The extreme mechanics of micro- and nanoarchitected materials
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troduction Micro- and nanoarchitected materials represent a new class of materials with rationally designed structural hierarchy down to the nanoscale. These materials are capable of achieving extreme mechanical and metamaterial properties. Architecture, constituent materials, and architectural feature sizes, which allow for exploiting material size effects, define the mechanical properties. The introduction of controlled micro- and nanoarchitecture allows for the creation of materials with properties that far exceed those of their bulk counterparts, such as strengths greater than 1 GPa with densities below that of water, deformability, recoverability despite being made from intrinsically brittle constituents, and an insensitivity to large flaws. Multiple hierarchical structures can be introduced to control material deformation, similar to what is observed in biological materials.1 The mechanical behavior of an architected material can be decoupled into its material behavior and its architectural behavior. Architecture is responsible for properties such as high specific stiffness, “ductility” caused by structural instabilities, and most metamaterial properties2 such as negative Poisson’s ratio (auxetic behavior) and near infinite bulk to shear modulus ratio (pentamode stiffness). Architectural behavior is scale independent, meaning two identical
architectures of different sizes with identical constituent material properties will have the same behavior. Metamaterial behaviors describe a subset of architectural behaviors and are both scale- and material-independent. The advantage of micro- and nanoarchitected materials is that their constituent materials experience size-affected property changes that are utilized to improve the overall mechanical response. Mechanisms governing the size effects are complex, but there is a general overall trend of “smaller is stronger.”3,4 In addition, macroscopically brittle materials experience an enhanced ductility at the nanoscale5 and materials such as metal nanowires have been observed to have notch insensitivity, resulting in the same failure strength independent of whether failure occurred at the notch or in the unnotched material.6,7 Microstructural and dimensional constraints can significantly change the mechanical,3,4,8 magnetic,8 thermal,9 and electrical10 properties of a material relative to its bulk properties. The field of mechanical metamaterials has benefited strongly from advances in three-dimensional (3D) additive manufacturing with light-based methods.11 Techniques such as projection micro-stereolithography (PμSL)12 and direct laser writing (DLW)13,14 provide control over complicated 3D geometries with the nanoscale feature sizes necessary for
R. Schwaiger, Institute of Energy and Climate Research IEK-2, Forschungszentrum Jülich GmbH, and Institute for Applied Materials, Karlsruhe Institute of Technology (KIT), Germany; [email protected] L.R. Meza, University of Washington, USA; [email protected] X. Li, Department of Engineering Mechanics, Tsinghua University, China
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