Development of Micromechanics-basd Elasto-plastic Constitutive Model for Granular Materials and its validation using SPring-8 Micro X-ray CT experiments

该研究项目旨在建立一个基于细观力学的颗粒材料本构方程，该本构方程可以涵盖从弹性响应到破坏的广泛行为。为此，在Chang和Misra(1990)等人提出的均匀应变模型中加入了一个描述散体材料中颗粒柱屈曲的简单模型。研究发现，只有一个控制屈曲的参数就能使致密和疏松散体材料在破坏区域内都能得到合理的响应。我们现在正在探索从颗粒材料的其他微观性质中以物理方式确定屈曲参数的方法。颗粒形状的影响可能是成功的一个关键问题，也是该项目的一个重要目标，即开发一种图像处理工具，从Spring8微X射线CT实验中获取有用的微观力学信息。Spring-8是一个同步辐射设备，其中有非常精确的微型X射线CT系统可用。我们开发的图像处理技术能够自动识别组件内部的每个不规则形状的颗粒。利用该技术获得了异形颗粒三维堆积结构的一些有用信息。此外，我们还开发了一种在加载过程中按时间顺序识别三轴试件中的颗粒的方法，这将为颗粒材料的微观力学提供各种有用的信息。此外，本文还将该细观力学研究应用于月壤的力学行为研究。

This research project aims to develop a micromechanics-based constitutive equation of granular materials such as soils and engineering powders that can cover a wide range of the behavior from elastic response to failure. For this purpose, a simple model to describe the buckling of granular column formed in granular material is incorporated into uniform strain model proposed by Chang and Misra (1990) and others. It is found that only one parameter to control a buckling leads to reasonable response of both dense and loose granular materials up to failure regime. We are now exploring the way to determine the buckling parameter from other micro properties of granular materials in a physical way. The effect of grain shape may be a key issue for success.It is also an important objective in the project to develop an image processing tool for getting useful micromechanical information from micro X-ray CT experiments at SPring-8. SPring-8 is a synchrotron radiation facility in which very precise micro X-ray CT system is available. The image processing technique we developed enables to automatically identify each irregularly-shaped grain inside an assembly. Some useful information on 3-D packing structure of irregularly-shaped grains is obtained using this technique. Moreover, we are also developing a method to identify the grains inside a triaxial specimen in time sequence during loading, which leads to various information useful for micromechanics of granular materials. Additionally, this micromechanics study is applied to investigate the mechanical behavior of lunar soil.