Re-entrant Cell Research Articles

Negative Poisson's ratio polyethylene foams

Various polyethylene foams were subjected to thermo-mechanical processing with the aim of transforming them into re-entrant materials exhibiting negative Poisson's ratio. Following transformation, large cell foams (cell sizes of 1 and 2 mm) exhibited re-entrant cell structure and negative Poisson's ratio over a range of processing times and temperatures. Poisson's ratio vs. strain for these foams was similar to prior results for reticulated polyurethane foams. Following processing, microcellular polyethylene foam was densified but cells remained convex; it did not exhibit a substantial negative Poisson's ratio. This foam had a different transition temperature as determined via DSC than the large cell foams.

Numerical and experimental uniaxial loading on in-plane auxetic honeycombs

Auxetic honeycombs show in-plane negative Poisson's ratio properties; they expand in all directions when pulled in only one, and contract when compressed. This characteristic is due to the reentrant shape of the honeycomb unit cell. The cell convoluteness gives a geometric stiffening effect that affects the linear elastic properties of the whole cellular solid. In this paper finite element simulations are carried out to calculate the in-plane Poisson's ratio and Young's moduli of re-entrant cell honeycombs for different geometric layout combinations (side cell aspect ratio, relative thickness and internal cell angle) subjected to uniaxial loading. The results show a high sensitivity of the mechanical properties for particular ranges of the geometric cell parameters. An image data detection technique is used to extract displacements and strains from an aramid paper re-entrant honeycomb sample in a tensile test. The comparison between numerical and experimental results shows good agreement.

THEORETICAL CHARACTERISTICS OF THE VIBRATION OF SANDWICH PLATES WITH IN-PLANE NEGATIVE POISSON'S RATIO VALUES

This paper is concerned with re-entrant cell honeycombs which show in-plane negative Poisson's ratio values, in which their anisotropic mechanical properties are described using the cellular material theory. Out-of-plane shear moduli are affected by the unit cell geometric parameters and, for some ranges of the latter, it is possible to obtain higher values of the shear moduli compared to those of a regular hexagonal honeycomb, in particular for cell geometries with a negative Poisson's ratio. A first order sandwich plate theory is applied in order to obtain the fundamental frequencies of sandwich laminates in cylindrical bending and for the simply supported case. Sensitivities of the frequencies per unit mass versus the geometric cell parameters are also calculated. The results suggest that the dynamic performance of a sandwich structure could be significantly improved with a proper design of the unit cell shape of the honeycomb. In particular, re-entrant cell cores offer improvements in bending stiffness capabilities for particular cell parameter ranges.

An Auxetic Filter: A Tuneable Filter Displaying Enhanced Size Selectivity or Defouling Properties

Micromachined polymeric honeycomb membranes having conventional and re-entrant cell geometries have been fabricated using femtosecond laser ablation. Mechanical properties characterization confirms that the re-entrant membrane is auxetic (possesses negative Poisson's ratios: νxy = −1.82 ± 0.05 and νyx = −0.51 ± 0.01) whereas the conventional membrane possesses positive Poisson's ratios (νxy = +0.86 ± 0.06 and νyx = +0.6 ± 0.1). Comparison with honeycomb theory confirms that the dominant deformation mechanism is flexure of the honeycomb ribs. The auxetic membrane has been challenged with single-sized glass chromatography beads such that the beads were initially resting on the re-entrant cells. Subsequent tensile loading of the membrane showed the auxetic cells opening during deformation, enabling the beads to pass through the membrane. We have modeled the pore-opening properties of both types of membranes, and the observed behavior for the auxetic membrane is consistent with the model. This is a clear proof-of-concept demonstration of the potential of auxetic materials and structures in filter defouling or cleaning operations. This paper, therefore, demonstrates the successful design and fabrication of a micromachined auxetic structure having specifically tailored mechanical properties that show enhanced functional performance over the conventional filter structure.

Indentation Resilience of Conventional and Auxetic Foams

Materials with a negative Poisson's ratio (auxetic materials) demonstrate the fascinating property of becoming fatter when stretched. This phenomenon is fundamentally important to the properties of a material and a significant change in the value of the ratio has significant effects on the material's mechanical performance. This article describes foams with a negative Poisson's ratio that show better indentation resilience than conventional foams with a positive Poisson's ratio. The unusual mechanical properties of auxetic foams are attributed to the deformation characteristics of re-entrant (concave polyhedral) cell structures.

Fabrication methods for auxetic foams

Auxetic materials have a negative Poisson's ratio, that is, they expand laterally when stretched longitudinally. Negative Poisson's ratio is an unusual property that affects many of the mechanical properties of the material, such as indentation resistance, compression, shear stiffness, and certain aspects of the dynamic performance. The unusual mechanical properties of auxetic foams are attributed to the deformation characteristics of re-entrant microstructures. One way of obtaining negative Poisson's ratio is by using a re-entrant cell structure. Auxetic foam was fabricated from a conventional polymeric foam. The fabrication method for making both small and large auxetic foam specimens is described.

