Statistical method for quantifying the strain localization process in Beishan granite under multi-creep triaxial compression based on distributed optical fiber sensing

To investigate the damage evolution caused by stress-driven and sub-critical crack propagation within the Beishan granite under multi-creep triaxial compressive conditions, the distributed optical fiber sensing and X-ray computed tomography were combined to obtain the strain distribution over the sample surface and internal fractures of the samples. The Gini and skewness (G-S) coefficients were used to quantify strain localization during tests, where the Gini coefficient reflects the degree of clustering of elements with high strain values, i.e., strain localization/delocalization. The strain localization-induced asymmetry of data distribution is quantified by the skewness coefficient. A precursor to granite failure is defined by the rapid and simultaneous increase of the G-S coefficients, which are calculated from strain increment, giving an earlier warning of failure by about 8% peak stress than those from absolute strain values. Moreover, the process of damage accumulation due to stress-driven crack propagation in Beishan granite is different at various confining pressures as the stress exceeds the crack initiation stress. Concretely, strain localization is continuous until brittle failure at higher confining pressure, while both strain localization and delocalization occur at lower confining pressure. Despite the different stress conditions, a similar statistical characteristic of strain localization during the creep stage is observed. The Gini coefficient increases, and the skewness coefficient decreases slightly as the creep stress is below 95% peak stress. When the accelerated strain localization begins, the Gini and skewness coefficients increase rapidly and simultaneously.