Creep Theory Research Articles

Creep mechanism in ductile dual-phase aluminum alloys with a fully continuous fiber state

To understand the creep behavior of a practical alloy that has a multiphase consisting of a matrix and several types of precipitate phases, the role of individual phases needs to be studied. However, it is difficult to elucidate the contribution of each phase to creep owing to their complex microstructural morphologies and very complex creep behavior. To understand the role of individual phases and the creep behavior, fundamental creep theories for ductile dual-phase (DDP) alloys must be established initially for specimens with simple microstructural morphologies. To apply these theories practically, they must also be experimentally verified. In this study, DDP alloys composed of 99.99 % Al (matrix phase) and an Al–Mg solid-solution alloy (reinforced phase) were fabricated as a fully continuous fiber state by accumulative roll bonding. Tensile and creep tests at elevated temperatures were performed on alloys composed of systematically varying volume fractions of the reinforced phase. Tensile data at 546 K indicated a linear relationship between the maximum stress and volume fraction of the reinforced phase; the lines were also connected to the data from single-phase materials of the matrix and reinforced phase. Thus, the amount of strain in the matrix and reinforced phases was the same as that during the tensile test, and the relationship between the maximum stress and volume fraction of a DDP alloy followed the classical linear law of mixtures (CLLM). The creep data for Al + Al–2Mg (DDP alloy) at 546 K were nearly equal to those calculated using the CLLM. Thus, the matrix and reinforced phases deformed at the same creep rate. Creep tests were also conducted for Al + Al-1M(5ARB), which had a fine-grained reinforced phase. The creep data for Al + Al-1M(5ARB) at 523 K were also a good approximation to the lines representing the data calculated using the CLLM. Hence, although the grain sizes of the two phases were very different, corresponding to different rate-controlling mechanisms, the creep data for a DDP alloy could be analyzed using the CLLM. Consequently, the creep behavior of the DDP alloys was a superposition of the creep behavior in both phases, and the interpretation of the creep data for DDP alloys using the creep theory of single-phase materials would lead to incorrect conclusions. These findings will contribute to a better understanding of the creep behaviors of practical alloys and will aid in the correct estimation of the creep lifetime.

Calculation of reinforced concrete structures: considerations on developing new theory (continued)

Introduction. The authors develop the results of their previously published study in order to build up a nonlinear dissipative theory of concrete compressive strength. The present paper explores possibilities to improve the theory of calculation of concrete and reinforced concrete structures in the light of the accumulated knowledge.Aim. To introduce an approach to developing a nonlinear dissipative theory of concrete compressive strength in the absence of regulatory documents for loading modes, using instead the standard classification of loads.Results. The authors refer to the non-linear hereditary creep theory as the most accurate but time-consuming, and keeping its positive properties, reduce its equation of state to the equation of ageless concrete with its simplicity in application. In addition, the equation of state of concrete is solved in elementary functions and closed form in practically important cases.The overwhelming majority of published scientific data on the behavior of creep curves in the area of reliable performance of reinforced concrete structures indicates smoothness and continuity of the curves. Considering that, the data obtained in short-term tests can be extended to the whole time interval, thus providing possibility for express analyses of the strain properties of concrete.The study estimated the reversibility of creep strain and found out dissipativity of elastic strain. The authors established a nonlinear relation between strain and constant stress and introduced an approach to developing a nonlinear dissipative theory of concrete compressive strength in the absence of regulatory documents for loading modes, using instead the standard classification of loads.Conclusions. The considered experimental data were recognized to provide qualitative information due to their apparent deficit. In order to obtain quantitative evaluation, the experiments should be reproduced a sufficient number of times.

Order–disorder phase transition and elastic-to-plastic vortex creep crossover in a triclinic iron pnictide superconductor (Ca_{0.85}La_{0.15})_{10}(Pt_3As_8)(Fe_2As_2)_5

