Method for correction of differential stress calculations from experiments using the solid salt assembly in a Griggs-type deformation apparatus

It is necessary to perform deformation experiments in appropriate temperature and pressure conditions equivalent to the inside of the earth to characterize rheological properties of rocks. There are several types of deformation apparatus using different confining media such as gases or weak solids. Solid medium apparatus is suitable to generate high pressures and temperatures to investigate mechanical properties of rocks in crust or mantle. However, stress accuracy of solid medium apparatus was not high. Recent calibration allowed us to obtain steady-state stresses accurately within ±. 30 MPa. However, it was not able to reproduce initial linear region, transient and post-yield behaviors because the calibration was obtained from the comparison of stresses only at 5% strain. In this study, we performed several axial compression experiments on metals to measure differential stresses using a Griggs-type deformation apparatus with solid salt assembly (SSA). Measured stresses are consistent with results of the prior research within ±. 30 MPa under the identical conditions, while the measured stresses increased with the confining pressures. Obtained mechanical data were analyzed based on the viscoelastic constitutive law. Master curves were constructed from identical materials between the Griggs and gas apparatuses under normalized temperatures, strains, and confining pressures. Therefore, it is considered that differences between both master curves are derived from distinction of rheology components of two apparatuses. From the comparison between master curves, a correction method for the stresses obtained in the Griggs apparatus with SSA was derived. Applying the correction method to stress measurements of metals using the Griggs apparatus with SSA, it became possible to reproduce the stress equivalent to ones measured using gas apparatus not only at steady-state but also at initial linear region, transient and post-yield behaviors within an error of ±. 30 MPa. Moreover, the correction can be extended to higher confining pressure up to 1500 MPa.