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

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 (< 15 kN or σ1 = 760 MPa on a 5 mm diameter sample); but becomes linear at loads greater than 15 kN. 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 (σGas apparatus = 0.73 × σGriggsMSC, +/−10 MPa, where differential stress σ = σ1–σ3) can be applied to the differential stresses obtained using the MSC in the Griggs apparatus to obtain sample flow strength. A similar linear calibration (σGas apparatus = 0.73 × σGriggsSSA − 48 MPa, +/−30 MPa) 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.

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► Calibrates differential stress measurements collected using Griggs rock deformation apparatus against the gas apparatus.
► Results indicate that the Griggs apparatus overestimates stresses, but a temperature and rate independent linear calibration can be applied that brings results of replicate experiments into coincidence.
► Calibrations can be used to explore previous studies performed using the Griggs apparatus.