Dem Simulations Of Soil Creep Due To Particle Crushing

published_or_final_versio

Parametric study of smooth joint parameters on the behavior of inherently anisotropic rock under uniaxial compression condition

Paper no. ARMA 15-536Inherently anisotropic rocks are modeled with the use of two-dimensional Discrete Element Methods (DEM). In the simulated anisotropic rock sample, the rock matrix is modeled as an assembly of rigid particles and the existence of weak layers is directly represented by imposing individual smooth joint (SJ) contacts with same orientation into the rock matrix. The properties of a SJ contact include normal and shear stiffness, normal strength, cohesion, and friction angle. A systematic study is conducted to investigate the influence of these parameters on the macro behaviors of anisotropic rocks with different anisotropy angles under uniaxial compression condition. The Young’s modulus is found to increase significantly with the SJ normal stiffness when the anisotropy angle is low (0o-30o). The USC increases with the SJ normal strength at high anisotropy angle (β>60°) while cohesion raises the UCS at medium anisotropy angle (30o-60o). The influence of friction angle is not significant. Understanding the influence of each parameter is of great importance for the calibration of micro parameters to represent certain type of rock. A general process for the calibration of micro parameters to reproduce the strength and deformation behaviors of different types of anisotropic rocks is proposed.postprin

DEM simulation of fracture process of inherently anisotropic rock under Brazilian test condition

Paper no. ARMA 15-535postprin

DEM simulations of thermally activated creep in soils

Discrete element modelling (DEM) has been used to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory. Soil stiffness is represented by a pair of values of linear spring stiffness normal and tangential to each intergranular contact, and the limiting coefficient of contact friction is described as varying linearly with the logarithm of sliding velocity. DEM simulations of an assembly of 3451 spheres reproduce a number of significant phenomena including: creep rate as a function of the mobilisation of deviatoric stress; initially linear decay of creep strain rate with time plotted on log-log axes and with a slope m in the range 20.8 to 21; and ultimate creep failure in triaxial simulations at high deviatoric stress ratios. Creep-induced failure is shown to occur at a unique axial strain for a given state of initial packing, and to be linked with dilatancy. The numerical results are compared quantitatively with the test data of soils from the literature. The effects of activation energy are considered in relation to the different magnitudes of creep encountered in sands and clays.published_or_final_versio

The effect of carrier-induced change on the optical properties of AlGaAs-GaAs intermixed quantum wells

The carrier-induced effects in the change of absorption and refractive index on the AlGaAs-GaAs intermixing modified quantum wells (QW's) have been investigated theoretically. Band-filling, bandgap shrinkage, and free-carrier absorption have been included for various carrier concentrations. The Schrodinger and the Poisson equations have been considered self-consistently. The polarized absorption coefficients are calculated using the Kane k·p method for a four band model and followed by the Kramers-Kranig transformation to obtain the refractive index change. The results obtained show a more enhanced bandgap renormalization and change of absorption, but a reduced change in refractive index for the larger intermixing extents. It is important to know the carrier-induced optical parameter changes the intermixed QW's because of their recent interests in photonics.published_or_final_versio

環保開發香港岩洞

Fulltext in: http://www.engineering.hku.hk/enggke/library/project-id-10/metro_book.pd

Analysis of switching dc-dc converters using a grid-point approach

Author name used in this publication: P. K. S. TamAuthor name used in this publication: F. H. F. LeungVersion of RecordPublishe

Challenges of simulating undrained tests using the constant volume method in DEM

Liquefaction during earthquakes can cause significant infrastructural damage and loss of life, motivating a fundamental study of undrained sand response using discrete element modeling (DEM). Two methods are widely used in DEM for simulating the undrained response of soil. One approach is to numerically couple the DEM code with a fluid model. Alternatively, if the soil is fully saturated and water is assumed to be incompressible, the volume of the sample can be held constant to simulate an undrained test. The latter has the advantage of being computationally straightforward, but the assumption of a constant volume can cause some issues which are discussed in this paper. Depending on the contact model selected, extremely high deviatoric stresses and pore water pressures can be generated for dense samples using the constant volume approach which are not observed in corresponding laboratory tests. Furthermore the results of these constant volume simulations tend to be sensitive to the strain rate selected. The evolution of particle size distribution caused by grain crushing is also ignored in most undrained simulations. For these reasons, authors often restrict the extent of the data presented to physically-realistic ranges and report results in non-dimensional terms, e.g., using stress ratios (q/p’) or stresses normalized by the initial confining pressure. This paper aims to highlight some of these issues, explore whether the constant volume approach is appropriate and make recommendations for future analysis of undrained soil behavior using DEM. © 2013 AIP Publishing LLCpublished_or_final_versio

High-resolution imaging of two bipolar proto-planetary nebulae

Sub-arcsecond resolution V and I images have been obtained for two proto-planetary nebulae. Both are found to show a definite bipolar morphology. A circumstellar disk is clearly seen in the V - I color image, suggesting that the bipolar lobes are due to starlight scattered into the polar openings. This indicates that bipolar morphologies develop early in the evolution of planetary nebulae, even before the onset of photoionization. © 1996. The American Astronomical Society. All rights reserved.published_or_final_versio

A variational approach for dissipative quantum transport in a wide parameter space

published_or_final_versio