Yield shear stress dependence on nanoindentation strain rate in bulk GaN crystal

2011 ◽  
Vol 8 (2) ◽  
pp. 429-431 ◽  
Author(s):  
Masaki Fujikane ◽  
Toshiya Yokogawa ◽  
Shijo Nagao ◽  
Roman Nowak
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Stress Dependence ◽  
Yield Shear Stress ◽  
Bulk Gan ◽  
Gan Crystal

Mechanical and Optical Properties Characterization of C-Plane (0001) and M-Plane (10−10) GaN by Nanoindentation and Luminescence

2015 ◽  
Vol 1792 ◽  
Author(s):  
Toshiya Yokogawa ◽  
Masaki Fujikane ◽  
Shijo Nagao ◽  
Roman Nowak
Keyword(s):  
Plastic Deformation ◽  
Shear Stress ◽  
Dislocation Density ◽  
Twin Boundary ◽  
Boundary Energy ◽  
Dynamics Simulation ◽  
Dark Spot ◽  
Yield Shear Stress ◽  
Gan Crystal

ABSTRACTYield shear stress dependence on dislocation density and crystal orientation was studied in bulk GaN crystals by nanoindentation examination. The yield shear stress decreased with increasing dislocation density which is estimated by dark spot density in cathodoluminescence, and it decreased with decreasing nanoindentation strain-rate. It reached and coincided at 11.5 GPa for both quasi-static deformed c-plane (0001) and m-plane (10-10) GaN. Taking into account theoretical Peierls–Nabarro stress and yield stress for each slip system, these phenomena were concluded to be an evidence of heterogeneous mechanism associated plastic deformation in GaN crystal. Transmission electron microscopy and molecular dynamics simulation also supported the mechanism with obtained r-plane (-1012) slip line right after plastic deformation, so called pop-in event. The agreement of the experimentally obtained atomic shuffle energy with the calculated twin boundary energy suggested that the nucleation of the local metastable twin boundary along the r-plane concentrated the indentation stress, leading to an r-plane slip. This nanoindentation examination is useful for the characterization of crystalline quality because the wafer mapping of the yield shear stress coincided the photoluminescence mapping which shows increase of emission efficiency due to reduction of non-radiative recombination process by dislocation.

Strain Rate Controlled Nanoindentation Examination and Incipient Plasticity in Bulk GaN Crystal

2013 ◽  
Vol 52 (8S) ◽  
pp. 08JJ01 ◽  
Author(s):  
Masaki Fujikane ◽  
Toshiya Yokogawa ◽  
Shijo Nagao ◽  
Roman Nowak
Keyword(s):  
Strain Rate ◽  
Incipient Plasticity ◽  
Bulk Gan ◽  
Rate Controlled ◽  
Gan Crystal

Fundamental Process Mechanics Common to Machining and Grinding Operations

2020 ◽  
Author(s):  
Wolfgang Lortz ◽  
Radu Pavel
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Plastic Flow ◽  
Material Behavior ◽  
Workpiece Material ◽  
Yield Shear Stress ◽  
Rate Versus ◽  
Wide Range ◽  
Process Mechanics ◽  
Grinding Operations

Abstract All different production processes have one thing in common: in each case a workpiece with characteristic material behavior, stress, strain, self-hardening and temperature will be produced by a tool with special geometry and individual kinematic conditions, with a wide range of energy in a designed machine tool which is working along programmed lines. For the workpiece material, it is not important from which machine the energy is coming. To be able to predict more accurate values of the production process, it will be necessary to focus more on the complex and difficult process mechanics. The result must have a strong physical base and be in good agreement with practical results To solve these problems, we have to uncover all previous simplification assumptions for the existing models. This leads in a first step to a new fundament in process mechanics, which is only based on mathematics, physics and material behavior with friction conditions, and resulting temperatures during metal plastic flow. The new mathematical equations developed for yield shear stress and strain rate will be presented and discussed in this paper. The plastic deformation is the only parameter that will not disappear after completing the operation. Therefore, this will be the base to compare the developed theoretical deformation with the experimental results for two operations: cutting and grinding. In addition, it could be shown that yield shear stress and corresponding strain rate versus temperatures have an interdependent relationship, which creates the opportunity to determine the temperatures during metal plastic flow.

scholarly journals Longitudinal Stress Gradients and Basal Shear Stress of a Temperate Valley Glacier

1979 ◽  
Vol 24 (90) ◽  
pp. 507-508 ◽  
Author(s):  
Robert Bindschadler
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Field Data ◽  
Numerical Models ◽  
Lateral Strain ◽  
Longitudinal Stress ◽  
Base Shear ◽  
Stress Gradients ◽  
Valley Glacier ◽  
Image Position

AbstractFor the first time field data from a temperate valley glacier, the Variegated Glacier, are used to investigate the behavior of longitudinal stress gradients predicted by the relation(1)whereHis the local depth, andysandybare the surface and bed elevations respectively. This equation is similar to one derived by Budd (1970) for plane strain-rate, to evaluate the importance of longitudinal stress gradients, but a shape factorfis included to account approximately for lateral strain-rate gradients. Predictive numerical models of valley glaciers require the local base shear stress to be known as accurately as possible. It has been argued on theoretical grounds that whenTis averaged over distances of more than five to ten times the depth, this term is negligible. At larger averaging scales, 2Gcan then be considered a correction to the simple geometric expression of base stress due to the presence of longitudinal stress gradients. Field data of velocity and geometry are used to evaluate the terms of Equation (1), whereτband 2Gare estimated asandat intervals of 100 m,Usis the measured surface center-line velocity,Aandnare the flow-law parameters, andis the surface longitudinal strain-rate. The expression for 2Gis an approximation proposed by Budd (1970).