Vortex matter in layered high-T_c superconductors, including iron-pnictides, undergo several thermodynamic phase transitions due to the complex interplay of pinning energy, thermal energy and elastic energy. Moreover, the presence of anisotropy makes their vortex physics even more intriguing. Here, we report a detailed vortex dynamics study, using dc magnetization measurements, in a triclinic iron-pnictide superconductor (Ca_{0.85}La_{0.15})_{10}(Pt_3As_8)(Fe_2As_2)_5, with a superconducting transition temperature, T_csim 31 K. A second magnetization peak (SMP) feature is observed for magnetic field perpendicular (Hparallelc) and parallel (Hparallelab) to the crystal plane. However, its fundamental origin is quite different in both directions. For Hparallelc, the SMP can be well explained using an elastic-to-plastic vortex creep crossover, using collective creep theory. In addition, a possible rhombic-to-square vortex lattice phase transition is also observed for fields in between the onset-field and peak-field related to the SMP. On the other hand, for Hparallelab, a clear signature of an order-disorder vortex phase transition is observed in the isothermal M(H) measurements at Tge 6 K. The disordered phase exhibits the characteristics of entangled pinned vortex-liquid. We construct a comprehensive vortex phase diagram by displaying characteristic temperatures and magnetic fields for both crystal geometries in this unique superconducting compound. Our study sheds light on the intricate vortex dynamics and pinning in an iron-pnictide superconductor with triclinic symmetry.

Metallographic Evaluation of Increased Susceptibility to Intermediate Embrittlement of Engine Valve Forgings Made of NCF 3015 High Nickel and Chromium Steel.

This paper focused on determining the increased tendency of cracking after the die forging process of high nickel and chromium steel. The increase in carbon content in austenitic nickel-chromium steel promoted the tendency of valve forgings to forging intergranular crack on the valve head. Attention was paid to issues related to the chemical composition of the material to be considered when hot forming nickel-chromium steel components. Optical and scanning electron microscopies were used to examine the microstructure and fracture features of the samples removed from a fractured valve head. The embrittlement was due to microcavity formation at grain boundaries. Creep theory at grain boundaries was used to explain crack formation. The tensile behavior was interpreted from the evolution of the microstructure during deformation and referred to intermediate brittleness to explain the effect of carbon. It was found that the increased carbon content of the nickel-chromium steel and the strong undercooling observed at the edges of the valve head are factors that promote a reduction in grain boundary cohesion and enhance intermediate temperature embrittlement. Finally, it was found that the formation of a heterogeneous structure manifested by the presence of grain boundary M23C6-type carbides in the austenitic matrix was most likely related to the occurring brittleness.

Dynamic Analysis of Train–Bridge Coupling System for a Long-Span Railway Suspension Bridge Subjected to Strike–Slip Fault

Long-span railway bridges crossing active faults are more vulnerable owing to the joint combination of pulse ground motions and surface dislocation. To study the dynamic effects resulting from the coupling of long-span railway suspension bridges crossing strike–slip fault and trains, a nonlinear model in which wheel–rail contact was established based on Hertz’s nonlinear theory and Kalker creep theory. To generate the ground motions across strike–slip fault, an artificial synthetic method, which considers both the fling-step effect with a single pulse and the directivity effect with multiple pulses, is employed. The effects of fault-crossing angles (FCAs) and permanent ground rupture displacements (PGRDs) are systematically investigated based on wheel–rail dynamic (derailment coefficient, lateral wheel–rail force, and wheel–load reduction rate). Conclusions are drawn and can be applied in the practical seismic design and train running safety assessment of long-span railway suspension bridges crossing strike–slip fault.

Stress Development Under Fast Charging and Its Effect on Degradation of Separator

