HR: 0800h
AN: T41F-1298    [Abstracts]
TI: Grain-scale Numerical Models of Mechanical Particle Size Reduction
AU: * Sparks, D W
EM: sparks@seaver.tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics, TAMU, College Station, TX 77843-3115 United States
AU: Lang, R A
EM: rlang@houston.oilfiled.slb.com
AF: Schlumberger Information Systems, 5599 San Felipe, Houston, TX 77056 United States
AU: Karner, S L
EM: karner@geo.tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics, TAMU, College Station, TX 77843-3115 United States
AB: When unconsolidated granular material is stressed (e.g. during sediment burial, or shear of gouge-filled faults) it can deform by a combination of particle rearrangement and grain fracture or crushing. The nature of these two mechanisms govern the particle size evolution, spatial distribution of grains and stresses, chemo-mechanical processes and strength characteristics of the material (e.g. rate of diagenesis, yield strength, frictional properties). We use discrete element numerical models to explore the relationships of grain-scale heterogeneity, grain breakage and rearrangement to macroscopic material strength, and intergranular porosity. We then compare our results to observations from laboratory deformation experiments on unconsolidated sand and simulated fault gouge. Our models consist of 70-1500 circular grains packed between rough rigid walls. We apply normal and shear stresses to induce slow deformation rates. The forces on each grain are calculated from elastic, viscous and frictional interaction laws at the grain contacts. When a grain experiences a critical distribution of contact forces, we break it into a set of seven new, smaller grains. We have examined two criteria for grain failure: (1) tensile stresses in the grain interior, which typically occur when a grain is subjected to two opposing contact forces, and (2) large stress concentrations at grain contacts, which are more likely in grains subjected to many contact forces.. We deformed granular systems under both hydrostatic and simple shear loading. During shear, the rate of grain breakage increases to a maximum at small strains and then decreases with continued shear. If failure criterion (1) is used, breakage occurs preferentially in small grains because surviving large grains have more contacting neighbors. With failure criterion (2), breakage is more uniformly spread across all particle sizes. Systems deformed under hydrostatic compression show compaction and grain breakage rates that vary non-linearly with the imposed stress, in good agreement with laboratory deformation tests on unconsolidated sand. Further, the numerical simulations allow for separate analysis of strains induced by grain-scale elastic compression, particle rearrangement and grain crushing.
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 5104 Fracture and flow
DE: 3210 Modeling
DE: 3220 Nonlinear dynamics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting