Experimental analysis of the extension to shear fracture transition in Berea Sandstone

To characterize low-pressure, brittle deformation in porous, granular rock, notchcut cylinders (30 mm neck diameter) of Berea Sandstone were extended in a triaxial apparatus from 10 to 160 MPa confining pressure at strain rates of 10-4 s-1 and 10-5 s-1. Acoustic emission counts were monitored when extending samples at a slow strain rate. Stress at fracture is characterized by the least compressive principal stress, ??3, and maximum compressive principal stress, ??1 (??1 = Pc). A change in strength dependence on pressure at Pc = 50 MPa corresponds to a change from pure macroscopic extension fracture to mixed-mode opening and shear fracture, and likely reflects the increase in mean stress that suppresses the propagation of extension fractures and the interaction between closely-spaced stepped cracks. Within the extension fracture regime (Pc < 50 MPa), ??3 at failure becomes slightly more tensile with an increase in Pc. At Pc > 50 MPa, ??3 at failure becomes more compressive with an increase in Pc and follows Coulomb behavior; however, the angle between the fracture surface and ??1 increases continuously with Pc. Fracture surfaces characteristic of the extension to shear fracture transition appear as linked, stepped extension fractures; the length of extensional segments decreases with increasing pressure. The onset of acoustic emissions and inelastic strain at fracture occurs at earlier points in the strain history with pressure, consistent with the Griffith prediction of the beginning of fracture growth