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MESO-SCALE KINEMATICS IN SHEAR BANDS AND IMPACT OF
MATERIAL HETEROGENEITY ON SHEAR BAND DEVELOPMENT IN
SAND
by
Sara Abedi
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Ful llment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CIVIL ENGINEERING)
May 2012
Copyright 2012 Sara Abedi
Object Description
| Title | Meso-scale kinematics in shear bands and impact of material heterogeneity on shear band development in sand |
| Author | Abedi Mashhadi Mighani, Sara |
| Author email | abedimas@usc.edu;srabedi@yahoo.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Civil Engineering |
| School | Viterbi School of Engineering |
| Date defended/completed | 2011-12-21 |
| Date submitted | 2012-03-02 |
| Date approved | 2012-03-02 |
| Restricted until | 2012-03-02 |
| Date published | 2012-03-02 |
| Advisor (committee chair) | Rechenmacher, Amy Lynn |
| Advisor (committee member) |
Ghanem, Roger G. Ben-Zion, Yehuda Lynett, Patrick J. |
| Abstract | This study aims to improve our understanding of strain localization in sands from the meso-scale perspective. First, this research contributes to characterizing and quantifying experimentally the micro- and meso-scale kinematic mechanisms associated with force chains and vortex structures in shear bands in real granular materials. The knowledge gained from this research is relevant for other dense granular flow problems (e.g. silo flow), where force chains control the transition from jammed to flowing states. Also, the results serve to reveal microstructural explanations for different global material responses between softening and critical state. ❧ The Digital Image Correlation (DIC) technique is used to obtain the two-dimensional, incremental surface displacements from digital images of deforming plane strain sand specimens. The aim herein is to use DIC results as a means to 1) evaluate meso-scale kinematic behavior inside shear bands through the course of large deformation, 2) infer the kinematic manifestation of force chains and vortex-structures and study their evolution and dissolution in sheared sands, and 3) conduct a preliminary investigation of length scales in granular shear. ❧ The obtained results reveal that softening/critical state transition is microstructurally defined by a coordinated, multi force chain collapse event which induces coherent vortices. The critical state microstructure, on the other hand, is characterized by the continual buildup of force chains at the conflux of opposing displacement vectors between adjacent vortices, then force chains collapse and formation of new vortices. The temporal changes in meso-scale kinematics are seen to additionally correlate with fluctuations in macroscopic shear stress. Auto correlation analysis and wavelet analysis are used to quantify spatial periodicity of vortices and the size of them. Our results suggest that the correlation length or the size of vortices is in the order of 6 to 7 grains and the spatial periodicity of the observed pattern is not only a function of median grain size, more research is required to explore the role of morphological and mechanical parameters in this regard. ❧ Second, we studied the physical mechanism underlying shear band formation in a supposedly uniform soil specimen, apart from the boundary influences or loading non-uniformities. Specifically we investigate how meso-scale sand density variations affect the onset of strain localization, providing insight as to why persistent shear bands form where they form in the soil specimen. The technique of X-Ray Computed Tomography (CT) is used to capture meso-scale density variations in plane strain specimens of sand. Digital image processing techniques are then used to transfer the CT results as input to the three-dimensional FE models. Finally, Digital Image Correlation (DIC) enables tracking of the in plane displacement of the sand specimen throughout plane strain compression tests performed on the CT specimens. The laboratory results are then compared with numerical predictions. This direct comparison should enable refinement in numerical models to achieve better predictability. ❧ The experimental technique that we used here could precisely capture the role of minor density variation on pre-peak response of specimen and shear band formation. The resulting FE predictions of shear band location further prove that spatial density variations have an influential role in the development of persistent shear band. |
| Keyword | digital image processing (DIC); force chain; laboratory testing; sand; strain localization; vortex |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m |
| Contributing entity | University of Southern California |
| Rights | Abedi Mashhadi Mighani, Sara |
| Physical access | The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright. The original signature page accompanying the original submission of the work to the USC Libraries is retained by the USC Libraries and a copy of it may be obtained by authorized requesters contacting the repository e-mail address given. |
| Repository name | University of Southern California Digital Library |
| Repository address | USC Digital Library, University of Southern California, University Park Campus MC 7002, 106 University Village, Los Angeles, California 90089-7002, USA |
| Repository email | cisadmin@dots.usc.edu |
| Archival file | uscthesesreloadpub_Volume6/etd-AbediMashh-479.pdf |
Description
| Title | Page 1 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@dots.usc.edu |
| Full text | MESO-SCALE KINEMATICS IN SHEAR BANDS AND IMPACT OF MATERIAL HETEROGENEITY ON SHEAR BAND DEVELOPMENT IN SAND by Sara Abedi A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Ful llment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CIVIL ENGINEERING) May 2012 Copyright 2012 Sara Abedi |
