HR: 0800h
AN: T41F-1297    [Abstracts]
TI: Experimental Analysis of Hybrid Fracture in Berea Sandstone
AU: * Bobich, J K
EM: jbobich@geo.tamu.edu
AF: Center for Tectonophysics and Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: Chester, F M
EM: chesterf@geo.tamu.edu
AF: Center for Tectonophysics and Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: Chester, J S
EM: chesterj@geo.tamu.edu
AF: Center for Tectonophysics and Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AB: Previous triaxial extension experiments investigating the transition from extension fracture to shear fracture in low porosity, polycrystalline Carrara marble demonstrate abrupt changes in strength and a continuous transition in fracture orientation and morphology with increasing confining pressure, Pc. New tests on Berea sandstone investigate the same transition in a porous aggregate. Notch cut cylinders (30 mm neck diameter) of Berea sandstone (18% porosity, 0.15 mm average grain size, 80% quartz, 20% feldspar, and trace rutile and kaolinite) were extended in a triaxial apparatus from 0 to 160 MPa confining pressure at a rate of 20 $\mu$m/s. Stress at fracture is characterized by the least compressive principal stress, S3, and maximum compressive principal stress, S1 (S1=Pc). An abrupt change in fracture strength at Pc=50 MPa corresponds to a change from pure macroscopic extension fracture to mixed-mode opening and shear (hybrid) fracture. Within the extension fracture regime, S3 at failure becomes slightly more tensile with an increase in Pc, unlike the constant tensile strength observed for marble. Within the hybrid and shear fracture regimes, S3 at failure becomes more compressive with an increase in Pc. The angle between the fracture surface and S1 increases continuously with Pc, consistent with the marble results. In both rock types, hybrid fractures appear as linked, stepped extension fractures; the length of extensional segments decreases with increasing pressure. The abrupt change in failure strength at the transition from extension to hybrid modes in both rock types likely reflects the increase in mean stress that suppresses the propagation of extension fractures, and the interaction between closely-spaced stepped cracks. In the extension fracture regime, the different dependence of fracture strength on Pc for sandstone and marble may reflect differences in grain scale deformation mechanisms.
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8025 Mesoscopic fabrics
DE: 8164 Stresses--crust and lithosphere
DE: 5104 Fracture and flow
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting