Collaborative Research: What Processes Cause State Evolution in Rate and State Friction?

合作研究：什么过程导致速率和状态摩擦的状态演化？

• 批准号: 2024766
• 负责人: Allan Rubin
• 金额: $ 9.6万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-08-01 至 2023-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2024766&HistoricalAwards=false
• 关键词: Collaborative; Research; What; Processes; Cause

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.

摩擦是在接触中的两个表面之间滑动的阻力，无论是在地震断层，门铰链的部分还是汽车发动机中的碎片。人们观察到，如果两个表面在更长的时间内接触，而滑动很少，则需要更大的力才能真正使它们再次滑动。尽管这已经知道了将近50年，但没有人确定为什么它是真的。实际上，在许多小接触点中，摩擦表面实际上只互相接触。为什么更长的接触时间使摩擦更高的最受欢迎的想法是，接触更长的时间增加了许多接触点的总面积或大小。但是，计划该研究项目的团队的最新结果表明，改变这些小接触点的强度或“质量”比改变其区域更为重要。实验者和理论家团队将进行和分析实验，以便更好地理解这一点很重要，因为摩擦强度的变化会影响许多实际重要性过程。其中包括两个表面稳定滑动还是经历其他粘性和滑动运动。这种替代动作发生，例如在地震期间或将弓伸到小提琴弦上时。对摩擦的更好理解对包括制造和运输在内的几个经济重要行业有影响。如果结果是预期的革命性，那么该项目将改变试图了解许多学科的生涩滑动的科学家的研究方向。它可能会影响追求接触区域变化可能导致摩擦变化的可能性的人们的研究方向。这表明了解摩擦接触点处的化学键很重要。这将导致适当方程式的新理论投资用于将实验室结果应用于地震断层。该项目还将提高本科生和研究生的技能，知识和网络，以及参与该项目理论部分的两位早期职业科学家的技能和网络。摩擦的速率和州本构方程所描述的行为已被认可了将近50年，并且已被广泛接受，在地震裂片的成核中被广泛接受。然而，关于该公式中“状态”变量代表的微机械含义仍然存在不确定性。换句话说，哪种物理或化学变化控制状态，决定其进化。从基本的科学角度来看，这是不令人满意的。这也不令人满意，因为如果未对国家进化的过程不理解和以基于过程的方程为特征，那么实验室结果的推断以了解地震力学并不基于牢固的基础。社区中的许多人认为，进化过程涉及摩擦滑动界面之间接触大小的时间依赖性增加，即接触数量的演变。但是，仅接触数量的演变无法解释正常应力步骤后对摩擦现象学的最新实验观察结果。相比之下，接触界面的质量，换句话说，每单位区域的接触剪切强度主要控制着正常应力步骤后的强度进化，这是一种变革性的观察。团队汇集了实验家和理论家。他们将对各种地质和其他材料进行大量实验，以及对实验结果的强化理论建模和反转，以确定接触数量和质量变化对状态进化的相对贡献。将包括颗粒状岩石的离散元素建模，最近已证明可以重现各种实验发现。该奖项反映了NSF的法定任务，并且使用基金会的知识分子优点和更广泛的影响审查标准，通过评估被认为是珍贵的支持。