Rock Failure Envelope and Behavior Using the Confined Brazilian Test

Complete 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 sigma(1)-sigma(3)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. Plain Language Summary We have carried out confined Brazilian tests and investigated the impact of confining pressure on tensile strength, the failure mechanism, and brittle-ductile transition. Four rock types have been tested, including limestone and three sandstones. Experimental results show that the complete strength envelope resembles an offset parabola. The tensile strength increases with confinement. Tensile fracturing disappears as the confining pressure increases, with the stressdisplacement curves exhibiting typical brittle-ductile transitions and a change in failure mode from extension to shear.

Automated summary

Confined Brazilian tests were conducted to investigate the impact of confining pressure on tensile strength, failure mechanisms, and brittle-ductile transition. Four rock types, including limestone and three sandstones, were tested. Results suggest that the complete strength envelope resembles an offset parabola, with an increase in tensile strength under confinement and a transition from extension to shear as the confining pressure increases.