This paper discusses the predicted stress when a pouch-type lithium-ion (Li-ion) cell undergoes cyclic fast charging and how the stress is related to the observed degradation of the separator. Previous experiments showed a significant degradation in the mechanical integrity of the separator after repeated fast charging. In this paper, an integrated multiphysics model is developed for prediction of stresses when the cell, under compression as in a pack assembly, is charged at a high rate. Creep and fatigue tests of the separator demonstrate a significant damage can be accumulated under the predicted stress level. The separator is one of the most crucial safety components inside the lithium-ion battery preventing contact between the positive and negative electrodes and internal short circuits. Therefore, the separator should be mechanically robust to withstand the mechanical stresses encountered during the battery assembly, battery operation, and under abuse conditions. Both high tensile and high puncture strengths are required to avoid rupture and penetration of electrode material which can cause short circuits in the battery. We reported a gradual and upto 50% decrease in ductility of the separator after the battery cell underwent 400, 800, and 1600 charge-discharge cycles involving 4C-rate charging [1]. Scanning electron micrographs (SEM) analysis of the cycled separators revealed accumulated damage in the separator in the form of pore closure along the thickness direction and fiber fracture and cracks along the machine direction. The degradation was attributed to the damage caused by mechanical creep which can accumulate over the cycles. This paper therefore studies the stress distribution in the separator when the battery cell under mechanical loading inside a battery pack is fast charged. For that purpose, a multiphysics model is developed consisting of three main components: (1) a one-dimensional (1D) electrochemical model, (2) a lumped-parameter model, and (3) a two-dimensional (2D) solid mechanics model. The 1D electrochemical model is used to predict the heat generated and lithium-concentration in the battery cell. The heat generated is used as an input for a lumped-parameter model which simulates the thermo-mechanical behavior of the cell inside a pack array. The predicted lithium-concentration and temperature are used in the 2D solid mechanics model to predict the expansions and stresses inside the battery cell. Therefore, the proposed multiphysics model allows predicting the stress distribution in the separator under various charging rates, thermal management conditions, and pack assembly conditions such as pre-compressions.A cycling test is conducted on a lithium-ion battery cell inside a custom fixture that mimics the constrained condition inside a pack assembly. The setup is equipped with temperature, load, and displacement measurements. The parameters of the lumped-parameter model are optimized based on the experimental data using a non-linear regression technique. The model predictions show good agreements with the experimental data.The developed multiphysics model is simulated to estimate the stress distribution in the separator under the 4C charging condition (see Figure 1). The maximum first principal stress is found to be 70 MPa which is considerably lower than the yield strength of the separator. Creep and fatigue tests are conducted on the pristine separators to test their behaviors under the 70 MPa stress. Both test results show a significant amount of damage can be accumulated in the separator under a repeated loading of 70 MPa as in charge-discharge cycling. These test results corroborate our theory of mechanical creep being the main degradation mechanism of the separator while simultaneously validating our multiphysics model for prediction of stress. Reference [1] Makki M, Ayoub G, Lee CW, Bae C, Colin X. Effect of battery fast cyclic charging on the mechanical and fracture behavior of lithium-ion battery separator. In Preparation.Figure 1. Predicted two-dimensional (2D) distribution of the first principal stress in the separator near the center of Li-ion battery cell. Figure 1

Creep of Concrete in Shell Structures: Nonlinear Theory.

The creep of concrete is one of the main problems threatening concrete structural development and the stability and safety of structures. However, the nonlinear theory is the key to solving the problem of taking into account the physical and mechanical properties of concrete creep in shell structures. To create such a theory, the original shell is replaced by a continuous equivalent elastic shell. To determine the stress-strain state of the structure, the equations of nonlinear creep and crack growth are derived, and a deformation model of the section is created. The behavior of the structure at all stages of the life cycle is investigated by solving the solving systems of differential equations of equilibrium, motion, and perturbation of the equivalent shell. The values of the ratios of dependence of long-term and short-term critical loads on deformations, forces, cracks, etc., are given. The accuracy of the solution of the developed nonlinear theory is compared with the linear theory of concrete creep as well as experimental data. The results show that, according to the linear theory, for the values for the short term and long term, up to 56% and up to 39% of critical loads are overestimated, respectively. The creep process in practical engineering can be effectively controlled by the results of the proposed theory.

МОДЕЛИРОВАНИЕ ОСТАТОЧНЫХ ДЕФОРМАЦИЙ КОТЛА ВАГОНА-ЦИСТЕРНЫ НА ОСНОВЕ ТЕОРИИ ПОЛЗУЧЕСТИ

The study objective is to determine the effect of residual deformations on the behavior of the boiler during operation.
 Tasks: to develop a creep model based on flow theory and a methodology for calculating creep deformations of a railway tank boiler that occur during long-term operation. The methods of elasticity and creep theory, finite element method are used.
 The novelty of the work is in the development of a methodology for determining residual deformations in the boiler of a tank car. The results of the study are the dependences of the value of residual deformations on various factors, the deformation of the boiler in the support zones, the selection of parameters of the creep model.
 Conclusions: in the study, the creep state equation is modeled, on which basis calculations of various technical solutions for reducing residual deformations in the boiler are carried out. The options are compared.

