Towards a Robust Constitutive Law for Calcite Rocks

EAR-0125669J。 Brian Evans 在这个项目中，研究人员重点研究涉及位错蠕变和扩散流的晶体塑性机制。我们对方解石岩石（通常是复杂的多相组合）的蠕变强度的理解主要基于来自一些相对纯净的石灰石和大理石、单相合成大理石、甚至单晶体的数据，并且，除了少数例外，仅基于非常小的应变增量。根据其他岩石和陶瓷变形的经验，人们可能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性质。标准范式，稳态幂律蠕变，应用于方解石岩石时，通常仅与温度、应力、应变率相关，而对于扩散蠕变或边界滑动过程，则与晶粒尺寸相关。对添加和不添加第二相的合成大理石以及天然岩石进行的测试的当前数据表明，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。如果岩石中的微观结构随着应变或时间而演变，则可能会出现一个特殊问题。这些结构变量可能包括平均晶粒尺寸、晶粒尺寸分布、晶粒取向以及随晶粒尺寸变化的位错结构。以前假设晶粒尺寸对高温位错蠕变的强度没有影响。在由多于一种相组成的材料中，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，将在高温和高压下合成和测试精心控制的大理石样品。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观结构与机械行为的相关性。高应变实验将使用简单剪切、传统三轴拉伸和受限扭转载荷进行。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.