通过颗粒形状转换细观颗粒塑性
Transforming mesoscale granular plasticity through particle shape
摘要：
当非晶材料的应变超过屈服点时，它会通过耗散的雪崩式滑动事件进入持续重构状态，从而减轻累积的局部应力。然而，这些事件的统计数据如何取决于组成单位之间的地方互动仍然存在争议。为了解决这个问题，我们在颗粒材料上进行实验，我们使用颗粒形状来系统地改变相互作用。颗粒状材料，在恒定压力的边界条件下，被单轴压缩，而应力测量和内部重排与X射线成像。我们引入波动性，一个经济理论中的量，作为一个强大的新工具来量化压力波动的大小，发现系统的，形状依赖的趋势。对于所有22个调查形状的幅度$s$的松弛事件是很好的拟合截断幂律分布$P(s)\sim {s}^{-τ} exp(-s/s^*)$，已提出的塑性模型的上下文中。所有形状的幂律指数$τ$测试聚类在$τ=$ 1.5左右，实验不确定性范围为1.3 - 1.7。$τ$的形状独立性及其与平均场模型的兼容性表明，该系统的粒度，而不是颗粒形状，修改后的应力重分布的滑移事件远离连续弹性。同时，特征最大事件大小$s^*$变化两个数量级，并跟踪波动的形状依赖。因此，颗粒材料中的颗粒形状是影响无定形系统从无标度临界状态运行的距离的一个强大的新因素。这些实验结果没有捕获当前的模型，并建议需要重新检查的机制，驱动非晶系统中的介观塑性形变。
英文摘要：
When an amorphous material is strained beyond the point of yielding it enters a state of continual reconfiguration via dissipative, avalanche-like slip events that relieve built-up local stress. However, how the statistics of such events depend on local interactions among the constituent units remains debated. To address this we perform experiments on granular material in which we use particle shape to vary the interactions systematically. Granular material, confined under constant pressure boundary conditions, is uniaxially compressed while stress is measured and internal rearrangements are imaged with x-rays. We introduce volatility, a quantity from economic theory, as a powerful new tool to quantify the magnitude of stress fluctuations, finding systematic, shape-dependent trends. For all 22 investigated shapes the magnitude $s$ of relaxation events is well-fit by a truncated power law distribution $P(s)\sim {s}^{-τ} exp(-s/s^*)$, as has been proposed within the context of plasticity models. The power law exponent $τ$ for all shapes tested clusters around $τ=$ 1.5, within experimental uncertainty covering the range 1.3 - 1.7. The shape independence of $τ$ and its compatibility with mean field models indicate that the granularity of the system, but not particle shape, modifies the stress redistribution after a slip event away from that of continuum elasticity. Meanwhile, the characteristic maximum event size $s^*$ changes by two orders of magnitude and tracks the shape dependence of volatility. Particle shape in granular materials is therefore a powerful new factor influencing the distance at which an amorphous system operates from scale-free criticality. These experimental results are not captured by current models and suggest a need to reexamine the mechanisms driving mesoscale plastic deformation in amorphous systems.
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