Complex AC Magnetic Susceptibility as a Tool for Exploring Nonlinear Magnetic Phenomena and Pinning Properties in Superconductors.

The versatile AC magnetic susceptibility technique offers a detailed insight into the complex electrodynamic phenomena in superconductors. In the present study, we outline the key effects related to the temperature, AC field amplitude and frequency variations of the fundamental and harmonic components for an investigation of the vortex dynamics in a flux-grown FeSe crystal. By means of higher harmonic (nonlinear) analysis, we have explored certain atypical, asymmetric features in the AC magnetic response. These effects were identified through the detection of an even (second) harmonic and an unusual temperature shift in the odd (third) harmonic, possibly due to the complex interactions related to the composite superconducting/magnetic morphology of the crystal. Using the high-frequency sensitivity of the third harmonic, the basic functional dependencies of the pinning activation energy, as the main mixed state parameter, were determined with the implementation of the Kim-Anderson Arrhenius relation in the framework of the collective creep theory.

The Invisible Workload of Open Research

It is acknowledged that conducting open research requires additional time and effort compared to conducting ‘closed’ research. However, this additional work is often discussed only in abstract terms, a discourse which ignores the practicalities of how researchers are expected to find the time to engage with these practices in the context of their broader role as multifaceted academics. In the context of a sector that is blighted by stress, burnout, untenable workloads, and hyper-competitive pressures to produce, there is a clear danger that additional expectations to engage in open practices add to the workload burden and increase pressure on academics even further. In this article, the theories of academic capitalism and workload creep are used to explore how workload models currently exploit researchers by mismeasuring academic labour. The specific increase in workload resulting from open practices and associated administration is then outlined, including via the cumulative effects of administrative burden. It is argued that there is a high chance that without intervention, increased expectations to engage in open research practices may lead to unacceptable increases in demands on academics. Finally, the individual and systematic responsibilities to mitigate this are discussed.

Study on Hot Flow Behaviors and Deformation/Diffusion Mechanisms of V and V–Ti Microalloyed Steels by Physical Constitutive Modeling

The hot compression tests of medium carbon V and V–Ti microalloyed steels are carried out at the temperature of 900–1100 °C and strain rate of 0.01–10 s−1. Three physical constitutive models based on creep theory are studied, the prediction accuracy is compared and analyzed by employing the correlation coefficient (R) and average absolute relative error (AARE), and the deformation/diffusion mechanisms are discussed. The results show that Ti has obvious hardening effect in V microalloyed steel and retards the onset of dynamic recrystallization. The physical constitutive model containing the theoretical value of creep exponent 5 can accurately describe the hot flow behavior of both steels, the prediction accuracy of which is comparable to that of the model containing exponent n, reflecting that the dominant deformation mechanism of both steels is dislocation climbing. Furthermore, a modified physical constitutive model combining diffusion mechanisms of lattice diffusion and grain boundary diffusion shows higher accuracy (R = 0.996, AARE = 3.81% of V steel and R = 0.994, AARE = 4.52% of V–Ti steel), indicating that the diffusion mechanism is controlled not only by lattice diffusion but also by grain boundary diffusion.

The semantic inflation of “trauma” in psychology

Trauma is an increasingly prominent concept in psychology and society at large. According to the theory of concept creep, it is one of several harm-related concepts that have undergone semantic inflation in recent decades, expanding to encompass new kinds of phenomena (horizontal expansion) and less severe phenomena (vertical expansion). Previous research has demonstrated that "trauma" has come to be used in a widening range of semantic contexts, implying horizontal expansion, but has not investigated vertical expansion. The present study developed a methodology for evaluating vertical expansion and implemented it using an English-language corpus of 825,628 scientific psychology article abstracts from 1970 to 2017. Findings indicate that "trauma" has come to be used in less severe contexts, and this trend may be linked to its rising frequency of use. These findings support the predictions of the concept creep theory and provide a new method for investigating the language dynamics of harm-related concepts.

