STRESS DISTRIBUTIONS IN POLYCRYSTALLINE QUARTZ USING RAMAN SPECTROSCOPY

The internal distribution of stress in an elastically anisotropic rock under a load is not well understood, however, the stress distribution is an important factor in how the rock will ultimately deform. The Reuss bound, when applied to a polycrystal, describes isostress on each grain resulting in homogeneous intragranular strain and heterogeneous intergranular strain. Through finite element modeling, Burnley [Nature Communications, DOI: 10.1038/ncommons3117 (2013)], finds that stress can vary intragranularly within the polycrystalline material. It is hypothesized that stress percolation through force chains best describes and explains the heterogeneous stress distribution in polycrystals that can then lead to shear localization and subsequent deformation in rocks.

Experimentally measuring the manifestation of stress is possible through Raman spectroscopy. Raman spectroscopy is capable of quantifying elastic strain changes in a crystal lattice by looking at a change in spectral peak position in a polycrystal with and without an applied load. A megapascal load was applied, using a uniaxial sample press equipped with a load cell, to a millimeter-sized rectangular parallelepiped consisting of Tiger's Eye quartz. Stress was calculated using the difference between the loaded and unloaded spectral peak position, and the Raman stress shift factor specific to quartz. A raster-style stress map that showed stress distribution of the polycrystalline rock was then constructed from the stress values and overlain on sample images. Previous Raman experiments have suggested that there are intergranular heterogeneous stress distributions between the grains in the polycrystal that resemble stress percolation patterning. New data with improved methodology shows an increased spatial resolution that results in a sharper image of intragranular stress showing the heterogenous stress distribution in higher resolution.