Energy dissipation in granular materials

自从库仑关于平衡和颗粒斜坡破坏的工作以来，颗粒土的摩擦力和强度变得牢牢地联系在一起。两个多世纪后，许多理论被提出，导致库仑摩擦角和土壤颗粒之间的摩擦角几乎是一对一的比例。对这些理论进行实验验证的尝试产生了一个令人惊讶的迹象：宏观和微观摩擦角之间的联系远比过去的理论所暗示的要弱得多。尽管颗粒土的剪切强度特性看起来类似于摩擦系统，但缺乏与颗粒间摩擦的直接联系是不容易合理化的。这指出了一个基本的知识鸿沟，这可能解释了通过实际可用的模型来描述土壤的工程性质的许多困难。拟议的项目旨在大幅弥合这一差距，无论是在对摩擦在颗粒材料行为中的作用的基本理解方面，还是在开发基于适当物理的本构模型方面。这两个目标都与更广泛的主题有关，即颗粒材料中能量耗散的物理机制及其用于建模目的的分析描述。该项目本质上是理论上的，但涉及粒子级模拟，以及利用传感技术的最新进展进行的颗粒级物理实验。*颗粒间摩擦作为宏观强度的控制因素的作用已通过粒子级模拟得到一定程度的阐明，因为模拟组件只需要很少的摩擦就能影响显著的宏观阻力。此外，增加摩擦系数以防止颗粒间滑移对颗粒集合体的行为几乎没有影响。由于在这种情况下，摩擦滑移不会耗散能量，显然还涉及到其他耗散机制。对模拟系统中颗粒运动的观察表明，力以高度不均匀的方式通过颗粒集合体传递，因此一些颗粒受到高度压力，而另一些颗粒则不携带任何载荷。这会导致局部稳定性和结构重排的自发损失，在此期间，粒子获得最终消散的显著动能。这种运动表现在实验室测试沙子时的声发射，沙子雪崩时的轰鸣声，以及沙漠沙丘的“歌唱”。*该项目的实验部分将涉及利用高灵敏度的激光干涉术来监测由石油熔融的石英颗粒和匹配折射率的孔隙液组成的“透明土壤”中单个标记颗粒的运动。预计被跟踪的粒子将经历表示系统中不稳定的虚假运动。

Ever since Coulomb's work on equilibrium and the failure of granular slopes, friction and strength of granular soils became firmly linked. More than two centuries later, many theories have been advanced resulting in a near one-to-one proportionality between Coulomb's friction angle and the angle of friction between soil grains. Attempts to verify these theories experimentally yield a surprising indication that the link between macro and micro friction angles is far weaker than past theories implied. Although shear strength characteristics of granular soils appear analogous to those of a frictional system, the lack of a direct link to interparticle friction is not easily rationalizable. This points to a fundamental knowledge gap that possibly explains the many difficulties in describing engineering properties of soils through practically usable models. The proposed project is to substantially close this gap, both in terms of the fundamental understanding of the role of friction in the behavior of granular material and in developing a constitutive model based on adequate physics. Both objectives are connected to the broader topic of the physical mechanisms of energy dissipation in granular materials and its analytical description for modeling purposes. The project is theoretical in its nature but involves particle-level simulations, as well as grain-scale physical experimentation utilizing recent advances in sensing technology.***The role of inter-particle friction as a controlling factor of macroscopic strength has been clarified to some extent through particle-level simulations when it was shown that very little friction is required to effect significant macroscopic resistance of simulated assemblies. Moreover, increasing the friction coefficient to prevent interparticle slip changes little in the behavior of a granular assembly. As no energy is dissipated in frictional slip in this case, other dissipative mechanisms are clearly involved. Observations of particles motions in simulated systems suggest that forces are transmitted through granular assemblies in a highly heterogeneous manner so that some particles are highly stressed, while others do not carry any load. This causes spontaneous losses of local stability and structural rearrangements during which particles gain significant kinetic energy that eventually dissipates. Such motions are manifested in acoustic emissions during laboratory tests on sand, booming sound during sand avalanches, and the "singing" of desert dunes.****The experimental part of the project will involve utilizing highly sensitive laser interferometry to monitor motions of a single marked particle in "transparent soil" composed of oil-fused quartz particles with the pore fluid of a matching refraction index. It is expected that the tracked particle will experience spurious motions indicative of instabilities in the system.**** **