Granular packings of nonconvex or elongated particles can form freestanding structures like walls or arches. For some particle shapes, such as staples, the rigidity arises from interlocking of pairs of particles, but the origins of rigidity for noninterlocking particles remains unclear. We report on experiments and numerical simulations of sheared columns of “hexapods,” particles consisting of three mutually orthogonal sphero-cylinders whose centers coincide. We vary the length-to-diameter aspect ratio, $\alpha$, of the sphero-cylinders and subject the packings to quasistatic direct shear. For small $\alpha$, we observe a finite yield stress. For large $\alpha$, however, the column becomes rigid when sheared, supporting stresses that increase sharply with increasing strain. Analysis of x-ray microcomputed tomography (micro-CT) data collected during the shear reveals that the stiffening is associated with a tilted, oblate cluster of hexapods near the nominal shear plane in which particle deformation and average contact number both increase. Simulation results show that the particles are collectively under tension along one direction, even though they do not interlock pairwise. These tensions comes from contact forces carrying large torques, and they are perpendicular to the compressive stresses in the packing. They counteract the tendency to dilate, thus stabilizing the particle cluster.

中文翻译：

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六足颗粒的剪切柱中的屈服，刚度和拉伸应力。

非凸或拉长颗粒的粒状堆积可以形成独立的结构，如墙壁或拱门。对于某些颗粒形状（例如订书钉），刚度是由成对的颗粒互锁产生的，但非互锁颗粒的刚度起源仍然不清楚。我们报告了“六足动物”（由六个相互正交的球圆柱体，其中心重合）组成的颗粒的剪切柱的实验和数值模拟。我们改变长径比，$\alpha$，然后对填料进行准静态直接剪切。对于小$\alpha$，我们观察到有限的屈服应力。对于大$\alpha$但是，柱子在剪切时会变硬，支撑应力会随着应变的增加而急剧增加。对在剪切过程中收集的X射线微计算机断层扫描（micro-CT）数据进行的分析显示，硬化与名义剪切平面附近的倾斜，扁圆的六足类簇相关，颗粒变形和平均接触数均增加。仿真结果表明，即使粒子不成对互锁，它们也始终沿一个方向处于拉伸状态。这些张力来自承受大扭矩的接触力，它们垂直于填料中的压缩应力。它们抵消了膨胀的趋势，从而稳定了颗粒团簇。