Strain Localization in Granite via Optical Fiber Sensing: Effects of Crack Heterogeneity and Stress Relaxation

Abstract Monotonic triaxial compression and multi-stage relaxation tests were conducted on both pristine and thermally fractured granite core samples to investigate the effect of crack heterogeneity and stress relaxation on strain localization. Full-field strain distributions across the sample surfaces were mapped using Distributed Optical Fiber Sensing in combination with a compact fiber configuration. Experimental results indicate that increased crack heterogeneity enhances both the strength and total strain accumulation of granite sample prior to macroscopic failure under confined conditions. Granite with greater heterogeneity exhibits an earlier onset, more prolonged, and slower progression of strain localization compared to less heterogeneous granite, ultimately resulting in a higher degree of strain localization prior to macroscopic failure. In highly heterogeneous granite, high-strain events are redistributed across the sample, preventing excessive strain localization in specific volumes, whereas in less heterogeneous granite, the strain localization zone remains fixed before system-size failure. Optical microscopic observations confirm that higher crack heterogeneity promotes the further development of pre-existing compact and distributed microcracks during mechanical loading, whereas lower heterogeneity promotes the formation of isolated, long microcracks. Additionally, stress relaxation facilitates subcritical crack growth, promoting strain accumulation in pre-damaged zones and enhancing strain heterogeneity within the granite samples. These findings highlight the significant role of stress state, pre-existing damage zones, and initial crack heterogeneities in governing strain localization and macroscopic failure behavior in the upper crust.