Dislocation theory of steady and transient creep of crystalline solids: Predictions for olivine

In applications critical to the geological, materials, and engineering sciences, deformation occurs at strain rates too small to be accessible experimentally. Instead, extrapolations of empirical relationships are used, leading to epistemic uncertainties in predictions. To address these problems, we construct a theory of the fundamental processes affecting dislocations: storage and recovery. We then validate our theory for olivine deformation. This model explains the empirical relationships among strain rate, applied stress, and dislocation density in disparate laboratory regimes. It predicts the previously unexplained dependence of dislocation density on applied stress in olivine. The predictions of our model for Earth conditions differ from extrapolated empirical relationships. For example, it predicts rapid, transient deformation in the upper mantle, consistent with recent measurements of postseismic creep.

ОПИСАНИЕ ЛИНЕЙНОЙ И НЕЛИНЕЙНОЙ ТЕРМОПОЛЗУЧЕСТИ БЕТОНА НА ОСНОВЕ РЕОЛОГИЧЕСКОЙ МОДЕЛИ И ЭНЕРГЕТИЧЕСКОЙ ТЕОРИИ НЕЛИНЕЙНОЙ ПОЛЗУЧЕСТИ

This article discusses the isolation of linear and nonlinear components of thermal creep deformations on the basis of the energy theory of nonlinear creep and prolonged strength proposed by V.D. Harlab. For the theoretical description of the thermal creep of concrete, a rheo-logical model with a variable number of elastic and viscous bonds (V.D. Kharlab, I.A. Kupriyanov) was used. Analysis of numerical calculations confirmed the assumption that the nonlinear component of creep is independent of temperature. The results are recommended for use in the design of concrete structures operating in conditions of temperature changes.

Flux migration across a closed HTS ring caused by the interaction of the current in the ring and a permanent magnet

By combining the Law of mass action, Arrhenius equation and the magnetic flux creep theory of Kim-Anderson, we proposed a model to depict the flux motion across a current-carrying high temperature superconductor (HTS) ring under an external magnetic field. In this model, the rate of flux motion is exponentially related to the thermal activation energy of trapped fluxes and the level of potential barriers in HTS material. The rate is also proportional to the number density of the flux lines enclosed by HTS ring. To qualitatively validate the rightness of the model, a test was carried out with a “double eye” shaped YBaCuO tape circuit and a permanent magnet at 77 K. The experimental results proved the validity of the model. This model may be helpful for understanding the flux relaxation in HTS loops under an external magnetic field and may account for the flux dynamics of HTS as well.

CALCULATION OF REINFORCED CONCRETE STRUCTURES: CONSIDERATIONS ON DEVELOPING NEW THEORY

Introduction. Current regulatory documents in the field of concrete and reinforced concrete are based mainly on research results obtained in the past century.Aim. This article addresses the approaches to improve the theory of calculation of concrete and reinforced concrete structures, in the light of the accumulated knowledge.Materials and methods. Since concrete structures belong to non-conservative systems, it is inaccurate to determine the forces arising in them using standard methods of structural mechanics. To achieve this goal, the deformation and relaxation problems for an axially compressed concrete fiber were solved following the A.A. Gvozdev and K.Z. Galustov’s two-component theory of creep, where deformations are categorized on the basis of reversibility.Results. Historical background was provided. Proposed by the authors of this article is a solution to the deformation and relaxation problems for the elementary part of a concrete structure for the case of axial compression, since such an event is impossible. In this regard, it was suggested that estimating the loss of stability by Euler is also incorrect. It was observed that the achieved maximum load-bearing capacity of the construction is equivalent to zero resistibility, with a progressive collapse occurring. For further research, it was proposed to use the hypothesis of a straight normal, assuming that this law is valid in the absence of tangential stresses in the studied sections of the structure.Conclusions. To improve the theory of calculation of concrete and reinforced concrete structures, it is necessary to revise drastically the existing approach to scientific research.

Study of Energy Dissipation in a Two-Phase Soil System

The dependence of energy dissipation coefficient on filtration properties and on creep characteristics of soil skeleton is examined. The study was carried out on the basis of the general solution of the joint task of creep and consolidation theory for a two-phase soil system obtained on the basis of the generalized model of bulk forces, taking into account the interaction of soil phases, changes over time in the general stress state at any point of the soil and additional pressures in pore water, and incomplete transfer of external pressure to the pore water. Interactions between phases are taken into account in the solution of the one-dimensional joint task of the theory of creep and consolidation, and the creep kernel is taken in the form of an exponential function. The formula for the deformation of two-phase soils is presented as the sum of two syllables due to primary and secondary consolidation of the soil. The strain formula uses experimentally obtained values of filtration characteristics and creep parameters of the soil skeleton. Using the formulas obtained, hysteresis loops under sinusoidal stress changes can be plotted and the energy expended per deformation cycle and the dissipation coefficient for two-phase soil can be obtained.

