Nonequilibrium Mechanism of Geomaterials
Abstract
Certain materials display surprisingly large dynamical nonlinearities, including conditioning and memory effects, appearing at wave strains smaller and parts per million. Geomaterials are prototypes for these apparently unrelated solids that include granular materials, sintered and damaged metals, some ceramics, and even high explosives. Rocks are hysteretic and can be strongly nonlinear; their relation between stress and strain depends on the rate of the measurement. These materials relax over very long durations after an initial impulse or wave disturbance, a nonequilibrium phenomenon termed "slow dynamics". The microphysics underlying slow dynamics remains unknown. Previous experiments by us established the existence of two strain regimes separated by a strain threshold. Below this surprisingly small threshold strain (lower than 1 part per million), the material is non linear and quasi-equilibrium thermodynamics applies; above this threshold the behavior becomes nonequilibrium, as well as nonlinear. Whereas the material behavior in the first (reversible) nonlinear regime is well-described by Landau theory, the second regime the intrinsic nonlinearities are hard to disentangle from the ongoing nonequilibrium relaxation process. In order to understand better the coupling between nonlinear and nonequilibrium effects we investigated the full dynamical range of phenomena considered: high frequency/low strain to almost static/high strain. We carried out two experimental procedures on a number of rocks samples: resonance bar methods, where we measure amplitude as a function of drive amplitude and frequency and quasi-static stress/strain measurements. In the quasi-static domain Barea sandstone has demonstrated for the first time that rate is important and that nonequilibrium effects can affect results. In order to disentangle conditioning and nonequilibrium effects from nonlinear effects in the domain of high frequency and low strain we run long term resonance bar experiments under extremely stable environmental conditions. We will present preliminary results obtained in this regime. Understanding these rates should help understand the physics of these kinds of rocks. Theoretical framework for implementing microstructure-motivated models is under development.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMNG23B1138T
- Keywords:
-
- 4499 General or miscellaneous