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

摩擦力是两个接触表面之间滑动的阻力，无论是在地震断层上，门铰链的部件，还是汽车发动机的部件。人们已经观察到，如果两个表面接触的时间较长，并且很少滑动，则需要更大的力才能真正使它们再次滑动。尽管人们已经知道这一点近50年了，但没有人知道为什么这是真的。摩擦表面实际上只在许多小的接触点上相互接触。关于为什么接触时间越长摩擦越大，最流行的观点是，接触时间越长，许多接触点的总面积或大小就越大。然而，计划这个研究项目的团队最近的结果表明，改变这些小接触点的强度或“质量”比改变它们的面积更重要。为了更好地理解哪一种解释是正确的，由实验学家和理论学家组成的团队将进行和分析实验。理解这一点很重要，因为摩擦强度的变化会影响许多具有实际重要性的过程。这些包括两个表面是否稳定滑动或经历交替的粘着和滑动运动。这种交替运动发生，例如，在地震中或当弓拉过小提琴弦时。更好地理解摩擦对包括制造业和运输业在内的几个经济上重要的行业都有影响。如果结果像预期的那样具有革命性，这个项目将改变科学家们试图理解许多学科中滑动的研究方向。这可能会影响人们的研究方向，他们正在追求接触面积的变化可能是摩擦变化的原因。这将表明理解摩擦接触点的化学键是很重要的。这将导致新的理论研究的适当方程，用于应用实验室结果地震断层。该项目还将增加一名本科生和一名研究生的技能、知识和网络，以及参与该项目的理论部分的两名早期职业科学家。摩擦的速率和状态本构方程所描述的行为已被公认了近50年，并被广泛认为是地震滑动成核的重要因素。然而，在这个公式中，“状态”变量所代表的微观力学意义仍然存在不确定性；换句话说，物理或化学变化控制状态，决定其演变。从基础科学的观点来看，这是不令人满意的。它也不能令人满意，因为如果状态演化过程不能被理解，不能用基于过程的方程来表征，那么通过实验室结果的外推来理解地震力学就不能建立在一个坚实的基础上。学界的许多人认为，这种演化过程涉及摩擦滑动界面上接触量随时间的增加，即接触量的演化。然而，接触量的演变不能单独解释最近的实验观察到的正常应力步骤后的摩擦现象。相比之下，接触界面的质量，即单位面积的接触剪切强度，主要控制着正常应力步骤后的强度演变，这是一个变革性的观察结果。这个团队汇集了实验学家和理论家。他们将对各种各样的地质和其他材料进行大量的实验，并对实验结果进行密集的理论建模和反演，以确定接触量和质量变化对状态演化的相对贡献。理论模型将包括颗粒状断层泥的离散元素模型，该模型最近被证明可以再现各种实验结果。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.