median surface, as well as angular edge fixings, on the cover life in creep are studied under atmospheric pressure creep conditions. The calculation data obtained made it possible to determine the dependence between the cover life in creep and its edge fixing conditions: this effect is strong by the latent fracture time and weak by the allowable deflection values.

Prediction of the corrosion cracking of structures under the conditions of high-temperature creep

On the basis of the continual model of corrosion crack growth proposed earlier and the well-known incremental-type creep theory, we make an attempt to predict the corrosion cracking of structures under the conditions of high-temperature creep. We propose the mathematical statement of the problem taking into account the influence on corrosion cracking of the properties of corrosive media and the redistribution of stresses in time caused by creep. The Bubnov–Galerkin method is applied for the solution of this problem. An example of prediction of the phenomenon of corrosion cracking in the case of creep of a pipe under the action of internal pressure is analyzed.

Versions of the theory of inelasticity

Several versions of the theory of inelasticity (thermoviscoplasticity), which is a generalization and development of the ideas contained in various versions of the theory of plasticity, creep, inelasticity, and damage accumulation, are considered. The ability of various versions of the theory of inelasticity to provide an adequate description of processes of inelastic strain and fracture of structural steels and alloys under various loading conditions are illustrated.

Taking into account grain growth during the finite-element modeling of the superplastic formation of promising construction materials

A way to take into account the influence of grain growth in the statement of the edge problem for the mechanics of a deformed solid as applied to the superplastic formation of microcrystalline and ultra-fine-grain materials is suggested. The concrete example of a numerical solution for the edge problem of creep theory in a medium of the ANSYS software complex according to the published experimental data is suggested. A satisfactory agreement between the results of the solution to the edge problem and the experimental data is obtained. This solution is analyzed, and certain features of the character of the stress-strained state in the deformation site are revealed. In particular, it is established that complex disproportionate loading occurs in the vicinity of the fastening contour. Under this loading, the stress trajectories in the Il’yushin space have a larger curvature, despite the fact that the curvature of the corresponding deformation trajectories in the deformation space is relatively small.

Model of viscoplasticity with parallel deformation mechanisms. Part 2. Creep and ratcheting

A model of viscoplastic deformation for strengthening materials is considered within which metal deformation is governed by two independent (parallel) mechanisms. Each of these mechanisms corresponds to anisotropic creep theory with linear strengthening into which an isotropic strengthening parameter is introduced common for both mechanisms. The concept of surface flow is not used. In the second part of the article, unilateral strain accumulation (ratcheting) is considered in structures at elevated temperature and with cyclic loading. The possibilities of the model are demonstrated with description of experiments on austenitic stainless steel specimens with different programs for a change in load.

Vibration simulation of a multi-story high-speed railway station

Station is an important building in high-speed railway, and its vibration and noise may significantly affect the comfort of waiting passengers. A coupling vibration model for train-structure system is established to analyze and evaluate the vibration level of a typical waiting hall under dynamic train load. The motion of a four-axle vehicle with two suspension system is modeled in multi-body dynamics with linear springs and dampers employed. The station is modeled as a whole finite element structure which is 113 m in longitudinal and 163.5 m in lateral, and the stiffness of the station foundation is considered. According to the assumptions that both wheel and rail are rigid bodies and keep contact to each other in vertical direction, and the wheel/rail interaction and displacement coordination in horizontal direction is defined by the simplified Kalker creep theory, the vehicle spatial vibration model has 27 degrees-of-freedom. An overall analysis procedure is made of the train moving through the station, by which the dynamic responses of the train and the station are calculated. According to the comparison between analysis and test results, the actual connection status between different parts of the station is estimated and the vibration level of the waiting hall is evaluated.

Model of viscoplasticity with parallel deformation mechanisms. Part 1. Constitutive equations and choice of calculation coefficients*

A model of viscoplastic deformation of strengthening material is proposed. It is assumed that metal deformation is determined by two independent (parallel) mechanisms. Each of these mechanisms corresponds to anisotropic creep theory with linear strengthening, within which an isotropic strengthening parameter is introduced for both mechanisms. The concept of surface flow is not used. The model describes material behavior at both normal and elevated temperature when thermal recovery effects develop. Results of standard tensile and creep tests are used in order to determine model parameters. The order of determining model parameters from standard tensile and creep tests is provided. Features of austenitic stainless steel cyclic strengthening are considered. An algorithm is given providing implementation of the proposed viscoplasticity theory in structural design using FEM computer packages.

