INTERACTION BETWEEN THE 30° PARTIAL DISLOCATION AND HEX-VACANCY IN SILICON

2012 ◽  
Vol 26 (24) ◽  
pp. 1250154 ◽  
Author(s):  
CHAOYING WANG ◽  
ZHENQING WANG ◽  
GUOJUN LI ◽  
QINGYUAN MENG
Keyword(s):  
Molecular Dynamics ◽  
Shear Stress ◽  
Dislocation Density ◽  
Partial Dislocation ◽  
Critical Shear Stress ◽  
Critical Value ◽  
Shear Stresses ◽  
Pinning Effect ◽  
Interaction Processes ◽  
Small Models

The molecular dynamics (MD) method is used to investigate the interaction between the 30° partial dislocation and hex-vacancy (V6) in Si . The interaction processes are described in detail and compared with the prior results of mono-vacancy (V1) and di-vacancy (V2). It is found that dislocations are pinned by V6 when the shear stresses are smaller than a definite critical value τc. It illustrates that the encountered two segments beside vacant sites parallel each other or annihilate. Moreover, it is shown that the critical shear stress τc is mainly determined by both the migration barrier of kink and the volume of vacancy. Although V6 cannot make dislocations move faster due to the present small models, it may lower the dislocation density in certain conditions due to the pinning effect.

Field evidence of resuspension in a mine tailings pond

2001 ◽  
Vol 38 (4) ◽  
pp. 796-808 ◽  
Author(s):  
Celestina Adu-Wusu ◽  
Ernest K Yanful ◽  
Mohammed H Mian
Keyword(s):  
Shear Stress ◽  
Mine Tailings ◽  
Critical Shear Stress ◽  
Field Measurements ◽  
Shear Stresses ◽  
Wind Speeds ◽  
Tailings Pond ◽  
Field Evidence ◽  
Water Cover ◽  
Solubility Of Oxygen

Flooding of tailings under shallow water covers is an effective method of decommissioning potentially acid generating mine tailings. The low diffusivity and solubility of oxygen in water are attractive features of this technology. However, wind-induced waves can resuspend flooded tailings and expose them to greater contact with dissolved oxygen, thereby increasing the potential for oxidation and acid generation. Field measurements of wind activity and waves under different water cover depths and associated resuspension for a mine tailings pond in Ontario are presented and discussed. The results show that wind speeds greater than 8 m/s above water covers that are shallower than 1 m create waves of height greater than 10 cm and bottom shear stresses greater than 0.2 Pa. Under these conditions the critical shear stress of the mine tailings was exceeded, resulting in erosion and subsequent resuspension.Key words: mine tailings, water cover, wind-induced waves, resuspension, wind speed, shear stress.

DISSOCIATED DISLOCATIONS AND THE SCHMID LAW OF RESOLVED SHEAR STRESS IN b.c.c. METALS

1967 ◽  
Vol 45 (2) ◽  
pp. 939-943 ◽  
Author(s):  
F. R. N. Nabarro ◽  
T. R. Duncan
Keyword(s):  
Shear Stress ◽  
Partial Dislocation ◽  
Shear Stresses ◽  
Screw Dislocations ◽  
The Core

The dissociation of screw dislocations on [Formula: see text] planes in a b.c.c. metal can lead to unequal shear stresses for glide in opposite directions, while dissociation on [Formula: see text] planes cannot. Glide will occur in the former configuration only if the radius of the core of a partial dislocation exceeds [Formula: see text] of the radius of a symmetrically dissociated dislocation. If this condition is not satisfied, one partial dislocation runs to infinity before the remaining two coalesce.

Measuring the onset of mine tailings erosion

2007 ◽  
Vol 44 (4) ◽  
pp. 473-489 ◽  
Author(s):  
M Haneef-Mian ◽  
Ernest K Yanful ◽  
Robert Martinuzzi
Keyword(s):  
Shear Stress ◽  
Power Law ◽  
Mine Tailings ◽  
Laser Doppler Anemometry ◽  
Critical Shear Stress ◽  
Laser Doppler ◽  
Shear Stresses ◽  
Cohesive Sediments ◽  
Fine Grained ◽  
Power Law Equation

