Auxetic System Based on Rotating Hexagons and Triangles

Incomplete bimaterial ring (a circular ring with a gap) capable of producing negative macroscopic thermal expansion is proposed and its behavior is analyzed. The ring exhibits negative thermal expansion (NTE) (in the plane of the ring) when the outer ring has higher thermal expansion coefficient than the inner one. When the thermal expansion coefficients are equal (monomaterial incomplete ring), the effective (macroscopic) planar thermal expansion becomes zero. (The complete thermal expansion will be positive but small.) It is the presence of the gap which is the basis of this thermal behavior. Similar effect can be achieved by spring or spiral structures where the role of the gap is played by the open ends. These structures will have higher stiffness than the incomplete bimaterial ring. The thermal expansion of the ring is characterized by the effective (macroscopic) coefficient of linear thermal expansion. The effective coefficient of linear thermal expansion depends on the temperature increase, making the thermal expansion nonlinear. Planar and 3D NTE structures are considered.

Negative Thermal Expansion Realized by an Incomplete Bimaterial Ring

Pasternak,

Dyskin

2024

Auxetic Metamaterial Model Made from Rotating Rectangles, Hexagons, and Triangles

Lim

2024

A perfectly auxetic 2D metamaterial model is described herein by means of interconnected rotating rigid units of three different shapes: hexagons, triangles, and rectangles. The in‐plane Young's modulus is obtained by matching the discrete elastic energy stored at the hinges during relative rotation of the rigid units with the overall elastic energy stored in the metamaterial's continuum equivalence. Results indicate that, in addition to the trivial effects from the rotational spring constant at the hinges, the in‐plane Young's modulus is highly influenced by the internal angle, followed by the relative size of the triangle with reference to the sizes of other rigid units. The effect from the rectangle aspect ratio presents the least influence. As such, there are several parameters to control the Young's modulus, from the coarse tuning by adjusting the internal angle to fine tuning by controlling the rectangle aspect ratio. The capability of the metamaterial's Young's modulus to be tuned from a spectrum of parameters that range from coarse to fine tuning permits this metamaterial's mechanical property to be precisely designed and manufactured in a cost‐effective manner.

Fine‐Tuning of Elastic Properties by Modifying Ordering of Parallel Nanolayer Inclusions in Hard Sphere Crystals

Narojczyk,

Smardzewski,

Kędziora

et al. 2024

It is known that changes on the microscopic, structural level of materials exert changes on their macroscopic properties, in particular elastic ones. This approach can be used to alter the elastic properties of materials. However, it is not easy to predict the impact of a particular change in the structure of crystals. The recent studies show that depending on the size, shape, and orientation of inclusions used, the resulting elastic properties may differ. Inclusions may enhance, weaken, or eliminate the auxetic properties in a hard sphere model. This study is focused on the influence of the spatial distribution of planar inclusions within the hard sphere crystal and its impact on its elastic properties. Periodic systems containing two nanolayer inclusions in the representative volume element, oriented orthogonally to ‐direction and in various spatial ordering, are considered. It has been shown that introducing layer inclusions causes the deterioration of auxetic properties in the ‐direction. However, the latter weakly depends on the spatial ordering of the inclusion layers. Moreover, changes in the size of the inclusion particles, combined with different ordering of the inclusion layers, can be used to coarsely and finely tune the elastic properties of the model crystal.