Hierarchical structures with constituents over multiple length scales are found in various natural materials like bones, shells, spider silk and others, all of which display enhanced quasi-static mechanical properties, such as high specific strength, stiffness and toughness. At the same time, the role of hierarchy on the dynamic behaviour of metamaterials remains largely unexplored. This study assesses the effect of bio-inspired hierarchical organization as well as of viscoelasticity on the wave attenuation properties of continuous mechanical metamaterials. We consider single-phase metamaterials formed by self-similar unit cells with different hierarchical levels and types of hierarchy. Results highlight a number of advantages through the introduction of structural hierarchy. Band gaps relative to the corresponding non-hierarchical structures are mostly preserved, while additional "hierarchically-induced" band gaps appear. Additionally, the hierarchical configuration allows the tuning of the band gap frequencies of regular metamaterial to lower frequencies, with a simultaneous significant reduction of the global structural weight. We show that even small viscoelastic effects, not treated in the current literature, are essential in determining this behaviour. The approach we propose allows the addition of hierarchical elements to existing metamaterial configurations, with the corresponding improvement of the wave damping properties, thus providing indications for the design of structures for practical applications.
Nature of short high amplitude pulses in a periodic dissipative laminate metamaterial
本文研究了周期性耗散层合板中高幅应力脉冲的演化，考虑了各组成部分的非线性本构方程及其耗散行为。由于铝和钨的声阻抗差异很大，铝本构方程在所研究的应力范围内具有显著的非线性，因此以铝和钨层板为例，讨论了其可能的实际应用。研究了具有不同单元尺寸的层压板，其控制内部时间尺度，受到具有不同厚度的板的影响，确定入射脉冲持续时间。已经观察到，输入脉冲的持续时间与内部特征时间的比率决定了高振幅耗散传播波、振荡冲击状轮廓、局部脉冲的训练或单个局部脉冲的性质。这些局部准定常波类似于孤立波，即使在耗散存在的情况下：不同初始条件下出现的相似脉冲表明它们是相应层合板的固有特性，其特征长度尺度由细观尺度和应力幅值决定，它们碰撞后大部分恢复形状，并发生相移，速度与振幅之间存在线性关系。基于Korteweg de Vries型方程和介观结构决定的色散项，提出了近似描述这些高振幅耗散脉冲的形状、长度尺度和速度的理论描述。非线性项是从铝和钨的Hugoniot曲线中提取的非线性本构方程导出的。
Programmed Internal Reconfigurations in a 3D-Printed Auxetic Metamaterial Enable Fluidic Control for a Vertically Stacked Valve Array
Department of Biology, College of Arts and Sciences, University of North Carolina at Greensboro（北卡罗来纳大学格林斯伯勒分校文理学院生物系）；The Early College at Guilford（吉福德早期学院）
Ultrafast interband transitions in nanoporous gold metamaterial
Umeå University（于默奥大学）；CNR NANOTEC Institute of Nanotechnology（意大利国家研究委员会纳米技术研究所）；University of Salento（萨莱诺大学）；University of Rome Tor Vergata（罗马托尔维加塔大学）；Università di Modena e Reggio Emilia（摩德纳和雷焦艾米利亚大学）；Istituto Italiano di Tecnologia（意大利理工大学）
