HR: 0830h
AN: T41D-0256    [PDF]
TI: Conditions for Localized Deformation in Porous Granular Materials Under Axisymmetric Loading Using a Two Yield Surface Model
AU: * Challa, V
EM: challav@clarkson.edu
AF: Clarkson University, Mechanical and Aeronautical Engineering, Potsdam, NY 13699-5727 United States
AU: Issen, K A
EM: issenka@clarkson.edu
AF: Clarkson University, Mechanical and Aeronautical Engineering, Potsdam, NY 13699-5727 United States
AB: Strain localization in porous granular rock occurs in field and laboratory settings. Compaction bands and dilation bands are of particular interest since localized deformation may increase (or decrease) porosity/permeability, possibly affecting fluid flow within geological formations and impacting drilling and extraction applications. Mollema and Antonellini (1996) first identified compaction bands as "thin planar zones of pure compressional deformation," oriented perpendicular to maximum compression. B‚suelle (2001), and Du Bernard, Eichhubl and Aydin (2002) recently reported dilation bands (oriented perpendicular to minimum compression) in laboratory and field settings, respectively. Rudnicki and Rice (1975) modeled strain localization as a bifurcation from homogeneous deformation using a single yield surface model to describe shear localization in low porosity rock. However, recent reexaminations of this model reveal that predicted band orientations do not agree with experimental observations of compaction bands in high porosity sandstone. Microstructural observations by Men‚ndez, Zhu and Wong suggest multiple active damage processes, prompting development of a two yield surface model by Issen to describe strain localization in high porosity sandstone. The first yield surface corresponds to a dilatant, frictional mechanism, while the cap corresponds to a compactant mechanism. This model successfully predicts the experimentally observed compaction bands under axisymmetric compression (ASC) when the slope of effective mean stress-inelastic volume strain curve is zero or slightly positive, corresponding to the stress plateau characteristic of compaction band formation. Determining conditions for dilation band formation under axisymmetric extension (ASE) using the two yield surface model is facilitated by certain mathematical symmetries with compaction band conditions for ASC. The conditions for dilation band formation though complex, depend largely on the dilation coefficient and slope of the shear yield surface with dilation bands being predicted for a wide range of probable material parameter values. These conditions are less restrictive than the analogous conditions for compaction band formation under ASC, suggesting that dilation band formation could be a common deformation mode for high porosity sandstone. Furthermore, since multiple stress paths are possible for a single stress state (e.g., ASC or ASE), the chosen stress path defines the region of the yield surface activated (and therefore the appropriate constitutive model), thus influencing whether the localization condition can be satisfied. This may have important implications for band formation in geological settings.
DE: 8100 TECTONOPHYSICS
DE: 8199 General or miscellaneous
SC: Tectonophysics [T]
MN: 2003 Fall Meeting