| This work explores the physical basis for Hart's [1970] mechanical equation of state behavior in rock salt and olivine. Specifically, the experiments were designed to identify a possible microstructural basis for the scaling relation and the “hardness” parameter, σ*, associated with each load relaxation curve, which Hart interpreted to represent constant “structure.” They were also designed to examine the relationship between creep and load relaxation and the significance of steady-state, power-law creep within the context of the mechanical equation of state.; Constant-stress creep, stress-change, and load-relaxation experiments were conducted on synthetic rock salt (NaCl) at 400°C and 700°C and on natural San Carlos (AZ) olivine ([Mg, Fe]2 SiO4) single crystals at 1500°C and = 10−10 atm. The two minerals behaved very differently, with rock salt exhibiting classic mechanical-equation-of-state behavior and olivine, under the conditions studied, exhibiting almost no path effects. We found that in the creep of rock salt at constant stress, the material passes through a continuum of structural states during transient creep, each state represented by a constant σ* curve measured using load-relaxation experiments. It was surmised that each load-relaxation curve corresponds to the behavior of the material at constant (or nearly constant) average subgrain diameter, and that subgrain size distributions are similar (or nearly similar) to each other. The hardness parameter is found to follow the relation σ* ≅ 50Gb/DI where G is the shear modulus, b is the Burgers vector, and DI is the mean intercept diameter. In contrast, it was found that for olivine, load-relaxation data are equivalent to steady-state creep data. In olivine, as in rock salt, the density of mobile dislocations responds rapidly to changes in stress, but, unlike rock salt, subgrain formation in olivine is very slow or nonexistent under the conditions tested, resulting in the direct correspondence between creep and load relaxation. |
