scholarly journals Study on the Porosity of Saturated Fragmentized Coals during Creep Process and Constitutive Relation

2020 ◽  
Vol 2020 ◽  
pp. 1-10
Author(s):  
Peng Gong ◽  
Zhanguo Ma ◽  
Yongheng Chen ◽  
Shixing Cheng ◽  
Kelong Li
Keyword(s):  
Grain Size ◽  
Stress Level ◽  
Creep Model ◽  
Creep Process ◽  
Deformation Characteristics ◽  
Test Device ◽  
Porosity Variation ◽  
Grain Sizes ◽  
Voigt Model ◽  
Creep Constitutive Model

Pore abundance and deformation characteristics of saturated fragmentized coals during creep process are of significant meaning to the study on ground sediment in the mined-out area. The law of porosity variation of saturated fragmentized coals during creep process and its creep constitutive model were studied by using the self-developed multiphase coupling creep test device. And, results have indicated that the porosity logarithm of fragmentized coal during creep process shows a linear negative correlation with the time ln(n−a) = −ct + lnb, and the porosity decrease is evidently divided into three phases. In addition, when the stress level is relatively low, the porosity decreases slowly; when the stress level rises up, the porosity decreases quickly; when the stress level remains stable finally, the porosity is smaller. Under the equal stress, as the grain size of fragmentized coals decreases, the porosity tends to decrease, and as the grain size of fragmentized coal tends to be stable, the porosity tends to increase; the creep constitutive equation of fragmentized coals with different grain sizes was established by using the Kelvin–Voigt model, and the correlation analysis shows that the Kelvin–Voigt creep model of fragmentized coals is reasonable.

Investigation of Deformation Characteristics of Micropins Fabricated Using Microextrusion

2005 ◽  
Author(s):  
Neil Krishnan ◽  
Jian Cao ◽  
Brad Kinsey ◽  
Sunal A. Parasiz ◽  
Ming Li
Keyword(s):  
Grain Size ◽  
Material Properties ◽  
Size Effects ◽  
Relative Size ◽  
Deformation Characteristics ◽  
Small Length ◽  
Base Diameter ◽  
Frictional Behavior ◽  
Grain Sizes ◽  
Measured Hardness

Microextrusion has recently emerged as a feasible manufacturing process to fabricate metallic micropins having characteristic dimensions of the order of less than 1 mm. At this length scale the deformation of the workpiece is dominated by the so-called ‘size effects’, e.g. material properties and frictional behavior vary at small length scales. In recent extrusion experiments performed to produce sub-millimeter sized pins having a base diameter of 0.76 mm and an extruded diameter of 0.57 mm, certain interesting deformation characteristics were observed. When a workpiece with a relatively large grain size of 211 μm was used, the billet tended to deform inhomogenously, and the extruded pins showed a tendency to curve. This phenomenon was not seen when workpieces with a smaller grain size of 32 μm were used. It is believed that the relative size and orientation of the large grains in the 211 μm grain size sample are responsible for this behavior and the aim of this paper is to investigate this phenomenon. Microindentation tests were performed on micropins extruded from workpieces of both grain sizes to obtain a measure of the distribution of induced strain. The results obtained from this analysis show that the deformation characteristics of the extruded pins are dominated by the size and location of specific grains leading to a non-uniform distribution of plastic strain and measured hardness.

scholarly journals A Creep Model of Asphalt Mixture Based on Variable Order Fractional Derivative

2020 ◽  
Vol 10 (11) ◽  
pp. 3862
Author(s):  
Wenbo Luo ◽  
Bo Li ◽  
Yongjun Zhang ◽  
Boyuan Yin ◽  
Jingling Dai
Keyword(s):  
Mechanical Properties ◽  
Stress Level ◽  
Asphalt Mixture ◽  
Primary Creep ◽  
Creep Model ◽  
Creep Process ◽  
Variable Order ◽  
Time Varying ◽  
Steady Creep ◽  
Creep Stage

In order to quantitatively describe the time-varying mechanical properties of asphalt mixture during creep process, a nonlinear viscoelastoplastic creep model was proposed, by using variable-order fractional calculus. The differential order of the variable-order fractional element of the model is no longer constant, but a variable that changes with time, which reflects the changes of the mechanical properties of the material during the creep process. Whereas the tertiary creep phase is modeled by the viscoplastic element with time-varying viscosity, which is attributed to damage evolution. The uniaxial creep compression tests of AC-13C asphalt mixture under different stress levels (0.7 MPa, 0.9 MPa, 1.1 MPa, 1.3 MPa, 1.5 MPa, 1.7 MPa) were carried out with MTS-809 testing machine at 25 °C, and the test results were analyzed by the model using Levenberg–Marquardt optimization algorithm. It is shown that creep damage occurs when the applied stress exceeds a certain critical value, and the damage incubation time depends on the applied stress level. The higher stress decreases the damage incubation time. The model is in good agreement with the experimental results, and is applicable to describe the entire creep process, which consists of primary, steady and tertiary stages. Moreover, the variation of the model parameter can describe the change of viscoelastic properties of the material during the creep process. The differential order of the variable-order fractional element is constant during the primary creep stage, indicating that the creep behavior of the asphalt mixture is linear viscoelastic in small strain range. For the same stress level, the fractional order of the steady creep stage is greater than that of the primary creep stage, and it increases with the increasing stress level, which shows that the viscous behavior in the steady creep is more remarkable than that in the primary creep, and the higher the stress level, the more prominent the viscous performance exhibits.

