Microfracturing, damage, and failure of brittle granites

The evolution of stress-induced damage and the eventual brittle failure are experimentally analyzed for Mount Scott granite of Oklahoma. We quantify the damage intensity in two methods and directly compare model predictions and actual damage. The 14 samples of the medium-grain-size granite were loaded triaxially at dry conditions, room temperature, and under 41 MPa confining pressure. Microfractures were mapped in five samples, and the majority of them (80%) belong to two groups: tensile microfractures trending subparallel to the loading axis and shear microfractures trending 11°-40° off the loading axis. The tensile microfractures dominate the low-stress stage, and they remain intragranular with a stress increase. The relative density of shear microfractures increases with increasing stresses, and they formed elongated, intergranular zones of coalescing microfractures. We compared two independent values of damage intensity: (1) the macroscopic, experimentally measured reduction of the deformation modulus and (2) the expected reduction of this modulus calculated with several damage models for the density of the mapped microfractures. Our fracture density data best fit the model of noninteracting cracks of Kachanov [1992].