SCEC Award Number 15088 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title A New Paradigm for Modeling Fault Zone Inelasticity: A coupled granular-bulk framework incorporating spontaneous localization and grain fragmentation
Name Organization
Ahmed Elbanna University of Illinois at Urbana-Champaign
Other Participants Xiao Ma, and Prof. Jean Carlson
SCEC Priorities 3a, 3c, 4b SCEC Groups FARM, Seismology, CS
Report Due Date 03/15/2016 Date Report Submitted 03/14/2016
Project Abstract
The scientific objective of this proposal was to develop a 2D model for inelastic deformation in fault gouge to predict the evolution of grain size and shear banding patterns across a wide range of scales spanning both laboratory-like and field-like conditions. Methodology: We use the Shear Transformation Zone theory to describe viscoplasticity in sheared granular layers. We implement an inhomogeneous ver-sion of the theory as a user defined subroutine VUMAT in the finite element software Abaqus and couple it with a finite deformation continuum model based on the updated Lagrangian formulation and Green-Naghdi stress rate. Main results: (1) A working validated implementation of the material subroutine, (2)Identification of brittle to ductile transition in sheared granular materials as a function of initial porosity and grain size, (3) Reproducing complex strain localization patterns while tracking their evolution history. Significance: The proposal is an important step towards developing a theoretically sound framework for inelasticity and shear banding in granular materials accounting for complex structural and loading condi-tions. It opens new opportunities for multiscale modeling of earthquake ruptures that couple large scale elastodynamics with small scale inelastic processes in fault gouge. Coupling the current formulation with pore fluids will enable investigation of poro-visco-plasticity in fault gouge at a level of details that has not been addressed before.
Intellectual Merit The project addresses short-term objectives in Fault and Rock Mechanics (3a, 3c, 4b and 3e) by developing models that quantifies the influence of small scale processes on large scale rupture re-sponse. A better quantification of this issue will aid long-term objectives in Earthquake Source Physics and Ground Motion, informing models of fault system evolution and dynamics, and physics-based hazard analysis. Understanding the complex behavior of fault zone and its influence on rupture dynamics is also essential for the interpretation of seismic observations and for the problem of seismic inversion. It is also essential for evaluating impacts of future seismic events on Southern California. The proposal develops a unique and novel methodology for modeling gouge viscoelasticity considering grain evolution characteristics, shear band complexity and gouge spatial heterogeneities.
Broader Impacts The activity contributed to the training of 1 graduate student Xiao Ma who is currently conducting his PhD at UIUC on inelasticity in amorphous solids. The activity supported the PI’s travel to attend the annual meeting and continue his interactions/ explore new collaboration opportunities with other SCEC scientists. The computational methods developed as part of this proposal have applications beyond fault mechanics as it is relevant to analyzing deformation and failure in a broad range of amorphous materials including metallic glasses and lithium ion batteries. The activity contributes to SCEC efforts in developing fundamental models for multiscale deformation in fault zones which will enhance our physics based earthquake rupture simulations and improve our ground motion prediction tools on the long run. This will contribute to combating the heavy toll that earthquakes take on our society through making better informed decisions in the context of seismic hazard and risks.
Exemplary Figure Figure: Evolution of compactivity (a state variable that is in one-to one-correspondence with porosity) and stress strain response in a sheared granular layer for different initial conditions. [Top]: Contour plots for compactivity in the case of high initial disorder (a) and low initial disorder. In the former case disorder is distributed across the sample and evolves almost uniformly. In the latter, disorder is localized and a shear band emerges. [Bottom] A layer with high initial disorder shows a ductile-like deformation with the stress increasing progressively towards the flow strength (red curve). Low initial disorder yields a brittle behavior and a visible stress drop (blue curve)