An Anomaly in Creep Property Dependence on Grain Size for Ultrafine Grain Nanocrystalline Nickel at Higher Creep Temperature

2020 ◽  
Vol 978 ◽  
pp. 477-486
Author(s):  
Md. Meraj ◽  
Snehanshu Pal
Keyword(s):  
Grain Size ◽  
Grain Growth ◽  
Deformation Process ◽  
Creep Property ◽  
Dynamics Simulation ◽  
Creep Process ◽  
Ultrafine Grain ◽  
Creep Properties ◽  
Grain Sizes ◽  
Creep Temperature

In this present study, molecular dynamics simulation of creep for ultrafine grain NC Ni specimens with different grain sizes have been carried out under a constant 1 GPa applied load for various creep temperatures to study the dependence of grain growth on creep temperature and grain size during creep process and its influence on creep properties. It is observed that the extent of grain growth in ultrafine grain NC Ni during creep deformation process is more if creep in creep temperature is higher. A noteworthy anomaly, that is NC Ni with smaller grain exhibits better creep property compared to NC Ni with larger grain, is observed in case of higher creep temperatures (i.e. around or greater than 1400K).

scholarly journals A Phenomenological Primary–Secondary–Tertiary Creep Model for Polymer-Bonded Composite Materials

Polymers ◽  
2021 ◽  
Vol 13 (14) ◽  
pp. 2353
Author(s):  
Xiaochang Duan ◽  
Hongwei Yuan ◽  
Wei Tang ◽  
Jingjing He ◽  
Xuefei Guan
Keyword(s):  
Composite Materials ◽  
Creep Behavior ◽  
Reference Model ◽  
Creep Model ◽  
Creep Process ◽  
Validation Dataset ◽  
Testing Conditions ◽  
Proposed Model ◽  
Testing Data ◽  
Bonded Composite

This study develops a unified phenomenological creep model for polymer-bonded composite materials, allowing for predicting the creep behavior in the three creep stages, namely the primary, the secondary, and the tertiary stages under sustained compressive stresses. Creep testing is performed using material specimens under several conditions with a temperature range of 20 °C–50 °C and a compressive stress range of 15 MPa–25 MPa. The testing data reveal that the strain rate–time response exhibits the transient, steady, and unstable stages under each of the testing conditions. A rational function-based creep rate equation is proposed to describe the full creep behavior under each of the testing conditions. By further correlating the resulting model parameters with temperature and stress and developing a Larson–Miller parameter-based rupture time prediction model, a unified phenomenological model is established. An independent validation dataset and third-party testing data are used to verify the effectiveness and accuracy of the proposed model. The performance of the proposed model is compared with that of an existing reference model. The verification and comparison results show that the model can describe all the three stages of the creep process, and the proposed model outperforms the reference model by yielding 28.5% smaller root mean squared errors on average.

scholarly journals Investigation of Movement of Nourishment Sand of Different Grain Sizes Using Model for Predicting Bathymetric and Grain Size Changes

2009 ◽  
Vol 65 (1) ◽  
pp. 691-695
Author(s):  
Yasuhito NOSHI ◽  
Akio KOBAYASHI ◽  
Takaaki UDA ◽  
Masumi SERIZAWA ◽  
Takayuki KUMADA
Keyword(s):  
Grain Size ◽  
Grain Sizes

The Method of Eliminating Banded Structure and the Effects of Banded Structure on the Transverse Properties in 20G Steel

2013 ◽  
Vol 347-350 ◽  
pp. 1171-1175 ◽  
Author(s):  
Bin Wang ◽  
Hong Mei Hu ◽  
Cui Zhou
Keyword(s):  
Heat Treatment ◽  
Grain Size ◽  
Stress Concentration ◽  
Base Metal ◽  
Metallic Inclusion ◽  
Banded Structure ◽  
Grain Sizes ◽  
Fine Grain ◽  
Treatment Processes ◽  
Metallic Inclusions

The transverse properties were inferior to the longitudinal properties for the existence of banded structure in 20G steel. In order to eliminate the banded structure and improve the transverse performance of 20G steel, different heat treatment processes were adopted. The results showed that conventional normalizing could reduce the banded structure and refine the grain sizes. When 20G was heated with 10°C/min heating rated and then held at 920°C for 2h, the banded structure in the steel was almost eliminated and the microstructure was homogeneous with fine grain size, the strength increased by 14%. The non-metallic inclusion and carbide in the microstructure leaded to stress concentration and separation with the base metal. To some extent, heat treatment can improve the distribution and form of non-metallic inclusions.

