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，门铰链的零件上，还是汽车发动机的零件上。人们已经观察到，如果两个表面接触更长时间而几乎没有滑动，则需要更大的力才能真正使它们再次滑动。虽然这已经知道了近50年，但没有人知道为什么它是真的。摩擦表面实际上只在许多小的接触点上相互接触。对于为什么接触时间越长摩擦力越大，最流行的观点是接触时间越长，接触点的总面积或大小就越大。然而，计划这项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。由实验家和理论家组成的团队将进行和分析实验，以便更好地理解哪种解释是正确的。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意义的过程。 这些包括两个表面是否稳定滑动或经历交替的粘附和滑动运动。例如，在地震期间或当弓拉过小提琴弦时，就会发生这种交替运动。更好地了解摩擦对包括制造业和运输业在内的几个重要经济行业具有重要意义。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试图理解许多学科中的急滑的研究方向。它可能会影响人们的研究方向，这些人正在追求接触面积的变化可能是摩擦力变化的原因。这将表明理解摩擦接触点处的化学键合是重要的。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，以及参与该项目理论部分的两名早期职业科学家的技能、知识和网络。摩擦的速率和状态本构方程描述的行为已经被认识了近50年，并被广泛认为是地震滑移成核的重要因素。然而，在这个公式中，关于“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，什么物理或化学变化控制状态，决定其演变。从基本科学的角度来看，这是不令人满意的。这也是不令人满意的，因为如果参与状态演化的过程不被理解，并以基于过程的方程表征，那么将实验室结果外推到理解地震力学就没有坚实的基础。许多业内人士认为，该演化过程涉及跨滑动界面的接触尺寸随时间的增加，即接触量的演化。然而，单独的接触量的演变不能解释最近的实验观察到的摩擦现象以下的正常应力步骤。相比之下，接触界面的质量，换句话说，单位面积的接触剪切强度，主要控制强度演变后的正常应力步骤，一个变革的观察。这个小组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，并对实验结果进行深入的理论建模和反演，以确定接触量和质量变化对状态演化的相对贡献。理论建模将包括离散元模型的粒状断层泥已被证明是最近重现了各种实验findings.This奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.