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

Rocks in the upper crust are naturally heterogeneous, which affects how they become damaged and eventually fail. As failure nears, deformation becomes more focused in certain areas, a process known as strain localization, which often occurs before the rock breaks completely. This study used distributed optical fiber sensing (DFOS) technology to map strain patterns and measure strain localization in rocks during multi-stage stress relaxation tests. To increase microstructural heterogeneity, we applied thermal treatment at 500°C. Our results show that greater microstructural heterogeneity reduces strain localization, delays failure, and increases the rock's peak strength, challenging the common view that more initial damage lowers strength. Additionally, the strain changes observed during relaxation demonstrate the effectiveness of DFOS in detecting fine-scale strain, revealing that relaxation further increases strain and strain localization.