Microscopic examination of the microstructure and deformation of conventional and auxetic foams

Auxetic materials have a negative Poisson’s ratio, that is, they expand laterally when stretched longitudinally. One way of obtaining a negative Poisson’s ratio is by using a re-entrant cell structure. Auxetic foam was fabricated from a conventional polymeric foam. Assuming similar mechanical properties for the solid material comprising the foams, the principle variable affecting the properties of the foam is the geometry of the cells. This means that the unusual mechanical properties of auxetic foams are attributed to the deformation characteristics of re-entrant microstructures. In this paper, the results of optical- and scanning electron-microscopic studies of the geometrical parameters for the different foams examined are presented. Examples of the microstructural deformation mechanisms observed are also presented. Comparison between the conventional foams and their auxetic conversions are also made.

Fracture toughness of re-entrant foam materials with a negative Poisson's ratio: experiment and analysis

Fracture toughness of re-entrant foam materials with a negative Poisson's ratio is explored experimentally as a function of permanent volumetric compression ratio, a processing variable. J IC values of toughness of negative Poisson's ratio open cell copper foams are enhanced by 80 percent, 130 percent, and 160 percent for permanent volumetric compression ratio values of 2.0, 2.5, and 3.0, respectively, compared to the J IC value of the conventional foam (with a positive Poisson's ratio). Analytical study based on idealized polyhedral cell structures, approximating the shape of the conventional and re-entrant cells, disclose for re-entrant foam, toughness increasing as Poisson's ratio becomes more negative. The increase in toughness is accompanied by an increase in compliance, a combination not seen in conventional foam, and which may be useful in some applications such as sponges.

Analysis of elastic modulus of conventional foams and of re-entrant foam materials with a negative Poisson's ratio

Foam materials with negative Poisson's ratio can be made by transforming the cell structure from the convex polyhedral shape of conventional foam cells to a concave or “re-entrant” shape. Mechanical behavior of re-entrant foams differs from that of conventional foams. Most of the difference is attributed to the change in cell shape. In this study, Young's moduli of conventional and re-entrant open cell foams are obtained by modeling the three-dimensional unit cell as an idealized 14-sided unit cell. Young's modulus of re-entrant foams decreases with permanent volumetric compression ratio in both modeling and experiments.

Nonlinear Analysis of the Poisson's Ratio of Negative Poisson's Ratio Foams

This article contains an analytic study of Poisson's ratio of re-entrant foam materials with negative Poisson's ratio. These materials get fatter when stretched and thinner when compressed. The Poisson effect is so fundamentally important to the properties of a material that a large change in the value of the ratio will have significant effects on the material's mechanical performance. Isotropic foam structures with negative Poisson's ratio have been fabricated through a permanent volumetric transformation. The cells were converted from the convex polyhedral shape of conventional foam cells to a concave or "reentrant" shape. Mechanical behavior of a re-entrant open cell foam material will differ from that of a conventional foam in ways not addressed by existing theoretical treatment. Poisson's ratio as a function of strain is obtained by modeling the three-dimensional unit cell as an idealized polyhedron unit cell. Poisson's ratio is predicted to approach the isotropic limit of −1 with increasing permanent volumetric compression ratio of idealized cells, in comparison with experimental values as small as −0.8.

Auxetic foams: Modelling negative Poisson's ratios

A finite element model for three-dimensional, open-called foams is developed. A reentrant cell shape is defined that has the unusual property of exhibiting a negative Poisson's ratio. By altering the cell shape it is possible to achieve different values for the elastic constants and different degrees of anisotropy. It is shown that simple two-dimensional approximations fail to model the properties of the three-dimensional network accurately and a full three-dimensional analysis is required. The reentrant cell used is fully auxetic, with negative Poisson's ratios exhibited when loaded in all three orthogonal directions.

Damping Test of the Higher-Order Modes of the Re-Entrant Accelerating Cavity

Higher-order modes were studied in a model cavity (500 MHz re-entrant single cell cavity) which was designed for the KEK-PF 2.5 GeV electron storage ring. The longitudinal electric field and the transverse electric and magnetic fields were measured near the beam axis for the higher-order modes. The observed parameters of the TMO-mode resonances were in reasonable agreement with the SUPERFISH calculation. The TM011-like resonance at 758 MHz has the highest shunt impedance of 3.0 MΩ. The observed transverse coupling impedances of the HEMl modes at 829 MHz (TM110-like) and 1071 MHz (TM111-like) were 12 NΩ/m and 27 MΩ/m, respectively. Damping couplers are being developed to prevent the cavities from driving the coupled-bunch instabilities. The impedances were reduced to less than 0.01 MΩ and 0.2 MΩ/m for almost all of the TM0- and HEMl-mode resonances, respectively, in a low power test.