Spatial Persistence of High Strain Events During Brittle Failure.

The onset of brittle failure in rocks includes dilatancy and strain localization. To better understand this nucleation process, we analyze the evolution of the local three‐dimensional strain tensor using X‐ray tomograms acquired during triaxial compression experiments on granite and sandstone. The onset of the localization of the compaction, dilation, and shear strain occurs when ∼65% of the rock volume experiences dilation. Tracking the locations of the high strains throughout loading suggests that the deformation that occurs early in loading influences the location of the system‐sized fracture network that produces macroscopic failure. This influence is larger in the sandstone experiments than the granite experiments, likely due to the microstructure of the sandstone. These results have important implications for detecting precursors to catastrophic failure. Plain Language Summary: We investigate the fundamental processes that lead to brittle failure in rocks. We deform two common types of crustal rocks, granite and sandstone, under upper crustal stress conditions. As the stress applied to the rock increases, the rock tends to expand (dilate) more than compact, particularly as it approaches catastrophic, macroscopic failure. A larger portion of the rock undergoes dilation when the strain field starts to localize, indicating that accelerating dilation is a precursor to macroscopic failure. We observe different localization patterns in the rocks: in sandstone, strain localization progresses monotonically with increasing stress, whereas phases of delocalization can occur in the granite. Two competing models describe the development of the system‐sized fracture network that produces macroscopic failure: the network develops from (a) the coalescence of fractures that form early in loading, or from (b) the propagation of a process zone of interacting fractures through relatively intact rock. We find that the high strain events persist at the same location throughout the experiments more than expected by chance, particularly in experiments on sandstone. The results provide perhaps the most robust experimental confirmation yet that the fracture network that causes macroscopic failure evolves from the deformation that occurs earlier in loading. Key Points: X‐ray tomography quantifies the evolving spatial evolution of high strain events during brittle failureAt the onset of strain localization, on average 65% of the volume of the rock cores undergo dilationLocalized zones of high strain events persist in space from the onset of loading [ABSTRACT FROM AUTHOR]