Controlled Unusual Stiffness of Mechanical Metamaterials

Abstract

Mechanical metamaterials that are engineered with sub-unit structures present unusual mechanical properties depending on the loading direction. Although they show promise, their practical utility has so far been somewhat limited because, to the best of our knowledge, no study about the potential of mechanical metamaterials made from sophisticatedly tailored sub-unit structures has been made. Here, we present a mechanical metamaterial whose mechanical properties can be systematically designed without changing its chemical composition or weight. We study the mechanical properties of triply periodic bicontinuous structures whose detailed sub-unit structure can be precisely fabricated using various sub-micron fabrication methods. Simulation results show that the effective wave velocity of the structures along with different directions can be designed to introduce the anisotropy of stiffness by changing a volume fraction and aspect ratio. The ratio of Young's modulus to shear modulus can be increased by up to at least 100, which is a 3500% increase over that of isotropic material (2.8, acrylonitrile butadiene styrene). Furthermore, Poisson's ratio of the constituent material changes the ratio while Young's modulus does not influence it. This study presents the promising potential of mechanical metamaterials for versatile industrial and biomedical applications.Recent advances in the fabrication of micro-/nano-structures have resulted in the development of interesting mechanical metamaterials like phononic (acoustic) 1 and meta-fluidic materials 2 . Phononic metamaterials have a negative effective bulk modulus 3 or negative effective mass densities 4 . These properties can be used in applications like acoustic subwavelength imaging 5 , superlensing 6 , and transforming acoustics 7,8 . Meta-fluidic materials have extremely large bulk modulus compared to shear modulus, which makes them behave like a fluid 2 . Similar concepts have also been explored to improve mechanical characteristics with new designs of micro lattices without changing the chemical composition or weight 9,10 . Ultralight and ultrastiff metamaterials have been reported, which maintain a nearly constant stiffness per unit mass density even at ultralow densities 10 . Anisotropic versions of meta-fluidic materials have been suggested, which can be used in elasto-dynamic cloaking devices 11 . Wave trapping in cellular metamaterial plates constructed by bending-dominated and stretch-dominated unit-cells has been studied based on the numerical simulation 12 . The interesting mechanical properties of mechanical metamaterials are a consequence of their deliberate structuring, and not of the bulk material that comprises them. Owing to the complicated patterns involved 10,13 , currently mechanical metamaterials are mostly fabricated by using three dimensional printing. Recently, triply periodic bicontinuous structures have received attention because their geometric characteristics can be precisely controlled in the sub-micro scale with various fabrication methods...

Multifunctional Mechanical Metamaterials with Embedded Triboelectric Nanogenerators

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Scalable additive manufacturing via integral image formation

“…1F). These examples of complex 3D microstructures with different degrees of connectivity can be extended to a variety of tissue scaffolds [30], mechanical metamaterials [31], feed spacers for water reuse system [32], or functional surfaces [33]. In addition to the periodic microstructures in The homogeneous light distribution on the lens array from the diffuser enables images from different perspectives (i.e.…”

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A Hierarchical Programmable Mechanical Metamaterial Unit Cell Showing Metastable Shape Memory

“…As a result of such advances, metamaterials can be designed with properties not yet found in nature and manipulated on size scales critical to their fundamental functions, with the topic being spearheaded through work in mechanical and elastic metamaterials in 1987 through foams based on reentrant unit cells . Although widespread applications of mechanical metamaterials are lacking to date, novel advances in the field's research have been prolific, with advanced lithographic techniques such as direct laser writing revealing new research potential at a rapid pace. Recent works have shown metamaterial lattices with “designer” Poisson's ratios, strength‐to‐weight ratios approaching diamond, and even self‐recovering shock absorbers .…”