Fish Cells, a new zero Poisson’s ratio metamaterial - part II: Elastic properties

Fish Cells as a new metamaterial with zero Poisson’s ratio in two planar directions is introduced with application in morphing aircraft skin. In order to tailor the design of this metamaterial for arbitrary loadings, equivalent elastic properties of the Fish Cells metamaterial are derived and analyzed using analytical and numerical methods. The admissible range of geometric parameters is presented and variation of elastic properties with parameters is studied. The effective elastic modulus of the metamaterial is derived analytically and verified with finite element models. The in-plane and transverse shear modulus of the metamaterial are evaluated using finite element analysis where accurate periodic boundary conditions for in-plane shear loading are investigated. The lower and upper bounds of the transverse shear modulus are derived based on strain and complementary energy relations which are verified with finite element results. As zero Poisson’s ratio behavior of the Fish Cells topology is proved, derivative geometries from this topology with zero Poisson’s ratio behavior are also presented

Fish Cells, a new zero Poisson’s ratio metamaterial - Part I: Design and experiment

A novel cellular mechanical metamaterial called Fish Cells that exhibits zero Poisson’s ratio in both orthogonal in-plane directions is proposed. Homogenization study on the Fish Cells tessellation is conducted and substantially zero Poisson’s ratio behavior in a homogenized tessellation is shown by numerical analysis. Experimental investigations are performed to validate the zero Poisson’s ratio feature of the metamaterial and obtain force–displacement response of the metamaterial in elastic and plastic zone. A detailed discussion about the effect of the numerical model approach and joints on the structural response of the metamaterial is presented. Morphing skin is a potential application for Fish Cells metamaterial because of the integration benefits of zero Poisson’s ratio design. The structural integrity of the Fish Cells is investigated by studying the stiffness augmentation under tension and in presence of constraints on transverse edges. Finally, geometrical enhancements for improved integrity of the Fish Cells are presented that result in substantially zero stiffness augmentation required for morphing skins

PhD in Aerospace

Morphing structures provide the ultimate aerodynamic efficiency for the aircraft wings during different phases of flight. Achieving a continuously deformed and smooth aerodynamic surface requires a sophisticated morphing skin that provides the necessary strain capacity, prevents stiffness augmentation under actuation, sustains aerodynamic loads, and presents anisotropic behaviour. Cellular metamaterials with zero Poisson’s ratio (ZPR) are able to satisfy the morphing requirements when used as a core in the sandwich morphing skin. Therefore, a new zero Poisson’s ratio metamaterial called “Fish Cells” with compliance in two planar directions is presented that supports the 2D morphing applications. The ZPR behaviour of the Fish Cells metamaterial is studied and proved analytically, numerically, and experimentally. The stress-strain curves of the Fish Cell metamaterial is studied experimentally and effect of geometrical details on the ZPR performance is investigated. In addition, the in-plane and out of plane homogenized equivalent elastic modulus of the Fish Cells are derived. Effect of Poisson’s ratio on dynamic characteristics of cellular structures is studied in two parts. First, hexagonal topology is considered that achieves negative to positive range of Poisson’s ratios by tuning the geometry. The modal properties of such metastructures are studied analytically, numerically, and experimentally. As a result, it is shown that modal properties including the damping ratio, natural frequencies and mode shapes can be tuned by tailoring the Poisson’s ratio. Second, the dynamic analysis of the Fish Cells metamaterial is considered where effect of gradient properties and pre-stress on the modal parameters are studied. It is found that under morphing strains, the ZPR is a crucial factor to save the stability of the cellular core. In addition, it is shown that Fish Cell geometry enables gradient properties that help in tuning modal properties while the global ZPR behaviour is maintained

The study of genetic diversity in some Iranian accessions of Hyoscyamus sp. using amplified fragment length polymorphism (AFLP) and retrotransposon/AFLP markers

Hyoscyamus sp. is well known as a natural source of two main tropan alkaloids including hyoscyamine and scopolamine. The environmental conditions make a very wide diversity of this herb in Iran. This study was conducted to evaluate the genetic diversity within a set of 45 Iranian accessions of Hyoscyamus sp. using amplified fragment length polymorphism and retro/AFLP markers. 18 primer combinations of AFLP markers and five retro/AFLP primer combinations were also used to estimate genetic diversity among accessions. Analysis of banding patterns of 18 AFLP primer combinations revealed 264 polymorphic bands. A total of 264 polymorphic fragments were scored with an average of 14.7 fragments per primer combination. The five retro/AFLP primer combinations generated 42 clearly scorable polymorphic bands across 45 genotypes. The number of polymorphic fragments for each primer pair varied from 5 to 12 with an average of 8.4 polymorphic fragments per primer combination. The cluster analysis discriminated the accessions based on the species and the origin of accessions and demonstrated a high level of genetic diversity in Hyoscyamus sp. accessions. According to cluster analysis, almost all accessions of Hyoscyamus niger and all accessions of Hyoscyamus reticulatus were placed in the same group and the accessions of Hyoscyamus puccillus from the Yazd province were classified in the same group. These results provide important information with regard to future domestication and breeding programs for management of germplasm resources.Key words: Hyoscyamus, retrotransposon, amplified fragment length polymorphism (AFLP), genetic diversity

Dynamic characterization of 3D printed mechanical metamaterials with tunable elastic properties

Mechanical metamaterials are advanced engineering materials that exhibit unusual properties that cannot be found in nature. The elastic properties (i.e., elastic modulus and Poisson's ratio) of mechanical metamaterials can be tuned by changing the geometry of their fundamental unit cells. This allows for the design of metamaterial lattices with targeted quasi-static properties. However, it is not clear how these freedoms contribute to the dynamic properties of mechanical metamaterials. We, therefore, used experimental modal analysis, numerical simulations, and analytical models to study the dynamic response of meta-structures with different values of the Poisson's ratio. We show that Poisson's ratio strongly affects the damping properties of the considered mechanical metamaterials. In particular, we found an inverse relationship between the damping ratio and the absolute value of the Poisson's ratio of the meta-structures. Our results suggest that architected meta-structures similar to those studied could be tailor-made to improve the dissipative performance of mechanical systems. Geometrical design could play an important role in this regard by providing the possibility to tune the various types of quasi-static and dynamic properties of such mechanical metamaterials. </p