Accurate differential stress measurement using the molten salt cell and solid salt assemblies in the Griggs apparatus with applications to strength, piezometers and rheology

摘要

Calibrations have been developed for the molten salt cell and solid salt assemblies used in tri-axial Griggs-type piston-cylinder deformation apparatus (Griggs apparatus) that enable accurate differential stress determinations in both assemblies at high pressures and high resolution differential stress measurements in the molten salt cell. Elastic response of the apparatus was characterized by loading alumina and WC rods, and flow strengths were measured for high-purity nickel and molybdenum, and a Ti-Zr-Mo alloy in replicate experiments in a Griggs apparatus and a Heard tri-axial gas apparatus (gas apparatus) at identical pressures, temperatures and strain rates. The elastic response of the Griggs apparatus is nonlinear at low total loads (<15kN or σ_1=760MPa on a 5mm diameter sample); but becomes linear at loads greater than 15kN. Plastic deformation experiments on water-insensitive metals indicate that differential stresses measured in the molten salt cell (MSC) and solid salt (SSA) assemblies in the Griggs apparatus are greater than those measured in the gas apparatus at all conditions, but vary systematically and reproducibly. A simple, linear calibration (σ_(Gas apparatus)=0.73×σ_(GriggsMSC), +/σ10MPa, where differential stress σ=σ_1-σ_3) can be applied to the differential stresses obtained using the MSC in the Griggs apparatus to obtain sample flow strength. A similar linear calibration (σ_(Gas apparatus)=0.73×σ_(GriggsSSA)σ48MPa, +/σ30MPa) can be applied to the differential stresses obtained using the Griggs apparatus with a solid salt assembly to obtain accurate sample flow strength. These calibrations have been tested by applying them to mechanical data for carbonates deformed in the Griggs apparatus and gas apparatus. The calibrations are also applied to data from previous studies performed in the Griggs apparatus on clinopyroxenites and quartzites and compared to results from similar experiments using the gas apparatus. When the experimental starting materials and conditions are the same, the recalculated flow strengths measured in the Griggs apparatus experiments agree with those measured in the gas apparatus. However, discrepancies in differential stress are still observed in cases where starting materials differ or water available to silicate samples varies, presumably through hydrogen diffusion through sample jackets and hydrogen fugacities of the surrounding confining media.