Institute of Applied Mechanics, University of Stuttgart, Stuttgart 70569, Germany.
Multi-Scale Mechanics, University of Twente, AE Enschede 7500, The Netherlands.
Proc Natl Acad Sci U S A. 2023 Jun 27;120(26):e2219999120. doi: 10.1073/pnas.2219999120. Epub 2023 Jun 20.
This research focuses on performing ultrasound propagation measurements and micro-X-ray computed tomography (µXRCT) imaging on prestressed granular packings prepared with biphasic mixtures of monodisperse glass and rubber particles at different compositions/fractions. Ultrasound experiments employing piezoelectric transducers, mounted in an oedometric cell (complementing earlier triaxial cell experiments), are used to excite and detect longitudinal ultrasound waves through randomly prepared mixtures of monodisperse stiff/soft particles. While the fraction of the soft particles is increasing linearly from zero, the effective macroscopic stiffness of the granular packings transits nonlinearly and nonmonotonically toward the soft limit, remarkably via an interesting stiffer regime for small rubber fractions between 0.1 ≲ ≲ 0.2. The contact network of dense packings, as accessed from µXRCT, plays a key role in understanding this phenomenon, considering the structure of the network, the chain length, the grain contacts, and the particle coordination. While the maximum stiffness is due to surprisingly shortened chains, the sudden drop in elastic stiffness of the mixture packings, at ≈ 0.4, is associated with chains of particles that include both glass and rubber particles (soft chains); for ≲ 0.3, the dominant chains include only glass particles (hard chains). At the drop, ≈ 0.4, the coordination number of glass and rubber networks is approximately four and three, respectively, i.e., neither of the networks are jammed, and the chains need to include particles from another species to propagate information.
这项研究专注于对不同组成/分数的双相单分散玻璃和橡胶颗粒混合物制备的预应力颗粒堆积进行超声传播测量和微 X 射线计算机断层扫描（µXRCT）成像。使用安装在压汞仪（补充早期的三轴压汞仪实验）中的压电换能器进行超声实验，通过随机制备的单分散硬/软颗粒混合物来激发和检测纵波超声。随着软颗粒分数从 0 线性增加，颗粒堆积的有效宏观刚度非线性且非单调地向软极限过渡，在橡胶分数为 0.1 ≲ ≲ 0.2 之间的有趣硬区中表现出显著的过渡。从 µXRCT 获得的密集堆积的接触网络在理解这种现象方面起着关键作用，考虑到网络结构、链长、颗粒接触和颗粒配位。虽然最大刚度归因于出乎意料的缩短链，但混合物堆积的弹性刚度突然下降（在 ≈ 0.4 时）与包含玻璃和橡胶颗粒的颗粒链有关（软链）；对于 ≲ 0.3，主要链仅包含玻璃颗粒（硬链）。在下降点 ≈ 0.4 时，玻璃和橡胶网络的配位数分别约为四和三，即两个网络都没有堵塞，并且链需要包含来自另一种物质的颗粒才能传播信息。