The present study gives details of a methodology for estimating the critical shear stress for erosion of mine tailings and other naturally occurring cohesive sediments. Erosion of a cohesive sediments bed occurs when the critical shear stress is exceeded to break the interparticle bond. Experiments were conducted in a 30 cm diameter laboratory column and calibrated using laser Doppler anemometry. The results showed that the erosion pattern of mine tailings particles was similar to those of fine-grained cohesive sediments. A power-law relation of the form E = α[(τ – τcr)/τcr]n is suggested for mine tailings, where E is the erosion rate, α is a coefficient, τ is the shear stress, τcr is the critical shear stress, and n is an exponent. The computed values of α, n, and τcr in the power-law equation were found to be comparable to values derived from experiments in a rotating circular flume. The derived expression for rate of erosion may be incorporated in resuspension and transport models for fine mine tailings of a similar nature.Key words: mine tailings, laser Doppler velocimetry, wall shear stresses, critical shear stress for erosion, erosion – shear stress relationship.

Development of a Three-Dimensional Iterative Methodology Using a Commercial CFD Code for Flow Scouring Around Bridge Piers

2012 ◽  
Author(s):  
Phani Ganesh Elapolu ◽  
Pradip Majumdar ◽  
Steven A. Lottes ◽  
Milivoje Kostic
Keyword(s):  
Shear Stress ◽  
Three Dimensional ◽  
Critical Shear Stress ◽  
Shear Stresses ◽  
Computational Domain ◽  
Time Step ◽  
Mesh Morphing ◽  
River Bed ◽  
Scour Hole ◽  
The Stability

One of the major concerns affecting the safety of bridges with foundation supports in river-beds is the scouring of river-bed material from bridge supports during floods. Scour is the engineering term for the erosion caused by water around bridge elements such as piers, monopiles, or abutments. Scour holes around a monopile can jeopardize the stability of the whole structure and will require deeper piling or local armoring of the river-bed. About 500,000 bridges in the National Bridge Registry are over waterways. Many of these are considered as vulnerable to scour, about five percent are classified as scour critical, and over the last 30 years bridge failures caused by foundation scour have averaged about one every two weeks. Therefore it is of great importance to predict the correct scour development for a given bridge and flood conditions. Apart from saving time and money, integrity of bridges are important in ensuring public safety. Recent advances in computing boundary motion in combination with mesh morphing to maintain mesh quality in computational fluid dynamic analysis can be applied to predict the scour hole development, analyze the local scour phenomenon, and predict the scour hole shape and size around a pier. The main objective of the present study was to develop and implement a three dimensional iterative procedure to predict the scour hole formation around a cylindrical pier using the mesh morphing capabilities in the STARCCM+ commercial CFD code. A computational methodology has been developed using Python and Java Macros and implemented using a Bash script on a LINUX high performance computer cluster. An implicit unsteady approach was used to obtain the bed shear stresses. The mesh was iteratively deformed towards the equilibrium scour position based on the excess shear stress above the critical shear stress (supercritical shear stress). The model solves the flow field using Reynolds Averaged Navier-Stokes (RANS) equations, and the standard k–ε turbulence model. The iterative process involves stretching (morphing) a meshed domain after every time step, away from the bottom where scouring flow parameters are supercritical, and remeshing the relevant computational domain after a certain number of time steps when the morphed mesh compromises the stability of further simulation. The simulation model was validated by comparing results with limited experimental data available in the literature.

Experimental study and molecular dynamics simulation of wall slip in a micro-extrusion flowing process

2011 ◽  
Vol 225 (5) ◽  
pp. 1175-1190 ◽  
Author(s):  
D Zhao ◽  
Y Jin ◽  
M Wang ◽  
M Song
Keyword(s):  
Molecular Dynamics ◽  
Shear Stress ◽  
Molecular Dynamics Simulation ◽  
Slip Velocity ◽  
Polymer Melts ◽  
Critical Shear Stress ◽  
Wall Slip ◽  
Dynamics Simulation ◽  
Desorption Mechanism ◽  
Adsorption Desorption

