85 research outputs found
On three-dimensional dilational elastic metamaterials
Dilational materials are stable three-dimensional isotropic auxetics with an
ultimate Poisson's ratio of -1. We design, evaluate, fabricate, and
characterize crystalline metamaterials approaching this ideal. To reveal all
modes, we calculate the phonon band structures. On this basis, using cubic
symmetry, we can unambiguously retrieve all different non-zero elements of the
rank-4 effective metamaterial elasticity tensor, from which all effective
elastic metamaterial properties follow. While the elastic properties and the
phase velocity remain anisotropic, the effective Poisson's ratio indeed becomes
isotropic and approaches -1 in the limit of small internal connections. This
finding is also supported by independent static continuum-mechanics
calculations. In static experiments on macroscopic polymer structures
fabricated by three-dimensional printing, we measure Poisson's ratios as low as
-0.8 in good agreement with theory. Microscopic samples are also presented.Comment: 8 figure
A characteristic lengthscale causes anomalous size effects and boundary programmability in mechanical metamaterials
The architecture of mechanical metamaterialsis designed to harness geometry,
non-linearity and topology to obtain advanced functionalities such as shape
morphing, programmability and one-way propagation. While a purely geometric
framework successfully captures the physics of small systems under idealized
conditions, large systems or heterogeneous driving conditions remain
essentially unexplored. Here we uncover strong anomalies in the mechanics of a
broad class of metamaterials, such as auxetics, shape-changers or topological
insulators: a non-monotonic variation of their stiffness with system size, and
the ability of textured boundaries to completely alter their properties. These
striking features stem from the competition between rotation-based
deformations---relevant for small systems---and ordinary elasticity, and are
controlled by a characteristic length scale which is entirely tunable by the
architectural details. Our study provides new vistas for designing, controlling
and programming the mechanics of metamaterials in the thermodynamic limit.Comment: Main text has 4 pages, 4 figures + Methods and Supplementary
Informatio
Cymatics for the cloaking of flexural vibrations in a structured plate
Based on rigorous theoretical findings, we present a proof-of-concept design for a structured square cloak enclosing a void in an elastic lattice. We implement high-precision fabrication and experimental testing of an elastic invisibility cloak for flexural waves in a mechanical lattice. This is accompanied by verifications and numerical modelling performed through finite element simulations. The primary advantage of our square lattice cloak, over other designs, is the straightforward implementation and the ease of construction. The elastic lattice cloak, implemented experimentally, shows high efficiency
Morphological study of two types of glands (Thoracic Gland and Mandibular Gland) in Larvae of Lepidoptera
A study of the structure and activity of the endocrine organs in Cerura vinula L. (Lepidoptera) during the pupation
A New Synthesis Of Coenzymically Active Water-Soluble Macromolecular Nad And Nadp Derivatives
Die Redoxpigmente von Carausius morosus und ihre Bedeutung f�r den morphologischen Farbwechsel
- …