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Modeling the Viscoplastic Deformation of Damaged Rocks Using a Perzyna Viscoplasticity Law

Description: 

Fault damage zones host abundant fractures, from microscopic to macroscopic scale, created by stress concentrations associated with the fault slip. Sliding and closing of these fractures could promote the overall viscous behavior of damage zone rocks, which cause evolution of fault strength and stress in the interseismic period. We observed the viscous behavior of damage zone analogues created by impacting samples.

Siltstone samples were impacted using a Split Hopkinson Pressure bar at strain rates comparable to those during earthquake ruptures. Samples were then deformed in a conventional triaxial apparatus by applying several steps of differential stresses, followed by a constant stress hold period, to measure the instantaneous and time-dependent responses of the rocks. The volumetric strain response of the specimens were modeled using a Perzyna viscoplasticity law that uses the Modified Cam Clay (MCC) model as the yield criterion (Haghighat et al. 2020). We also alter the relation between plastic strain rate and overstress by raising overstress to the power of ‘n’ to better fit the experimental data. Given the stress history from the experiments as the boundary conditions, the volumetric strain is modeled and fit to the experimental data to recover constitutive parameters that describe the elastic/inelastic compliances, yield strength, overstress exponent, and parameters used to describe how the apparent viscosity changes with plastic strain.

We find that the Perzyna-MCC formulation fits the experimental data well but observed tradeoffs between parameters such as the overstress exponent and the apparent viscosity. This implies that a unique set of parameters are not constrained in some cases although minimal parameters are defined to describe basic features of the viscoplastic behavior. The correspondence of these constitutive parameters to physical attributes of the rock samples is required to constrain the appropriate magnitudes of these parameters and to eliminate the tradeoffs.

Session: Above the Seismogenic Zone: Fault Damage and Healing in the Shallow Crust

Type: Oral

Date: 4/19/2023

Presentation Time: 04:30 PM (local time)

Presenting Author: Hiroki Sone

Student Presenter: No

Invited Presentation: Yes

Authors