Wall slip is one of the most important characteristics of polymer melts’ elasticity behaviours as well as the most significant factor which affects the flow of polymer melts. Based on the traditional Mooney method, through a double-barrel capillary rheometer, the relationship between velocities of wall slip, shear stress, shear rate, diameters of dies, and temperature of polypropylene (PP), high-density polyethylene (HDPE), polystyrene (PS), and polymethylmethacrylate (PMMA) is explored. The results indicate that the velocities of the wall slip of PP and HDPE increase apparently with shear stress and slightly with temperature. Meanwhile, the rise of temperature results in the decrease of critical shear stress. The wall-slip velocities of PS and PMMA are negative which means that the Mooney method based on the adsorption–desorption mechanism has determinate limitation to calculate the wall-slip velocity. Based on the entanglement–disentanglement mechanism, a new wall-slip model is built. With the new model, the calculation values of velocity of PP and HDPE correspond to the experimental values very well and the velocities of PS and PMMA are positive. The velocities of PS and PMMA increase obviously with the rise of shear stress. The rise of temperature results in the increase of velocity and decrease of critical shear stress. Then, the molecular dynamics simulation is used to investigate the combining energy between four polymer melts and the inside wall. The results show that at the given temperature and pressure, the molecules of PS and PMMA combine with atoms of the wall more tightly than those of PP and HDPE which means when wall slip occurs, the molecules of PS and PMMA near the wall will adsorb to the surface of the wall. However, those of PP and HDPE will be easy to slip. Therefore, the wall-slip mechanism of PP and HDPE is the adsorption–desorption mechanism, and that of PS and PMMA is the entanglement–disentanglement mechanism. According to the different wall-slip mechanisms of four polymers, an all-sided calculation method of wall-slip velocity is raised which consummates the theory of wall slip of polymer melts.

Study on Static Shear-Rheologic Characteristics of Semi-Solid AZ91D Magnesium Alloy by Reheating

2006 ◽  
Vol 116-117 ◽  
pp. 738-741
Author(s):  
Ze Sheng Ji ◽  
Mao Liang Hu ◽  
Xiao Ping Zheng
Keyword(s):  
Shear Stress ◽  
Magnesium Alloy ◽  
Critical Shear Stress ◽  
Critical Value ◽  
Holding Time ◽  
Az91d Magnesium Alloy ◽  
Coarse Grains ◽  
Small Grains ◽  
Semi Solid ◽  
Static Shear

Static shear-rheology for self-made semi-solid AZ91D magnesium alloy slurry was studied by using the tester and a universal electronic machine. Shear-stress along with cylindrical surface in the sample was produced by using the tester and then shear-rheologic deformation happened. It showed that on the condition of the same loading, the longer the loading time and holding time were, the larger the deforming rate was. When the holding time attained a certain value, small grains acquired enough energy to grow up or amalgamate with prolonging the holding time and coarse grains started to melt from intergranular or grain boundaries, but the critical shear-stress kept a constant. The sample didn’t flow but appeared to instantaneous shear-strains with starting to load. When the loading exceeded the critical value, the sample started to flow and had the phenomena of elastic after-working, elastic before-working and remained deformation. The rheologic characteristics of semi-solid AZ91D magnesium alloy were expressed by the five element mechanical model: H1—(N1/H2)—(N2/S).

Initiation of motion conditions for shale sediments

1983 ◽  
Vol 10 (3) ◽  
pp. 549-554 ◽  
Author(s):  
L. Magalhaes ◽  
T. S. Chau
Keyword(s):  
Shear Stress ◽  
Critical Shear Stress ◽  
Shear Stresses ◽  
Single Particles ◽  
Channel Stability ◽  
High Lift ◽  
Flume Tests ◽  
Load Transport ◽  
Bed Load Transport ◽  
Bed Material

Critical shear stresses for erosion of alluvial shale particles were investigated in a laboratory flume. Tests in the incipient motion conditions of individual particles showed that entrainment of shale bed particles takes place at mean shear stress values 40–50% smaller than other types of non-cohesive and coarse granular material. The low density and platy shape of the shale particles may induce high lift forces, which would account for a decrease in the resistance to erosion of shale channels and greater rates of bed-load transport. Keywords: channel stability, critical shear stress, flume tests, initiation of motion, shale gravels, single particles, weak bed material movement.

Critical Shear Stresses of Sunken Oils

2014 ◽  
Vol 2014 (1) ◽  
pp. 300241
Author(s):  
Charles Watkins ◽  
Olivia Jobin ◽  
Nancy Kinner ◽  
Thomas Ballestero ◽  
Neil W. Thomas ◽  
...  
Keyword(s):  
Shear Stress ◽  
Three Dimensional ◽  
Critical Shear Stress ◽  
Water Current ◽  
Shear Stresses ◽  
High Definition ◽  
Annular Flume ◽  
The University ◽  
University Of New Hampshire ◽  
Sunken Oil

As observed in several recent cases (e.g., DBL-152, Enbridge-Kalamazoo), under certain circumstances, spilled oil can sink to the bottom of a water body. Once on the bottom, the oil can move or even remobilize into the water column. The critical shear stress (CSS) is used to accurately predict the movement of sunken oil along and off the bottom. Unfortunately, shear stress has only been measured for one sunken oil (Hibernian Crude API = 34). The Coastal Response Research Center (CRRC) at the University of New Hampshire (UNH) has an annular flume equipped with high-definition cameras and an acoustic velocimeter that can be used to estimate CSS by measuring the instantaneous, three-dimensional water current velocities at which sunken oils move and erode as visible oil droplets. The results reported are for an Alberta bitumen, tested at temperatures between 5° and 28°C in freshwater.

