Kazuyoshi Iwashita and Masanobu Oda Micro-deformation mechanism of shear banding process based on modified distinct element method

Micro-deformation mechanism of shear banding process based on modified distinct element method

Kazuyoshi Iwashita and Masanobu Oda
Department of Civil and Environmental Engineering
Saitama University,
Urawa, Saitama 338-8570,
Japan
Source : http://www.sciencedirect.com/science...

Abstract

Numerical simulation tests were carried out using the distinct element method DEM. by paying much attention to the micro-deformation mechanism leading to the development of shear bands. To do this, the conventional DEM was modified slightly such that the effect of rolling resistance at contact points could be taken into account called MDEM.. It is found that MDEM can be a powerful tool for simulating not only the generation of large voids inside a shear band but also the high gradient of particle rotation along the shear band boundaries, in a quite similar manner to those observed in natural granular soils. It is concluded, based on the numerical simulation tests, that the basic micro-deformation mechanism ending up with the formation of shear bands is in the generation of a column-like structure during the hardening process and its collapse in the softening process. 2000 Elsevier Science S.A. All rights reserved.

Introduction

In a plane strain test, a dense granular soil is first deformed homogeneously. Around a peak stress, however, the deformation suddenly localizes into narrow bands called the shear bands., and the stress drops sharply down to a residual stress state. Understanding this behavior is of great importance to solve some engineering problems related to soil stability. For example, slope stability is commonly analyzed on the assumption that the failure occurs along a circular shear band e.g., Ref. w17x.. In order to clarify the mechanical meanings of the shear band, many research works have already been done for the past four decades in the field of soil mechanics e.g., Refs. w18, 20,21,29x.. More importantly, the strain localization is still a hot topic in the theoretical, as well as experimental, study of mechanics of granular materials e.g., Refs. w10,12,19,32,33,37,39x.. It should be emphasized here that in spite of such extensive studies, the micro-deformation mechanism leading to the development of shear bands is not yet well understood. This is partially because we did not have any skill to observe the micro-process taking place inside. Recently soil engineers are paying more attention to numerical simulation methods to overcome the present difficulty e.g., Refs. w3,6,4,8,9,13,16,35,34,36x.. To do this successfully, however, a question must be answered: does such a conventional numerical simulation method provide a sound basis for simulating the micro-process of the shear band development, as well as the overall stress–strain behavior Oda w24x and Oda and Kazama w26x recently observed the microstructure of shear bands developed in several natural sands, by means of X-ray application and an optical method using a microscope and thin sections. The two results are worth noting here: 1. very large voids are generated inside a shear band, while shear strain is being concentrated, and as a result of this, the void ratio or porosity. can exceed the corresponding maximum one determined by a standard method; and 2. particles rotate extensively in the shear band so that a high particle rotation gradient is generated along the boundaries. An important point is that nobody has succeeded in reproducing such huge voids and a high rotation gradient by any conventional numerical method. In other words, there might be something to be done in order to improve such conventional methods. This is the motivation of starting the present study. Each particle can move against neighboring particles by sliding andror rolling at contact points. The dominant role of sliding was considered, in success, in classical theories of strength and dilatancy of granular soils. Some experimental results tell us that rolling, rather than sliding, is a dominant micro-deformation mechanism leading to extensive dilatancy of granular media e.g., Ref. w28x.. Even in conventional numerical simulation analyses, particles can move actively by rolling. The problem is that the microstructure developed in such a simulation analysis is different from that observed in natural sands. Oda w24x and Oda and Kazama w26x have suggested that the difference may arise from the fact that rolling occurs without mobilizing any resistance at contact points in such conventional analyses. Rolling occurs freely provided particles are in contact with their neighboring through points contacts . However, real particles show rough surface texture, and are even covered with a thin film of weathered products . If this is the case, the particles may be in contact with their neighboring ones through contact surfaces, not pure points, so that rolling resistance can play a role to some extent in the contact behavior. Bardet and Huang w5x showed analytically, as well as experimentally, that the rolling resistance exists even at contacts among cylindrical particles. Recently, Iwashita and Oda w14x proposed a modified version of the distinct element method called MDEM., which is capable of dealing with the rolling resistance at contact points, and showed that MDEM provides a powerful tool in the simulation of the micro-structure development as well as the overall stress strain relation. In the present study, we focus on the micro-deformation mechanism leading to the development of shear bands, based on analyses using MDEM, and examine with seeking the reason why the huge voids as well as the high gradient of particle rotation is generated in the shear bands.