Energy dissipation in granular materials

自从库仑的工作平衡和破坏的粒状边坡，摩擦和强度的粒状土壤成为牢固地联系在一起。两个多世纪后，许多理论已经提出，导致库仑摩擦角和土壤颗粒之间的摩擦角之间接近一比一的比例。试图验证这些理论的实验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远弱于过去的理论暗示。尽管粒状土的抗剪强度特性与摩擦系统的抗剪强度特性相似，但颗粒间摩擦力与其缺乏直接联系的观点并不容易合理化。这指出了一个基本的知识缺口，可能解释了通过实际可用的模型描述土壤工程特性的许多困难。拟议的项目是要大大缩小这一差距，无论是在颗粒材料的行为中的摩擦作用的基本理解，并在开发一个本构模型的基础上充分的物理。这两个目标都连接到更广泛的主题的物理机制的能量耗散颗粒材料和其分析描述建模的目的。该项目本质上是理论性的，但涉及粒子级模拟，以及利用传感技术最新进展的颗粒级物理实验。颗粒间摩擦作为宏观强度的控制因素的作用已经通过颗粒级模拟在一定程度上得到澄清，当它被证明是非常小的摩擦需要影响模拟组件的显着的宏观阻力。此外，增加摩擦系数，以防止颗粒间的滑移变化不大的行为的颗粒组装。 由于在这种情况下，摩擦滑移没有耗散能量，显然还涉及其他耗散机制。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传递，因此一些颗粒受到很大的应力，而另一些颗粒则不承受任何载荷。这导致局部稳定性和结构重排的自发损失，在此期间，粒子获得显著的动能，最终消散。这种运动表现在实验室对沙子的测试中的声发射，沙崩时的轰鸣声以及沙漠沙丘的“歌声”中。该项目的实验部分将涉及利用高灵敏度激光干涉测量法，监测“透明土壤”中单个标记颗粒的运动，“透明土壤”由油熔石英颗粒和折射率相匹配的孔隙流体组成。预计被跟踪的粒子将经历指示系统中的不稳定性的虚假运动。**

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.**** **