Testing Stress Percolation as a Model for Stress Transmission in Rocks

测试应力渗透作为岩石中应力传递的模型

• 批准号: 1417218
• 负责人: Pamela Burnley
• 金额: $ 32.8万
• 依托单位: University of Nevada Las Vegas
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-09-01 至 2019-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1417218&HistoricalAwards=false
• 关键词: Testing; Stress; Percolation; Model; Transmission

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.

该项目将测试应力渗流假说及其在处理地质材料方面的实用性。科学家和工程师都假设，当一块岩石受到构造力的加载或在工程项目中加载时，载荷在整个岩石中均匀地承担。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类似于水流过干燥沙子产生的模式的模式分布在岩石上。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到一大类表现出渗流行为的现象中，包括在颗粒材料中观察到的应力分布模式。因此，这一假设的一个含义是，所有地球材料的变形都有一个统一的物理模型。了解岩石中的应力分布的价值在于，它将使我们能够建立更好的岩石变形模型，更好地预测岩石的力学行为，这对于加深我们对各种地球过程的理解以及工程应用具有重要意义。应力渗流的存在以及对它在岩石中如何运作的精细理解，对地球物理学以外的许多学科都有巨大的影响。材料科学、冶金和冲击物理的研究人员都在与所谓的多晶问题作斗争--就像岩石一样，许多固体材料是由具有各种性质的晶体颗粒的集合体组成的。理解其中许多材料对载荷的响应提出了类似于那些研究岩石变形的人所经历的挑战。因此，这项研究将有助于更好地理解所有这些学科中的力学问题。通过对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范围的图案，这可能是由应力渗流引起的。这些图案是材料中机械部件(颗粒和晶界)上的弹性和塑性特性的异质性和统计分布的函数。非均质性程度越高，密度越低的模式的应力集中程度越高。弹性非均质性程度越低，传递的应力模式越密集，调制幅度也越小。与颗粒材料中剪切带的发展相平行，应力模式直接导致剪切局部化。该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不均匀，以及在为模拟这些岩石而调整的有限元模型中观察到的应力和应变不均匀。具体来说，有限元模型预测的局部应力张量取向的变化将与孪晶和扭带等对压缩方向敏感的微观组织进行比较，有限元模型预测的局部压应力的大小分布将与来自原位变形实验的同步辐射X射线衍射数据得出的局部压应力分布进行比较。要考察的模型设计元素包括颗粒的形状、晶界和颗粒内部的流变性，以及模型的尺寸和高宽比。还将研究使用2D模型与3D模型的影响。此外，还将在实验变形的多晶板表面上观察到的微应变变化模式与为匹配板材表面的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验证应力渗流假说，并为进一步研究应力渗流提供基础。