Experimental and Theoretical Investigation of Deformation in Granular Materials: A Micromechanics Approach

Washington State University, Pullman WA

Investigators

Abstract

The deformation of soils in the field and laboratory are commonly observed to concentrate into shear bands formed during strain localization. While this mechanism is widely appreciated by engineers and researchers, it remains difficult to model, predict and analyze. The main difficulty can be attributed to the fact that while classical continuum mechanics models that do not include microstructure length scales can predict the onset of instability, they cannot predict the size and evolution of the shear bands. In particular, the classical theories of plasticity break down in the post-bifurcation regime. The main objective of this study is to develop a microstructure based continuum model to study deformation and localization in granular materials. The model is based on crystal plasticity but includes two microstructure length scales; one associated with the plastic curvature (orientation re-distribution) and the other related to the porosity re-distribution, both of which can be directly quantified by experiments. The study is unique in that the microstructure model parameters are determined directly from measurements. Granular specimens are hardened by impregnation with resin and their microstructure captured by means of non-invasive x-ray computer tomography. Evolution of the microstructure model parameters is monitored at various stages of shear deformation. The model will be implemented into a 3-D finite element code and used to identify deformation patterning, softening, and instabilities (shear banding and liquefaction) in boundary value problems. The experimental and analytical program will lead to a better understanding of the phenomenon of bifurcation and localization and to analytical methods for analyzing strain localization not only in laboratory specimens, but also in practical boundary value problems in geotechnical engineering. The outcome of this work would also have implications to the modeling of other type of materials such as metals and composites that exhibit deformation instabilities and shear banding.