Dynamic analysis of a train-bridge system under multi-support seismic excitations

A numerical solution for the dynamic responses of a train-bridge interaction system subjected to multi-support seismic loads was studied. The train vehicle was modeled by the rigid-body dynamics method, and the bridge was modeled by the finite element method. The vertical and lateral wheel-rail interaction forces were defined according to the wheel-rail corresponding assumption and the simplified Kalker creep theory. Three-dimensional seismic accelerations were incorporated using the large mass method. In a case study, the dynamic responses were simulated for a high-speed train traversing a steel truss cable-stayed bridge with different seismic intensities and different train speeds, and train safety was evaluated.

Subcritical crack growth in an aging plate with variable elastic modulus

The paper addresses subcritical growth of a crack in a thin isotropic plate made of an aging viscoelastic material with time-dependent elastic modulus. The behavior of the material is described by Arutyunyan’s creep theory. To simulate fracture, a modified Leonov-Panasyuk-Dugdale model and a critical crack opening displacement criterion are used. An equation describing the subcritical growth of the crack is derived assuming that Poisson’s ratio is constant. As an example, the critical loads are determined, and curves of subcritical crack growth are plotted for a specific material. The results are compared with the case of constant elastic modulus

NOVEL PHASES OF VORTICES IN SUPERCONDUCTORS

An overview is given of the new theories and experiments on the phase diagram of type II superconductors, which in recent years have progressed from the Abrikosov mean field theory to the "vortex matter" picture. We then detail some theoretical tools which allow to describe the melting of the vortex lattice, the collective pinning and creep theory, and the Bragg glass theory. It is followed by a short presentation of other glass phases of vortices, as well as phases of moving vortices.

Justification of the energy variant of the theory of creep and long-term strength of metals

The hypotheses underlying the energy variant of the theory of creep and long-term strength of metals are formulated and justified experimentally.

Rabotnov’s two-layer model of a shell and critical time of shell buckling during creep

The main propositions of Rabotnov’s two-layer model of a shell are given. It is shown that the Rabotnov’s functional can be obtained from the mixed variational principle of creep theory. The notion of critical time is introduced and a procedure for obtaining an explicit formula for it using a variational equation is described.

Effect of tangent track buckle on vehicle derailment

In order to investigate the effect of a tangent track buckle on the dynamic derailment of a railway vehicle, a coupled vehicle/track dynamics model is developed, in which the vehicle is modeled as a 35 D.O.F. multibody system and the track is modeled as a 3-layer discrete elastic support model. Rails are assumed to be Timoshenko beams supported by discrete sleepers, and the effects of vertical and lateral motions and rolling of the rail on the wheel/rail creepages are taken into account. The sleepers are treated as Euler beams on elastic foundation for the vertical vibration, while as lumped masses in the lateral direction. A moving sleeper support model is developed to simulate the effect of the periodical discrete sleepers on the vehicle/track interaction. The vehicle and the track are coupled by wheel/rail contacts whereas the normal forces and the creep forces are calculated using the Hertzian contact theory and the nonlinear creep theory by Shen et al., respectively. The equations of motion of the coupled vehicle/track system are solved by means of an explicit integration method. A tangent track buckle is simulated with a cosine function, which describes the misalignment of the track with different lengths due to its buckling. In the analysis the effects of the buckle wavelength and amplitude and of the vehicle speed on the dynamic behavior of the coupled vehicle/track system are considered. The present paper analyzes in detail the conventional derailment coefficients which include the ratio of the wheel/rail lateral force to the vertical force, the wheel load reduction, and the new criteria indicating the wheel/rail contact point traces and the wheel rise with respect to the rail. These criteria are simultaneously used to evaluate the risk of derailment of the whole vehicle. The numerical results obtained indicate that the track misalignment caused by the buckle and the vehicle speed have a great influence on the whole vehicle running safety when the vehicle passes through the buckled tangent track.

A VEHICLE–BRIDGE LINEAR INTERACTION MODEL AND ITS VALIDATION

A linear wheel–rail interaction model for the vehicle–bridge coupling system is presented in this paper. The vehicle subsystem is modeled by the rigid-body dynamics method, the bridge subsystem by both the direct stiffness method and superposition method, and the wheel–rail interaction by the "corresponding assumption" and simplified Kalker creep theory. Based on the above modeling for the wheel–rail interaction system, linear simultaneous equations are established and solved. As a case study, the response of the Pioneer train traversing a 24 m simply–supported girder bridge is calculated. The proposed analysis procedure is validated through comparison of the calculated results with the measured ones. Also, some vibration characteristics of the system are discussed.