Rock Failure Envelope and Behavior Using the Confined Brazilian Test

Abstract

AbstractComplete rock strength envelopes spanning both the tensile and compressive stress domains are essential to engineering design in Rock. However, the exact shape of the envelope in the tensile zone is not known. The major impediment to establishing the true profile of the envelope is the inherent difficulties in conducting tension tests under confinement. In this study, we use the confined Brazilian test to investigate the impact of confining pressure on tensile strength, the failure mechanism, and the brittle‐ductile transition. By applying a constant confining pressure, a state of triaxial stress is created in the disc such that the disc center is under three nonzero and unequal principal stress components. By increasing the confining pressure, the least principal stress changes from tensile to compressive so that rock failure can be observed over a wide range of stress conditions. Four lithologies have been tested in this study, including limestone and three sandstones. Experimental results suggest that the complete strength envelope resembles an offset parabola opening to the right with its vertex above the horizontal axis in the domain. The tensile strength shows a pronounced increase under confinement. For all lithologies tested, tensile fracturing disappears as the confining pressure increases, with the stress‐displacement curves exhibiting typical brittle‐ductile transitions and a change in failure mode from extension to shear. Observation of the progressive increase in the fracture angle with confining pressure also provides laboratory evidence for the continuous transition of failure from extension to shear.