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

在现场和实验室中，土的变形通常都集中在应变局部化过程中形成的剪切带中。 虽然这种机制受到工程师和研究人员的广泛赞赏，但仍然难以建模，预测和分析。主要的困难可以归因于这样一个事实，即经典的连续介质力学模型，不包括微观结构的长度尺度可以预测的不稳定性的发生，他们不能预测的剪切带的大小和演变。 特别是，经典的塑性理论在后分叉制度崩溃。本研究的主要目的是发展一个基于微观结构的连续介质模型来研究颗粒材料的变形和局部化。该模型是基于晶体塑性，但包括两个微观结构的长度尺度;一个与塑性曲率（取向重新分布）和其他相关的孔隙度重新分布，这两者都可以直接量化的实验。该研究的独特之处在于，微观结构模型参数直接从测量中确定。 颗粒状试样通过浸渍树脂硬化，并通过非侵入性X射线计算机断层扫描仪捕获其微观结构。 在剪切变形的各个阶段的微观结构模型参数的演变进行监测。 该模型将实施到一个三维有限元代码，并用于识别变形模式，软化，和不稳定性（剪切带和液化）的边界值问题。实验和分析程序将导致更好地理解分叉和局部化现象，并导致分析应变局部化的分析方法，不仅在实验室标本，也适用于岩土工程中的实际边值问题。 这项工作的结果也将对其他类型的材料，如金属和复合材料，表现出变形不稳定性和剪切带的建模。

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.