Evolution of the grain structure of metals and alloys under intense plastic deformation: multilevel models

It is well known that the performance properties of products made of metals and alloys are determined mainly by the meso- and microstructure of the latter. The structure of materials is formed and undergoes significant changes in the processes of manufacturing parts and structures using thermomechanical processing methods. A very important parameter that determines the physical and mechanical characteristics of materials is the grain structure (size, shape, relative positions of grains and inclusions of various phases). In recent decades, in this regard, special attention has been paid to the processes of severe plastic deformation (SPD), which make it possible to obtain a submicro- and nanocrystalline grain structure, which provides a significant increase in the performance properties of products made of metals and alloys. The development of SPD technologies in modern conditions is unthinkable without mathematical modeling of the processes under consideration; the most important component in the development of such a "toolkit" are constitutive relations (or, more broadly, constitutive models). In connection with the foregoing, the latter should be able to describe the evolutionary structure at various scale levels. Until now, the practice of developers of materials processing technologies has been dominated by the use of macrophenomenological models based on classical continuum theories of plasticity, viscoplasticity, and creep. From the second half of the 20th century to the present, various improvements to the constitutive models of the above class have been proposed, in which additional parameters and kinetic equations are introduced for them, describing certain characteristics of the structure of materials. As a rule, such models make it possible to obtain an adequate picture of the changing structure, however, for specific materials and methods of thermomechanical treatment. At the same time, such models, unfortunately, do not have the necessary universality; when changing the material or processing method, they have to be significantly “customized” to specific conditions, up to a complete change in the relationships included in the model. A brief review of works devoted to the creation and application of models of this class is given in the previous article by the authors. The most promising and possessing a significant degree of universality, according to the authors, are currently multilevel constitutive models based on the introduction of internal variables and physical theories of plasticity (elastoviscoplasticity). A review of works that consider various aspects of the formulation, modification, numerical implementation and application of such models is proposed in this article. The main attention is paid to models focused on the description of changes in the structure of materials due to dislocation-disclination mechanisms; a brief note is given on models that take into account thermally activated diffusion mechanisms, due to which the processes of recovery and recrystallization are realized.

Non-equilibrium and nonlinear processes in robustness potential evaluation of reinforced concrete structural systems in ultimate states

The problems of non-equilibrium and nonlinear processes in the evaluation of reinforced concrete structural systems robustness potential in ultimate states are considered. The definition of concept of “robustness exposition” is given for a quantitative assessment of the robustness potential. A calculation model based on the generalization of the well-known classical relationship between the current relative deficit change rate of the reinforced concrete stress-strain state with respect to each fixed time value is proposed to describe in time non-equilibrium processes of structural materials force resistance depending on the mode and level of loading. On the basis of the linear creep theory, aging materials, an algorithm was developed to determine the measure of creep, corrosion-damaged concrete and reinforced concrete and to determine the parameter “robustness exposition” of a reinforced concrete statically indeterminate structural system, taking into account non-equilibrium and nonlinear processes of its deformation in time. An example of a single-span rigidly clamped reinforced concrete beam calculating the robustness potential from the position of a special limiting state criterion is considered.

Compressive Experimental Analysis and Constitutive Model of Sintered Nano-Silver

Abstract Aiming at the potential high-temperature packaging material of the wide band gap semiconductors, experimental and theoretical analysis on the compressive properties of sintered nano-silver was performed. The viscoplastic properties of sintered nano-silver were investigated by compressive experiments with five loading rates, and the effects of loading rate on the ultimate strength and elastic modulus of sintered nano-silver were analyzed. According to the microstructure characteristics of sintered nano-silver, the damage framework including void volume fraction was developed by extending the Gurson–Tvergaard–Needleman model, and the mathematical model between Bonora damage law and void volume fraction was proposed, in which the internal void was assumed to be sphere and cube. A modified constitutive model including the damage model was developed based on the unified creep and plasticity theory for describing the compressive properties of sintered nano-silver. The accuracy of the proposed model was verified by comparing it with the experimental data.