Testing Stress Percolation as a Model for Stress Transmission in Rocks

本项目将测试应力渗透假说及其在地质材料中的应用。科学家和工程师都认为，当岩石受到构造力或工程项目的载荷时，载荷是均匀地承受在整个岩石上的。PI提出了另一种假设，即岩石上的外部载荷产生的应力按照一种模式分布在整个岩石上，这种模式类似于水流经干燥的沙子所产生的模式。如果这一假设被证明是正确的，那么岩石中的应力分布将加入一大类表现出“渗透”行为的现象，包括在颗粒材料中观察到的应力分布模式。因此，该假设的一个含义是，存在一个统一的物理模型来解释所有地球物质的变形。了解岩石中的应力分布的价值在于，它将使我们能够创建更好的岩石变形模型，更好地预测岩石的力学行为，这对加深我们对各种地球过程以及工程应用的理解具有重要意义。应力渗透的存在以及对其在岩石中如何作用的精确理解对地球物理学以外的许多学科都有巨大的影响。材料科学、冶金学和冲击物理学的研究人员都在与所谓的“多晶问题”作斗争——像岩石一样，许多固体材料是由具有各种性质的结晶颗粒的聚集体组成的。了解这些材料对载荷的响应，与研究岩石变形的人所经历的挑战类似。因此，这项研究将有助于更好地理解所有这些学科中的机械问题。利用多晶材料的有限元模拟，PI最近表明应力和应变的局部变化参与了大规模的模式，可能是由应力渗透引起的。这些模式是材料中机械部件（晶粒和晶界）的弹性和塑性性能的非均质性和统计分布的函数。更大程度的异质性导致更强烈的应力集中在一个较低密度的模式。较低程度的弹性非均质导致更密集的应力传递模式，携带较小的调制。与颗粒状材料中剪切带的发展平行，应力模式直接导致剪切局部化。拟议的项目将比较在实验变形的单相岩石中观察到的应力和应变非均质性与在模拟这些岩石的有限元模型中观察到的应力和应变非均质性。具体而言，将有限元模型预测的局部应力张量方向变化与孪晶和扭带等对压缩方向敏感的微观结构进行比较，并将有限元模型预测的局部压应力大小分布与原位变形实验中同步x射线衍射数据得出的局部压应力分布进行比较。要检查的模型设计元素包括颗粒形状，晶界和颗粒内部流变学，以及模型尺寸和纵横比。使用2D和3D模型的影响也将被检查。将在实验变形的多晶板表面观察到的微应变变化模式与为匹配板表面的EBSD图而建立的有限元模型的结果之间进行额外的直接比较。这些模型与实验数据的比较将为应力渗透假说提供检验，并为进一步研究应力渗透奠定基础。

This project will test stress percolation hypothesis and its utility for working with geologic materials. Both scientists and engineers have assumed that when a rock is loaded by either tectonic forces or within the context of engineering projects, the load is borne evenly throughout the rock. The PI has developed an alternative hypothesis that the stresses produced by external loads on a rock are distributed throughout the rock according to a pattern that resembles the pattern created by water flowing through otherwise dry sand. If the hypothesis proves correct, then stress distribution in rocks will join a large class of phenomena that exhibit 'percolation' behavior, including the patterns of stress distribution observed in granular materials. Thus one implication of the hypothesis is that there is a single unifying physical model for deformation of all earth materials. The value in understanding the stress distribution in rocks is that it will allow us to create better models of rock deformation and better predict the mechanical behavior of rocks, which has implications for deepening our understanding of a variety of Earth processes as well engineering applications. The existence of stress percolation and a refined understanding of how it operates in rocks has tremendous implications for many disciplines beyond geophysics. Researchers in materials science, metallurgy, and shock physics, all struggle with the so called 'polycrystalline problem' - like rocks, many solid materials are composed of aggregates of crystalline grains with a variety of properties. Understanding the response to loading of many of these materials has presented challenges similar to those experienced by those working on rock deformation. Thus, this research will contribute to a better understanding of mechanical problems in all of these disciplines. Using finite element (FEM) simulations of a polycrystalline material the PI has recently shown that local variations in stress and strain participate in large-scale patterns, likely caused by stress percolation. The patterns are a function of the heterogeneity and statistical distribution of elastic and plastic properties across the population of mechanical components (grains and grain boundaries) in the material. Greater degrees of heterogeneity lead to more intense stress concentrations across a less dense pattern. Lower degrees of elastic heterogeneity lead to a denser pattern of stress transmission that carries smaller modulations. Paralleling the development of shear bands in granular materials, the stress patterns lead directly to shear localization. The proposed project will compare the stress and strain heterogeneity observed in experimentally deformed mono-phase rocks with the stress and strain heterogeneity observed in FE models tuned to simulate these rocks. Specifically, the variation in the orientation of local stress tensor as predicted by FE models will be compared with microstructures that are sensitive to the compression direction such as twins and kinkbands and the distribution of the magnitude of the local compressive stress predicted by the FE models will be compared with local compressive stress distributions derived from synchrotron x-ray diffraction data from in-situ deformation experiments. Model design elements to be examined include the shapes of grains, grain boundary and grain interior rheology, as well as model size and aspect ratio. The impact of using 2D vs 3D models will also be examined. Additional direct comparisons will be made between the pattern of variation in microstrains observed on the surface of experimentally deformed polycrystalline slabs and the results from FE models built to match EBSD maps of the slab surfaces. These comparisons between models and experimental data will provide a test of the stress percolation hypothesis and provide a foundation for further investigations of stress percolation.