Reactive Milling and Mechanical Alloying in Electroceramics

2010 ◽  
Vol 63 ◽  
pp. 420-424
Author(s):  
Riva Rivas-Marquez ◽  
Carlos Gomez-Yanez ◽  
Ivan Velasco-Davalos ◽  
Jesus Cruz-Rivera
Keyword(s):  
Grain Size ◽  
Mechanical Activation ◽  
Raw Materials ◽  
Planetary Mill ◽  
Fabrication Technique ◽  
Nonlinearity Coefficient ◽  
Grain Sizes ◽  
Zno Varistor ◽  
Reactive Milling ◽  
Good Homogeneity

Using Mechanical Activation it is possible to obtain small grain size and good homogeneity in a ceramic piece. For ZnO varistor devices Mechanical Activation appears to be a good fabrication technique, since good homogeneity and small grain sizes are advantageous microstructural features. The typical formulation is composed by ZnO, Bi2O3, Sb2O3, CoO, MnO2 and Cr2O3 as raw materials, and during sintering, several dissolutions and reactions to form pyrochlore and spinel phases occur. When Mechanical Activation is applied to the entire formulation, it is difficult to know what processes are being mechanically activated due to the complexity of the system. The aim of the present work was to clarify how the mechanical activation is taking place in a typical ZnO varistor formulation. The methodology consisted in the formation of all possible combinations of two out of the five oxides above mentioned and to apply mechanical activation on the mixture of each pair of powders. The results showed that systems containing Bi2O3 are prone to react during mechanical activation. Also, reduction reactions were observed in MnO2. In addition, the powder mixture corresponding to the whole formulation was milled in a planetary mill, pressed and sintered, and varistor devices were fabricated. Improvement in the nonlinearity coefficient and breakdown voltage was observed.

Analysis of Thick-Walled Pipeline Elements Operating in Creep Conditions

2015 ◽  
Vol 712 ◽  
pp. 63-68
Author(s):  
Przemysław Osocha ◽  
Bohdan Węglowski
Keyword(s):  
Finite Element ◽  
Test Data ◽  
Creep Test ◽  
Power Plants ◽  
Steam Pressure ◽  
Creep Model ◽  
Creep Process ◽  
Fe Model ◽  
Element Analysis ◽  
Wide Range

In some coal-fired power plants, pipeline elements have worked for over 200 000 hours and increased number of failures is observed. The paper discuses thermal wear processes that take place in those elements and lead to rupture. Mathematical model based on creep test data, and describing creep processes for analyzed material, has been developed. Model has been verified for pipeline operating temperature, lower than tests temperature, basing on Larson-Miller relation. Prepared model has been used for thermal-strength calculations based on a finite element method. Processes taking place inside of element and leading to its failure has been described. Than, basing on prepared mathematical creep model and FE model introduced to Ansys program further researches are made. Analysis of dimensions and shape of pipe junction and its influence on operational element lifetime is presented. In the end multi variable dependence of temperature, steam pressure and element geometry is shown, allowing optimization of process parameters in function of required operational time or maximization of steam parameters. The article presents wide range of methods. The creep test data were recalculated for operational temperature using Larson-Miller parameter. The creep strain were modelled, used equations and their parameters are presented. Analysis of errors were conducted. Geometry of failing pipe junction was introduced to the Ansys program and the finite element analysis of creep process were conducted.

The Processing of Ultrafine-Grained Materials Using High-Pressure Torsion

2007 ◽  
Vol 558-559 ◽  
pp. 1283-1294 ◽  
Author(s):  
Cheng Xu ◽  
Z. Horita ◽  
Terence G. Langdon
Keyword(s):  
Plastic Deformation ◽  
Grain Size ◽  
High Pressure ◽  
High Pressure Torsion ◽  
Metallic Materials ◽  
Grain Sizes ◽  
Ultrafine Grained ◽  
Wide Range ◽  
Ultrafine Grained Materials ◽  
Thin Disks

It is now well-established that processing through the application of severe plastic deformation (SPD) leads to a significant reduction in the grain size of a wide range of metallic materials. This paper examines the fabrication of ultrafine-grained materials using high-pressure torsion (HPT) where this process is attractive because it leads to exceptional grain refinement with grain sizes that often lie in the nanometer or submicrometer ranges. Two aspects of HPT are examined. First, processing by HPT is usually confined to samples in the form of very thin disks but recent experiments demonstrate the potential for extending HPT also to bulk samples. Second, since the strains imposed in HPT vary with the distance from the center of the disk, it is important to examine the development of inhomogeneities in disk samples processed by HPT.

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