Rock Failure Behavior and Brittleness Under the Confined Brazilian Test Available to Purchase

The Brazilian test, an indirect tensile test involving diametrical compression of a rock disc, is normally conducted at zero confining pressure. In this study, the Brazilian test is extended to confined conditions and the impact of confining pressure on tensile strength is investigated for a suite of rocks and the impact of confining pressure on their failure mechanism and brittle-ductile transition is studied. The samples are wrapped with copper jacket such that the confining fluid does not communicate with the rock matrix. Then, 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 investigated over a wide range of stress conditions. Six lithologies are tested; these include Indiana limestone, Scioto sandstone, Tennessee sandstone, Barnett shale, Eagleford shale and Marcellus shale. Experimental results suggest that the discs fail under complex stress conditions, yielding the failure envelope in the tensile and compressive domains. For all lithologies, tensile fracturing disappears as confining pressure increases, exhibiting a typical brittle-ductile transition. The brittle-ductile transition plots of six lithologies are obtained, the inclination angles of the fracture plane with respect to s₁ are observed to increase with confining pressure. Observation of the progressive increase in fracture angle with confining pressure, from extension fracture at low pressure to shear fracture at high pressure, provides laboratory evidence for the a continuous transition of failure from extension to shear fractures. Finally, we propose a new approach for brittleness evaluation, which is based on damage-controlled Brazilian test, and use the crack opening displacement (COD) as the brittleness index. Results for three shales are compared and it is found that the brittleness correlates well with the mineralogical contents.