| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | July 20, 2020 |
| Latest Amendment Date: | July 20, 2020 |
| Award Number: | 2024766 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Eva Zanzerkia
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | August 1, 2020 |
| End Date: | July 31, 2023 (Estimated) |
| Total Intended Award Amount: | $95,993.00 |
| Total Awarded Amount to Date: | $95,993.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1 NASSAU HALL PRINCETON NJ US 08544-2001 (609)258-3090 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
NJ US 08544-2020 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Geophysics |
| Primary Program Source: |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Friction is the resistance to sliding between two surfaces in contact, whether it be on an earthquake fault, the parts of a door hinge, or pieces in an automobile engine. People have observed that if two surfaces are in contact for longer times with little sliding, greater forces are required to really get them sliding again. Although this has been known for nearly 50 years, no one is sure why it is true. Frictional surfaces actually only touch each other in many small contact spots. The most popular idea for why longer contact times make friction higher is that being in contact longer increases the total area or size of the many contact spots. However, recent results by the team planning this research project suggest that changing the strength or ?quality? of those small contact spots is more important than changing their area. The team of experimentalists and theoreticians will conduct and analyze experiments in order to better understand which explanation is correct. It is important to understand this because the variations in frictional strength influence many processes of practical importance. These include whether two surfaces slide steadily or undergo alternating sticking and slipping motion. Such alternating motions occur, for example, during earthquakes or when a bow is pulled across a violin string. A better understanding of friction has implications for several economically-important industries, including manufacturing and transportation. If the results are as revolutionary as anticipated, this project will alter the research directions of scientists trying to understand jerky sliding in many disciplines. It could influence the research directions of people who are pursuing the possibility that changes in contact area are responsible for changes in friction. It would show that it is important to understand the chemical bonding at frictional contact spots. It would result in new theoretical investigations of the appropriate equations to use for applying lab results to earthquake faults. The project will also increase the skills, the knowledge, and the networks of an undergraduate student and a graduate student, as well as of the two early-career scientists involved in the theoretical parts of this project.
The behavior described by rate and state constitutive equations for friction has been recognized for nearly 50 years and is widely accepted as being important in the nucleation of earthquake slip. Nevertheless, there is still uncertainty regarding the micromechanical meaning of what is represented by a ?state? variable in this formulation; in other words, what physical or chemical changes control state, dictating its evolution. This is unsatisfactory from a fundamental scientific point of view. It is also unsatisfactory because if the processes involved in the evolution of state are not understood and characterized by process-based equations, then extrapolation of laboratory results to understanding earthquake mechanics does not rest on a firm foundation. It is believed by many in the community that the evolution process involves time-dependent increase in the size of contacts across a frictionally sliding interface, namely evolution of contact quantity. However, evolution of contact quantity alone cannot explain recent experimental observations of friction phenomenology following normal stress steps. In contrast, the quality of the contact interface, in other words the contact shear strength per unit area, dominantly controls strength evolution following normal stress steps, a transformative observation. The team brings together experimentalists and theoreticians. They will conduct a large suite of experiments on a wide variety of geological and other materials, together with intensive theoretical modeling and inversion of the experimental results, to determine the relative contributions of changes in contact quantity and quality to evolution of state. The theoretical modeling will include discrete element modeling of granular gouge which has been recently shown to reproduce a variety of experimental findings.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Whether a fault slips in brief, potentially damaging earthquakes, or slides steadily at the rate dictated by the motions of tectonic plates, depends on small variations in frictional strength with sliding speed and recent fault slip history. Earthquake generation, for example, requires the frictional strength of the fault to decrease with increasing slip speed. The modern understanding of time-varying friction takes as its starting points the ideas that (1) the true contact area between two sliding surfaces is a small fraction of their apparent surface area, and (2) frictional resistance comes from the need to break atomic bonds across those microscopic "asperities" that bridge the two surfaces. For several decades, the conceptual framework used to describe the evolution of friction falls under the heading of "rate- and state-dependent friction", where the rate-dependence refers to the current sliding speed, and the state-dependence refers to the "state" of the fault surface, meaning a combination of the true contact area and intrinsic strength of the chemical bonds at those contacts. However, the equations used to describe how "state" evolves with time are largely empirical, and do not adequately describe all the robust results of laboratory experiments. This makes it difficult to extrapolate the results of laboratory rock friction experiments to faults in the Earth.
In this project we investigated the nature of fault "state" by subjecting a sliding surface between two rock samples to a range of sudden increases in the "normal" force, or force perpendicular to the sliding surface, while trying to maintain a constant sliding velocity. The sudden increase in normal force increases the true contact area by a nearly equal factor (e.g., a near-doubling of contact area for a doubling of the normal stress), which acts to reduce the slip speed initially. We find that the initial reduction in slip speed is significantly less than would be expected if the newly-formed contact area had the same strength as the existing area. In fact, to match the observed reduction in sliding speed, we find that the new contact area can only be about 20% as strong as the previously-existing area, and that the surface subsequently strengthens ("state" increases) at roughly constant contact area. This implies that the dominant source of state evolution following the normal stress step is contact quality, a conclusion which runs counter to the conventional wisdom that the evolution of "state" at constant normal stress mostly reflects changes in contact area.
Broader impacts: In addition to learning something new about the evolution of fault "state", understanding the response of fault frictional strength to changes in normal stress is important because changes in normal stress are an expected byproduct of slip on a non-planar fault. The response of the fault to these changes is important, for example, in models of how earthquakes develop. In addition, the phenomenology of rate- and state-dependent friction is common to many sliding surfaces of societal relevance, including metals, ceramics, polymers, and pharmaceutical powders. Understanding state evolution in rock is therefore likely to lead to insight into state evolution in other materials of economic interest.
Last Modified: 12/28/2023
Modified by: Allan M Rubin
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