Dry granular materials, such as sand, gravel, pills, or agricultural grains, can become rigid when compressed or sheared. Under isotropic compression, the material reaches a certain jamming density and then resists further compression. Shear jamming occurs when resistance to shear emerges in a system at a density lower than the jamming density. Although shear jamming is prevalent in frictional granular materials, their stability properties are not well described by standard elasticity theory and thus call for experimental characterization. We report on experimental observations of changes in the mechanical properties of a shear-jammed granular material subjected to small-amplitude, quasistatic cyclic shear. We study a layer of plastic disks confined to a shear cell, using photoelasticimetry to measure all interparticle vector forces. For sufficiently small cyclic shear amplitudes and large enough initial shear, the material evolves to an unexpected “ultrastable” state in which all the particle positions and interparticle contact forces remain unchanged after each complete shear cycle for thousands of cycles. The stress response of these states to small imposed shear is nearly elastic, in contrast to the original shear-jammed state.

中文翻译：

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超稳定剪切堵塞颗粒材料

干燥的颗粒材料，例如沙子、砾石、药丸或农用谷物，在压缩或剪切时会变得坚硬。在各向同性压缩下，材料达到一定的堵塞密度，然后抵抗进一步的压缩。剪切干扰当系统中出现的剪切阻力低于干扰密度时，就会发生这种情况。尽管剪切干扰在摩擦颗粒材料中很普遍，但标准弹性理论并未很好地描述其稳定性，因此需要进行实验表征。我们报告了受到小幅度准静态循环剪切的剪切堵塞颗粒材料的机械性能变化的实验观察结果。我们研究了限制在剪切单元中的一层塑料盘，使用光弹性测量法来测量所有粒子间矢量力。对于足够小的循环剪切幅度和足够大的初始剪切，材料演变为一种意想不到的“超稳定”状态，在这种状态下，所有粒子位置和粒子间接触力在每个完整的剪切循环数千个循环后保持不变。这些状态对施加的小剪切的应力响应几乎是弹性的，与原始剪切卡住状态相反。