Micro-macro mechanisms of cohesionless granular media : particle shape, fabric, crushing and loading paths

Pao Yue-kong Library Electronic Theses Database

Micro-macro mechanisms of cohesionless granular media : particle shape, fabric, crushing and loading paths

 

Author: Yang, Yi
Title: Micro-macro mechanisms of cohesionless granular media : particle shape, fabric, crushing and loading paths
Degree: Ph.D.
Year: 2015
Subject: Granular materials.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xv, 188 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2827779
URI: http://theses.lib.polyu.edu.hk/handle/200/8297
Abstract: Granular materials are applied in many fields, such as pharmaceuticals, agriculture, mining industry, geotechnical engineering and others. They are unusual media that cannot be directly categorized as solid, liquid or gas. The mechanical responses of a granular material are notoriously complicated to describe or predict. Although many previous researchers have attempted to explore the granular medium through different directions, the mechanisms of these materials under various conditions are still far from well understood. The nature of granular materials is composited by discrete grains and surrounded voids, which crucially determine the macroscopic mechanical behavior of granular media. Hence, it is extremely important to know the variation of particle contact information at the micro-scale. This study aims to deeply explore the variation of microscopic contact information with many new insights under different loading and boundary conditions, including biaxial drained, "undrained", and one dimensional compression. In the current thesis, the discrete element method (DEM) is adopted as a robust tool to carry out a series of numerical tests, which can be considered to explain the relationships between the microscopic information and the macroscopic response. Particle shape plays an important role in both the microscopic and macroscopic responses of a granular assembly. A more suitable shape descriptor SF is suggested for the quantitative analysis of the macro-scale strength indexes and contact parameters for non-convex grains. The critical state friction angle increases linearly with the SF value. It is found that particle shape can directly influence the strain localization pattern, micro-scale fabric distribution, micro-scale mobilization indexes and probability density function (PDF) of the normalized contact normal force. Additionally, the accuracy of the stress-force-fabric (SFF) relationship can be overestimated or underestimated by the average normal force and the distribution of contact vectors. A series of numerical tests on the quasi-static deformation of dense granular media are performed to investigate the characteristics of mixtures of shapes that are composed of round and non-convex (Elongate) particles. Four respective contact types, which are divided into circle-circle contacts (CC), circle-elongate contacts (CE), simple elongate-elongate contacts (EE1) and multiple elongate-elongate contacts (EEm), are applied to interpret the macro granular mechanical response from the information of the effective contacts on a deeper micro-scale. The contact force ratio (K) of these four contacts can also be considered to explain the variation of the steady friction angle, particularly at the decreasing ranges of (0.2, 0.3) and (0.6, 0.7). Another interesting finding is that the PDF of friction mobilization (Im) for the CE contacts nearly overlaps at the mixture of 30%~60% Elongated particles. For the end of the strong force chains, the probability of the CC contacts is approximately the same. However, the pdf curves of the normalised contact forces of EE1 and EEm show a nearly linear increasing relationship with the increased percentage of Elongated particles. It should be point out that the variation of contact normal anisotropic coefficient is insensitive with the contact portions of EE1 and EEm.
A traditional rolling resistance model has been implemented to compare with the simplified irregular clumps under the "undrained" shearing conditions. The numerical results indicate that the effects of rolling resistance model are limited to achieve the critical shear strength of irregular assemblages even when the rolling resistance coefficient α>0.5. Moreover, the artificial rolling resistance effect cannot easily generate an apparent liquefaction as the loose packing of irregular particle. Furthermore, the main weights of anisotropic parameters for "undrained" Elongate medium sample (UEM) and Triangular medium sample (UTM) are related to contact normal. However, for the rolling resistance samples, the anisotropy of contact normal force dominates the macro shear strength. In addition, it should be noted that the patterns of the probability distribution of friction mobilization as well as the combined with the force class for the rolling resistance samples and irregular assemblages at the quasi-static or the critical state are obviously different. The contact forces and coordination number distribution for crushable granular materials during one-dimensional compression are investigated using the DEM. A simple method is developed to reveal the fractal distribution of contact forces in comminuted granular materials. When the contact force distribution shows a log-normal relation, a fractal pattern of contact forces will emerge. The fractal contact force distribution can be considered to explain the evaluation of particle size distribution with a fractal feature. Moreover, systematically parametric studies show that the size ratio of the initial maximum size and the minimum crushable size, Weibull modulus, inter-particle frictional coefficient and permitted tensile stress of the initial largest particle can influence the patterns of contact force distribution at the ultimate state. Furthermore, the coordination number distribution for each particle displays a unified distribution within the fractal size distribution of granular assemblages at the ultimate state. The effects of initial particle size distribution (PSD) for the crushable granular materials are investigated under one dimensional compression within the loading-unloading-reloading procedure. The numerical macro results can qualitatively match with previous experimental tests for the initial uniform, bimodal and fractal packings. Many interesting post-processing results have been investigated to explore the variation of the macro responses. It is noticeable that the contact force distribution would gradually decrease with the reloading procedure, which would also increase the fractal dimension of the particle size distribution. However, the variation of the coordination number distribution is insensitive to the repetitive loading as well as the initial particle size distribution.

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