Towards a Robust Constitutive Law for Calcite Rocks

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

EAR-0125669 J. Brian Evans In 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.