Acoustic Probes of Granular States

*技术摘要*很难评估颗粒材料的稳定性，也很难确定是否即将发生故障。非侵入性地表征颗粒材料力学状态变化的能力将有助于我们理解向失效的转变。在普通的原子/分子系统和理想的阻塞系统中，态密度提供了关于系统状态的丰富信息。声学测量是对颗粒材料进行类似表征的一条很有希望的途径，因为它们能够将振动能量传递到材料的主体中，并反过来收集信息。我们将在静态和剪切系统中开发这种技术，在这些系统中可以看到内部应力，以及具有自然和工业重要性的更真实的三维材料。目标是：(1)模拟和分析预测，低频模式的增加与即将失去的刚性有关。真实的颗粒状材料是否表现出早期破坏的普遍特征，例如在剪切作用下？(2)具有不同形状(圆形、椭圆形和二聚体)的模拟堵塞材料各自具有特征的状态密度。真实的颗粒材料，例如那些有角的材料，是否表现出类似的形状依赖特征？(3)力和接触网络的性质可能取决于系统的维度。剪切实验的结果可能会加深我们对地震成核和破裂的理解。根据当前关于前震、地震、触发和监测断层破坏的概念，通过分析结果可以与地球物理学联系起来。*非技术摘要*许多与能源相关的行业依赖于颗粒材料：从油砂中提取、在制造设施内处理球团、石油、天然气或二氧化碳封存储藏层周围岩石的稳定性，或在沸腾床反应器内流动。很难评估颗粒系统的稳定性：故障迫在眉睫吗？为了避免破坏，承载桩的最稳定方向是什么？非侵入性地表征颗粒材料机械状态变化的能力将为评估这些问题提供一种手段。物理学家已经建立了对泡沫、乳液和颗粒状材料等一大类材料的机械稳定性的定量理解。尽管在计算机模拟中进行了十年的测量，但由于在不透明的颗粒材料内部进行测量的难度，实验进展一直很慢。由于声学技术能够穿透不透明材料的内部，因此是一条很有前途的途径。我们将开发新的声学测量方法，这将为颗粒材料的非破坏性测试开辟道路，这项研究将由本科生和研究生进行。这些新技术将使我们能够寻求声学信号，以了解系统离滑动有多近，最大作用力的方向，或粒子排列/无序的程度。除了在能源行业的应用，这些新技术可能能够提高我们对构造板块之间的颗粒状物质如何变得不稳定并引发地震的理解。研究小组还将开发一套相关的动手活动，供当地女童子军使用。

****Technical Abstract****It is difficult to assess the stability of a granular material, or to determine whether failure is imminent. The ability to non-invasively characterize changes in the mechanical state of a granular material would aid our understanding of the transition to failure. In both ordinary atomic/molecular systems and idealized jammed systems, the density of states provides a wealth of information about the state of the system. Acoustic measurements are a promising route to a similar characterization for granular materials, due to their ability to transmit vibrational energy into the bulk of the material, and to gather information in return. We will develop such techniques both in static and sheared systems where internal stresses are visible, as well as in more realistic three-dimensional materials which are of natural and industrial importance. Objectives are: (1) Simulations and analytics predict that an increased abundance of low-frequency modes is associated with an impending loss of rigidity. Do real granular materials exhibit this feature as a universal hallmark of incipient failure, e.g. under shear? (2) Simulated jammed materials with different shapes (circles vs. ellipses vs. dimers) each have a characteristic density of states. Do real granular materials, for instance those with corners, exhibit similar shape-dependent features? (3) The properties of force and contact networks may depend on the dimensionality of the system. Results from shear experiments may improve our understanding of earthquake nucleation and rupture. Connections to geophysics are possible by analyzing results in light of current ideas about foreshocks, tremors, triggering, and monitoring fault damage.****Non-Technical Abstract****Many energy-related industries rely on granular materials: extraction from oil sands, the handling of pellets within manufacturing facilities, the stability of rocks surrounding oil, gas, or carbon dioxide sequestration reservoirs, or flows within fluidized-bed reactors. It is difficult to assess the stability of a granular system: is failure imminent? what is the most stable direction to load the pile to avoid failure? The ability to non-invasively characterize changes in the mechanical state of a granular material would provide a means to evaluate such questions. Physicists have built a quantitative understanding of the mechanical stability of a broad class of materials such as foams, emulsions, and granular materials. Despite a decade of measurements within computer simulations, experimental progress has been slowed by the difficulty of making measurements in the interior of an opaque granular material. Acoustic techniques are a promising route due to their ability to penetrate the interior of opaque materials. We will develop new acoustical measurements which will open avenues for non-destructive testing of granular materials, research which will be conducted by undergraduate and graduate students. These new techniques will allow us to seek acoustical signatures of how close a system is to slipping, the direction of largest applied forces, or the degree of particle alignment/disorder. Beyond applications in the energy industry, the new techniques may be able to improve our understanding of how the granular material between tectonic plates becomes unstable and triggers earthquakes. The research team will also develop a related set of hands-on activities for use with local Girl Scouts troops.