Quantifying Localized and Delocalized Rock Deformation in Multi-Stage Relaxation Experiments Using Distributed Optical Fiber Sensing

Multi-stage uniaxial and triaxial stress relaxation tests were performed on Weschnitz granodiorite, Beishan granite, and Jinping dolomite marble to investigate the deformation evolution before system-size failure, and to study stress relaxation responses. Optical fiber sensing was used to measure distributed strain for full-field strain reconstruction across the sample surface. Strain heterogeneities due to imperfect boundary conditions are detected before and during the linear elastic deformation phase in all samples. The initial strain heterogeneity in the elastic phase is found to control the subsequent inelastic strain localization in granodiorite and granite samples. Macroscopic brittle splitting or faulting in granitic samples eventually occurs within or at the boundaries of the strain localization zones. In contrast, dolomite marble has a more homogeneous strain distribution, with increased differential stress promoting strain delocalization. The reduced axial strain rates during stress relaxation promote time-dependent deformation mechanisms, leading to different spatial distributions of strains. Stress relaxation does not significantly change the degree of strain localization in granite, but it promotes strain delocalization in marble after the onset of dilatancy. In multi-stage tests, inelastic strain accumulates mainly during stress relaxation in granite samples and during stress ramping in marble samples. The different strain distributions and relaxation responses between rock samples result from different deformation mechanisms: localized strain in granite results from clustered microcracking, whereas distributed strain in dolomite marble is driven by both microcracking and low-temperature plasticity (e.g., dislocation glide). These results suggest that lithological differences may result in different precursor signals before system-scale failure and postseismic bulk relaxation responses.