Strain localization and time-dependent deformation in granodiorite characterized by distributed optical fiber sensing

A multi-stage stress relaxation test was performed on a granodiorite sample to understand the deformation process prior to the macroscopic failure of brittle rocks, as well as the transient response during stress relaxation. Distributed optical fiber sensing was used to measure strains across the sample surface by helically wrapping the single-mode fiber around the cylindrical sample. Close agreement was observed between the circumferential strains obtained from the optical fibers and the extensometer. The reconstructed full-field strain contours show strain heterogeneity from the crack closure phase, and the strains in the later deformation phase are dominantly localized within the previously high-strain zones. The Gini coefficient was used to quantify the degree of strain localization and shows an initial increase during the crack closure phase, a decrease during the linear elastic phase, and a subsequent increase during the post-yielding phase. This behavior corresponds to a process of initial localization from an imperfect boundary condition, homogenization, and eventual relocalization prior to the macroscopic failure of the sample. The transient strain rate decay during the piston displacement holding phase was quantified using the p-value in the "Omori-like" power law function. A higher initial stress at the onset of relaxation results in a lower p-value, indicating a slower strain rate decay. As the sample approaches macroscopic failure, the lowest p-value shifts from the most damaged zone to adjacent areas, suggesting stress redistribution or crack propagation in deformed crystalline rocks under stress relaxation conditions.