HR: 0800h
AN: T41F-1305    [Abstracts]
TI: Compaction Localization in Notched Samples of Bentheim and Berea Sandstones: Mechanical Experiments, Microstructural Observations and Stress Analysis
AU: Vajdova, V
EM: vvajdova@ic.sunysb.edu
AF: SUNY at Stony Brook, Department of Geosciences , Stony Brook, NY 11794-2100 United States
AU: * Tembe, S
EM: stembe@ic.sunysb.edu
AF: SUNY at Stony Brook, Department of Geosciences , Stony Brook, NY 11794-2100 United States
AU: Zhu, W
EM: zwei@ic.sunysb.edu
AF: SUNY at Stony Brook, Department of Geosciences , Stony Brook, NY 11794-2100 United States
AU: Wong, T
EM: teng-fong.wong@sunysb.edu
AF: SUNY at Stony Brook, Department of Geosciences , Stony Brook, NY 11794-2100 United States
AB: Compaction band formation has been documented in field and laboratory studies as a localized failure mode in porous sandstones. To simulate the development of compaction bands induced by local stress concentration, we introduced a V-shaped circumferential notch in cylindrical samples of Bentheim and Berea sandstones and conducted triaxial compression tests at a range of confining pressures from 150 to 350 MPa. Our mechanical and microstructural data indicate that the stress concentration caused a compaction band to initiate at the notch tip as a cluster of fractured. The critical differential stress for its initiation decreased with increasing confining pressure, similar to the yield stress for an unnotched sample that maps out a cap with negative slope in the stress space. At the initiation stage, the asymptotic stress field in the vicinity of the notch can be evaluated using linear elastic fracture mechanics. Near the notch the stress path is approximately linear with a slope dependent only on the Poisson's ratio. The localized mean stress is significantly enhanced relative to the remote loading. Plastic yield initiates when this stress path intersects the yield cap determined for unnotched samples. From the asymptotic stress analysis, we infer that the initiation of localized yield, as indicated by a surge in acoustic emission in a notched sample, typically involves a damage zone extending from the tip to one or two grains, in agreement with our microstructural observations. Beyond initiation, compaction localization was observed to propagate differently for the two sandstones. While the bands propagated as "anti-cracks" in a direction subperpendicular to the maximum principal stress in Bentheim sandstone, they developed as conjugate bands at an angle of \sim$55\deg$ in Berea sandstone. This difference is possibly related to the different modes of localization observed in unnotched samples: Bentheim sandstone develops discrete compaction bands with episodic stress drops, whereas Berea sandstone develops diffuse compaction bands that are significantly thicker accompanied by strain hardening.
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 5114 Permeability and porosity
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting