Towards a Robust Constitutive Law for Calcite Rocks

EAR-0125669J。布莱恩·埃文斯在这个项目中，研究人员重点研究了晶体塑性机制，包括位错蠕变和扩散流。我们对方解石岩石蠕变强度的了解，通常是复杂的多相组合，主要基于几个相对纯的石灰岩和大理石、单相合成大理石甚至单晶的数据，除了少数例外，仅在非常小的应变增量上。根据其他岩石和陶瓷的变形经验，人们可能会认为岩石的强度高度依赖于分散的第二相、溶质杂质、先前存在的孔隙度、粒度和粒度分布，以及温度、压力和孔隙-流体化学。应用于方解石岩石的标准范式，稳态幂定律蠕变，通常只与温度、应力、应变率有关，而对于扩散蠕变或边界滑动过程，则与颗粒尺寸有关。目前在人造大理石和天然岩石上的实验数据表明，标准的幂定律方程不能准确地预测位错流动区域中常规三轴力学实验中观察到的应变率的应力敏感性。如果岩石中的微结构随着应变或时间的变化而演变，可能会出现一个特别的问题。这些结构变量可能包括平均颗粒尺寸、颗粒尺寸分布、颗粒取向和随颗粒尺寸变化的位错结构。以往的假设是，晶粒度对高温位错蠕变的强度没有影响。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，将在高温和高压下合成并测试经过严格控制的大理石样品。研究人员打算系统地研究镁、铁、锰和锶的基质颗粒尺寸、第二相含量和固溶体对强度的影响。这项工作的一个重要方面是晶粒和位错微结构与力学行为的关联。高应变试验将在简单剪切、常规三轴拉伸和约束扭转加载下进行。扩展的数据集将用于构建更稳健的本构定律，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.