Tensile properties of rocks in four-point beam tests under confining pressure

The strength and elastic properties of rocks were measured with a fourpoint beam device placed inside a pressure vessel. The experiments were conducted with Tennessee sandstone, Indiana limestone, and Berea sandstone. The tensile Young's modulus is nonlinear and best represented by σ_t = A ϵ_t, where σ_t, ϵ_t are the tensile stress and tensile strain and A and B are constants. B ranges from 0.56 for tests without confinement to 0.85-0.9 for tests with confinement of 10 MPa or more. The tensile strength depends only slightly on the confining pressure and it ranges from -8.8 MPa to -5.1 MPa. The yielding envelope agrees with the parabolic shape predicted by Griffith. The fractures observed under 20 MPa of confinment are invariably tensile with no indication for transition to shear fractures (faults).