SCEC Award Number 13038 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Compactivity, Comminution, Heating, and Disorder -- The Physics of Granular Fault Gouge
Name Organization
Jean Carlson University of California, Santa Barbara
Other Participants Charles Lieou, Ahmed Elbanna
SCEC Priorities 3, 4, 2 SCEC Groups FARM
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
We combined the Shear Transformation Zone (STZ) theory with fracture mechanics to describe grain fragmentation in shear flow. We have shown that the resulting dynamics for grain size reduction shares common qualitative features with those seen in several simulations – namely, that grain splitting dominates at small shear strains and grain abrasion dominates at large slip displacements. We have also found a feedback between strain localization and grain fragmentation which explains the formation of a thin gouge layer with particles with characteristic size several orders of magnitude smaller than those outside the shear band. In addition, we generalized the STZ theory to include temperature dependence of material properties such as the yield stress to capture additional causes of weakening and localization. We have applied our results to fit high-speed frictional experiments and make predictions for steady and transient frictional response of sheared gouge layers. Finally, we have developed a preliminary thermodynamic model that explicitly takes into account additional dissipative mechanisms and system-specific features such as inter-particle friction, vibration generated by acoustic waves, and shape effects, which are found in experiments to have important implications on the dynamics of faulting and rupture.
Intellectual Merit In this reporting period we continued our work investigating the physics of plastic deformation and strain localization and the corresponding implications for dynamic earthquake problems. We are extending Shear Transformation Zone (STZ) theory to include new features, previously omitted in the model, and have begun a series of direct comparisons to laboratory experiments involving frictional weakening, shear banding, and auto-acoustic compaction. These new features are expected to be important to earthquake physics at multiple scales. Our work addresses several SCEC priority science objectives in Fault and Rupture Mechanics (3c,3e and 4b) by developing physical constitutive laws for the fault zone, and evaluating their impact on rupture dynamics, faulting, and energy balance.
Broader Impacts Funds from the project were used to support the training and education of graduate student Charles Lieou at UCSB, who is expected to complete his PhD June, 2015. We also continue our collaboration with Professor Ahmed Elbanna, who has recently begun as an Assistant Professor at University if Illinois in Champaign-Urbanna. Results of this project provide a first-principles, physics-based understanding of frictional mechanisms associated with faulting and rupture dynamics, which will enable more accurate estimation of seismic hazards.
Exemplary Figure Figure 2: (a) Temporal evolution of characteristic grain size showing that grain comminution is most severe at the early stage, immediately upon the onset of irreversible plastic deformation. It slows down dramatically afterwards, suggestive of the dominance of grain abrasion at later stages. (b) At a fast shear rate, grain fragmentation significantly reduces the flow stress, under generic assumptions on the rate-strengthening behavior of the material. Strain localization also provides another weakening mechanism. (c) Sample grain size profile (close-up) and (d) Shear rate profile across the material, at various shear strain displacements. There is a feedback between shear localization and grain fragmentation accounting for the formation of a thin gouge layer at the shear band. Note the difference in horizontal scale.

Lieou, C. K. C., A. E. Elbanna, and J. M. Carlson (2014), Grain fragmentation in sheared granular flow: Weakening effects, energy dissipation, and strain localization, Phys. Rev. E 89(2), 022203.