A 3D Finite Element Study of Fatigue Life Dispersion in Rolling Line Contacts

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Nick Weinzapfel ◽  
Farshid Sadeghi ◽  
Vasilios Bakolas ◽  
Alexander Liebel
Keyword(s):  
Finite Element ◽  
Shear Stress ◽  
Material Properties ◽  
Rolling Contact Fatigue ◽  
Contact Fatigue ◽  
Critical Shear Stress ◽  
Rolling Contact ◽  
Shear Stresses ◽  
Rolling Element Bearings ◽  
Rolling Element

Rolling contact fatigue of rolling element bearings is a statistical phenomenon that is strongly affected by the heterogeneous nature of the material microstructure. Heterogeneity in the microstructure is accompanied by randomly distributed weak points in the material that lead to scatter in the fatigue lives of an otherwise identical lot of rolling element bearings. Many life models for rolling contact fatigue are empirical and rely upon correlation with fatigue test data to characterize the dispersion of fatigue lives. Recently developed computational models of rolling contact fatigue bypass this requirement by explicitly considering the microstructure as a source of the variability. This work utilizes a similar approach but extends the analysis into a 3D framework. The bearing steel microstructure is modeled as randomly generated Voronoi tessellations wherein each cell represents a material grain and the boundaries between them constitute the weak planes in the material. Fatigue cracks initiate on the weak planes where oscillating shear stresses are the strongest. Finite element analysis is performed to determine the magnitude of the critical shear stress range and the depth where it occurs. These quantities exhibit random variation due to the microstructure topology which in turn results in scatter in the predicted fatigue lives. The model is used to assess the influence of (1) topological randomness in the microstructure, (2) heterogeneity in the distribution of material properties, and (3) the presence of inherent material flaws on relative fatigue lives. Neither topological randomness nor heterogeneous material properties alone account for the dispersion seen in actual bearing fatigue tests. However, a combination of both or the consideration of material flaws brings the model’s predictions within empirically observed bounds. Examination of the critical shear stress ranges with respect to the grain boundaries where they occur reveals the orientation of weak planes most prone to failure in a three-dimensional sense that was not possible with previous models.

Computational Fluid Dynamics Characterization of a Bioreactor Mixing Device for the Animal Cell Culture

2011 ◽  
Vol 6 (1) ◽  
Author(s):  
Valérie Gisèle Gelbgras ◽  
Christophe E Wylock ◽  
Jean-Christophe Drugmand ◽  
Benoît Haut
Keyword(s):  
Fluid Dynamics ◽  
Cell Culture ◽  
Computational Fluid Dynamics ◽  
Shear Stress ◽  
Cell Viability ◽  
Animal Cell ◽  
Scale Up ◽  
Critical Shear Stress ◽  
Shear Stresses ◽  
A Cell

During a cell culture in a bioreactor, the cells are exposed to the shear stresses mainly generated in the culture medium by the mixing device. Beyond a critical shear stress, this exposition induces cell damages. Therefore, the limitation of the shear stress is an important criterion for the design of bioreactors. An accurate modeling of the flow and the induced shear stresses in the medium is a tool to achieve an effective design of a bioreactor. In this work, a new design of a mixing device is considered. The aims of this work are to develop a methodology to study the flow and the induced shear stresses in the device, to study and to model the relation between the flow, the induced shear stresses and the cell viability, to use the developed model as an optimization tool, and to study the design of the bioreactor mixing device and its scale-up. In a first step, the flow and the induced shear stresses in the device are simulated by Computational Fluid Dynamics. In a second step, the model of the influence of the flow and the induced shear stresses on the cell viability is established by a comparison between the computed flow and the induced shear stresses and experimental measurements of cellular viabilities for different impeller rotation speeds. Finally, the influence of another design of the mixing device on the cell viability is studied.

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