Towards a Robust Constitutive Law for Calcite Rocks

耳朵- 0125669 j。在这个项目中，研究人员专注于涉及位错蠕变和扩散流动的晶体塑性机制。方解石通常是复杂的多相组合，我们对其蠕变强度的理解主要基于一些相对纯净的石灰石和大理岩，单相合成大理岩，甚至单晶的数据，除了少数例外，只有在非常小的应变增量下。根据其他岩石和陶瓷变形的经验，人们可能会认为岩石强度高度依赖于分散的第二相、溶质杂质、预先存在的孔隙度、晶粒尺寸和晶粒尺寸分布，以及温度、压力和孔隙流体化学。应用于方解石的标准模式——稳态幂律蠕变，通常只涉及温度、应力、应变速率，而对于扩散蠕变或边界滑动过程，则只涉及晶粒尺寸。目前对添加和不添加第二相的合成大理岩以及天然岩石的测试数据表明，标准幂律方程无法准确预测在位错流动状态下传统三轴力学实验中观察到的应变率的应力敏感性。如果岩石的微观结构随应变或时间而变化，可能会出现一个特殊的问题。这些结构变量可能包括平均晶粒尺寸、晶粒尺寸分布、晶粒取向以及与晶粒尺寸成比例的位错结构。以前认为晶粒尺寸对高温位错蠕变强度没有影响。在由多个相组成的材料中，第二相颗粒的平均晶粒尺寸、形状和分布以及第二相的总量也可能影响强度。在这项工作中，精心控制的大理石样品将被合成并在高温高压下进行测试。研究人员打算系统地研究基体晶粒尺寸、第二相含量以及镁、铁、锰和锶的固溶体对强度的影响。工作的一个重要方面是晶粒和位错微观组织与力学行为的相关性。高应变试验将采用简单剪切、常规三轴拉伸和受限扭转加载方式进行。扩展的数据集将用于构建一个更健壮的本构律，适用于更大的热力学条件集。

EAR-0125669J. Brian EvansIn this project the investigators focus on crystal plastic mechanisms involving dislocation creep and diffusion flow. Our understanding of the creep strength of calcite rocks, which are usually complex polyphase assemblages, is largely based on data from a few relatively pure limestones and marbles, from single-phase synthetic marbles, or even from single crystals, and, with a few exceptions, only over very small strain increments. Based on experience with deformation of other rocks and ceramics, one might expect rock strength to be highly dependent on dispersed second phases, solute impurities, preexisting porosity, grain size, and grain-size distribution, as well as temperature, pressure, and pore-fluid chemistry. The standard paradigm, steady-state power-law creep, as applied to calcite rocks, typically relates only temperature, stress, strain rate, and for diffusion creep or boundary sliding processes, grain size. Current data from tests on synthetic marbles with and without an added second phase, and on natural rocks suggest that the standard power-law equation fails to predict accurately the stress sensitivity of strain rate observed in conventional triaxial mechanical experiments in the dislocation flow regime. One particular problem may arise if the microstructure in the rock evolves with strain or time. Such structural variables might include average grain size, grain-size distribution, grain orientation, and dislocation structures that scale with grain-size. Previously it has been assumed that grain size had no influence on strength for high-temperature dislocation creep. In materials composed of more than one phase, the average grain size, shape, and distribution of second-phase particles, and the total amount of the second phase may also affect strength. In this work, carefully controlled samples of marble will be synthesized and tested at high temperature and pressure. The investigators intend to systematically investigate the effect on strength of matrix grain size, second-phase content, and solid solutions of magnesium, iron, manganese, and strontium. An important aspect of the work is the correlation of grain and dislocation microstructure with mechanical behavior. High strain experiments will be conducted using loading in simple shear, conventional triaxial extension, and confined torsion. The expanded data set will be used to construct a more robust constitutive law, applicable to a larger set of thermodynamic conditions.