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.

本计画将测试应力逾渗假说及其在地质材料处理上的效用。 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，载荷会均匀地分布在整个岩石上。 PI提出了另一种假设，即岩石上的外部载荷产生的应力分布在整个岩石中，其分布模式类似于水流过干燥沙子时产生的模式。 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“渗流”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，该假说的一个含义是，所有地球物质的变形都有一个统一的物理模型。了解岩石中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，并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。 应力渗流的存在以及对它在岩石中如何作用的精确理解，对物理学之外的许多学科都有巨大的意义。 材料科学、冶金学和冲击物理学的研究人员都在努力解决所谓的“多晶问题”--就像岩石一样，许多固体材料是由具有各种性质的晶粒聚集体组成的。 理解这些材料对载荷的响应提出了与那些从事岩石变形工作的人所经历的挑战类似的挑战。 因此，这项研究将有助于更好地了解所有这些学科的机械问题。使用有限元（FEM）模拟的多晶材料的PI最近表明，局部变化的应力和应变参与大规模的模式，可能是由应力渗流。这些模式是材料中机械部件（晶粒和晶界）群体的弹性和塑性特性的非均匀性和统计分布的函数。 更大程度的不均匀性导致更强烈的应力集中在一个不太密集的模式。 较低程度的弹性不均匀性导致更密集的应力传递模式，其携带较小的调制。 应力模式直接导致剪切局部化，它与剪切带的发展相平行。 拟议的项目将比较在实验变形单相岩石中观察到的应力和应变不均匀性与在调整模拟这些岩石的FE模型中观察到的应力和应变不均匀性。 具体而言，FE模型预测的局部应力张量的取向的变化将与对压缩方向敏感的微观结构（例如孪晶和扭结带）进行比较，并且FE模型预测的局部压缩应力的大小的分布将与从来自原位变形实验的同步加速器X射线衍射数据得到的局部压缩应力分布进行比较。模型设计要素包括晶粒形状、晶界和晶粒内部流变学，以及模型尺寸和长宽比。还将研究使用2D与3D模型的影响。额外的直接比较将在实验变形的多晶板坯的表面上观察到的微应变的变化模式和从FE模型建立匹配EBSD地图的板坯表面的结果之间。这些模型和实验数据之间的比较将提供一个测试的应力渗流假设，并提供了一个基础，为进一步研究应力渗流。