Bulk Gan Crystal With Low Defect Density Grown By Hydride Vapor Phase Epitaxy

1997 ◽  
Vol 482 ◽  
Author(s):  
A. Usui
Keyword(s):  
Vapor Phase ◽  
Flat Surface ◽  
Defect Density ◽  
Vapor Phase Epitaxy ◽  
Hydride Vapor Phase Epitaxy ◽  
Extended Defect ◽  
New Approach ◽  
Bulk Gan ◽  
Gan Crystal ◽  
Reflectance Measurements

AbstractA new approach to grow thick GaN layers by hydride vapor phase epitaxy (HVPE) is described. Selective growth is carried out at the beginning of growth. The coalescence of selectively grown facet structures makes it possible to achieve a flat surface over the entire substrate. As a result, crack-free GaN films with mirror-like surfaces are successfully grown even to a thickness of about 100 μm on a 2-inch-diameter sapphire substrate. The extended defect density is as low as 6×107 cm−2. The reduction mechanism for dislocation is discussed based on TEM observation. The high optical properties of FIELO GaN are confirmed by 5 K photoluminescence and reflectance measurements.

scholarly journals Erebus Glacier Tongue, Mcmurdo Sound, Antarctica

1974 ◽  
Vol 13 (67) ◽  
pp. 27-35 ◽  
Author(s):  
G. Holdsworth
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Shear Strain ◽  
Longitudinal Strain ◽  
Strain Rates ◽  
Shear Strain Rate ◽  
Mcmurdo Sound ◽  
Glacier Tongue ◽  
The Past ◽  
Image Position

Examination of the past and present behaviour of the Erebus Glacier tongue over the last 60 years indicates that a major calving from the tongue appears to be imminent. Calculations of the regime of the tongue indicate that bottom melt rates may exceed 1 m a−1. By successive mapping of the ice tongue between the years 1947 and 1970, longitudinal strain-rates were determined using the change in distance between a set of 15 teeth, which are a prominent marginal feature of the tongue. Assuming a flow law for ice of the form where τ is the effective shear stress and is the effective shear strain-rate, values of the exponent n = 3 and B = 1 × 108 N m−2 are determined. These are in fair agreement with published values.

scholarly journals The Mechanical Properties of Single Crystals of Pure Ice

1969 ◽  
Vol 8 (54) ◽  
pp. 463-473 ◽  
Author(s):  
S. J. Jones ◽  
J. W. Glen
Keyword(s):  
Mechanical Properties ◽  
Strain Rate ◽  
Single Crystals ◽  
Fracture Stress ◽  
Constant Strain Rate ◽  
Tensile Creep ◽  
Stress Dependence ◽  
Creep Tests ◽  
Dislocation Multiplication ◽  
Compressive Tests

AbstractResults obtained from tensile and compressive tests on pure ice single crystals at various temperatures down to −90°C are reported. At −50°C tensile creep tests give a continually increasing creep rate until fracture, as observed at higher temperatures. The stress dependence of the strain-rate is discussed. Fracture stress increases with decreasing temperature. Results from constant strain-rate compressive tests are compared with theoretical curves computed from Johnston’s (1962) theory of dislocation multiplication. A dislocation velocity of the order of 0.5×10−8 m s−1 is deduced for ice at −50°C.

Drag reduction and turbulent structure in two-dimensional channel flows

1991 ◽  
Vol 336 (1640) ◽  
pp. 19-34 ◽  
Keyword(s):  
Shear Stress ◽  
Drag Reduction ◽  
Strain Rate ◽  
Reynolds Stress ◽  
Polymer Concentration ◽  
Turbulent Structure ◽  
Channel Flows ◽  
Channel Height ◽  
Normal Velocity ◽  
Two Dimensional

A two-component laser velocimeter has been used to determine the effect of wall strain rate, polymer concentration and channel height upon the drag reduction and turbulent structure in fully developed, low concentration, two-dimensional channel flows. Water flows at equal wall shear stress and with Reynolds numbers from 14430 to 34640 were measured for comparison. Drag reduction levels clearly depended upon wall strain rate, polymer concentration and channel height independently.However, most of the turbulent structure depended only upon the level of drag reduction. The slope of the logarithmic law of the wall increased as drag reduction increased. Similarly, the root-mean-square of the fluctuations in the streamwise velocity increased while the r.m.s. of the fluctuations in the wall-normal velocity decreased with drag reduction. The production of the streamwise normal Reynolds stress and the Reynolds shear stress decreased in the drag-reduced flows. Therefore it appears that the polymer solutions inhibit the transfer of energy from the streamwise to the wall-normal velocity fluctuations. This could occur through inhibiting the newtonian transfer mechanism provided by the pressure-strain correlation. In six drag-reducing flows, the sum of the Reynolds stress and the mean viscous stress was equal to the total shear stress. However, for the combination of highest concentration (5 p.p.m.), smallest channel height (25 mm) and highest wall strain rate (4000 s - 1 ), the sum of the Reynolds and viscous stresses was substantially lower than the total stress indicating the presence of a strong non-newtonian effect. In all drag-reducing flows the correlation coefficient for uv decreased as the axes of principal stress for the Reynolds stress rotated toward the streamwise and wall-normal directions.

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