Analysis of residual stress distributions in olivine using HR-EBSD mapping and EVP-FFT simulations

Deformation of rocks involves stresses that are heterogeneously distributed at the grain scale and below. Residual stress in the constituent minerals may arise from anisotropic thermal expansion, elasticity, and viscoplasticity, and from elastic strains associated with dislocation substructures. These stresses are likely intimately linked to the microphysical processes of high-temperature creep. However, mapping heterogeneous distributions of internal stress has been hindered by a lack of high-resolution techniques, and interpreting their origins and significance is challenging due to the variety of potential causes. We employ high-angular resolution electron backscatter diffraction (HR-EBSD) and elastoviscoplastic fast fourier transform (EVP-FFT) simulations to characterise residual stress in aggregates of olivine and infer their origins and implications for creep. We analyse hotpressed aggregates of olivine that were either macroscopically undeformed or were deformed by dislocation-accommodated grain-boundary sliding. Stress fields were mapped using HR-EBSD based on cross-correlation of diffraction patterns within each grain. Maps of crystal orientations obtained from EBSD were used as input for EVP-FFT simulations of the deformation, which include elastic and viscoplastic anisotropy but not elastic strain associated with dislocation structures or thermal contraction. The undeformed sample contains stress heterogeneities with magnitudes typically on the order of hundreds of MPa arising from thermal contraction. The range of residual stresses is significantly broader in deformed samples than in the undeformed sample, confirming that deformation induces stress heterogeneity beyond that due to thermal contraction. Stresses in deformed samples, corrected for stresses arising from thermal contraction, are still larger than those predicted in simulations, indicating that a portion of the residual stress arises from the dislocation substructure. Stress heterogeneities arising from dislocation content and from interactions between grains with anisotropic mechanical properties can be comparable in magnitude to externally applied stresses and therefore likely impact processes including dislocation motion and recrystallisation.