Earth Materials and Processes: Shearing and Healing of Earth Materials, from Grains to the Geological Scale
Abstract
Earth materials including rocks and sediment show a frictional response that is rate-dependent, contains a memory of loading history, and demonstrates healing with shear and time. The commonly accepted framework for modeling this frictional behavior is known as "Rate and State-dependent Friction , or RSF. This framework is developed for low sliding speeds (where thermal effects are not important) and embodies the notion that frictional strength depends upon a nebulous property termed "state , a function of recent slip history, as well as the current slip rate. Constitutive laws for the evolution of state are largely empirical, and none proposed thus far adequately describes the full range of laboratory observations. Furthermore, the state evolution law that does the best job of describing laboratory data (the "Slip law) has the least well developed physical justification, while the state evolution law that has the best developed theoretical justification (the "Aging law) describes very little laboratory data. This state of affairs severely limits our ability to confidently apply laboratory-derived friction laws to frictional slip in the Earth. In this proposal, we will explore the extent to which laboratory observations of state evolution can be described by models of granular friction with no time-dependent plasticity or adhesion at the contact scale. We note that laboratory experiments on even initially bare rock surfaces develop a granular (gouge) layer through mechanical wear. We will perform numerical simulations of a confined sheared frictional granular layer that are designed to mimic rock laboratory experiments. We will compare the results of our simulations to (i) abundant laboratory data currently available for frictional behavior of sedimentary and igneous rocks, and (ii) the RSF framework. Note that by leaving out time-dependence at the contact scale we are discarding what is conventionally considered to be the primary source of time-dependence in rock friction; all of the time dependence in our simulations arises from particle accelerations and wave propagation within the rock and gouge. Nonetheless, initial results show that these models are surprisingly successful at reproducing the results of laboratory experiments. As part of this proposal we will run simulations designed to mimic a wider range of experimental protocols. To the extent that our simulations accurately describe laboratory experiments, they are well-suited to help us understand why, in that the model output can be examined at the scale of individual grains or averages over many grains. We will employ a comprehensive set of tools that include studying (i) the geometrical properties and spatiotemporal evolution of contact and force networks in granular packings, (ii) the analysis of energy and stress landscapes, and (iii) the influence of grain shape and grain size distribution, to better understand the physical origins of rate dependence and healing in rock friction. We will perform granular acoustic measurements to obtain an independent measure for comparing the granular physics model with lab data, and for studying wave-rock interaction. We will implement the new physical insights we obtain in continuum models of granular materials and complex fluids. Such models will significantly advance state of the art for predicting frictional behavior of rocks and Earth materials at the geological scale, by explicitly accounting for mechanisms of healing and rate-dependence consistent with laboratory observations. The funding will be used for training a postdoctoral scholar at the interface of materials sciences and geophysics.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 09, 2020
- Source ID
- W911NF2010154
Entities
People
- Allan M. Rubin
Organizations
- Army Contracting Command
- Princeton University
- United States Army