Poisson's Ratio Foams Research Articles

Impact properties of uniaxially thermoformed auxetic foams

This work describes large strain quasi-static and impact tests of a new class of low-cost, uniaxially thermoformed, transverse isotropic auxetic foams with compression ratio ranging from 20 to 80%. A custom drop tower rig with high-speed cameras suitable for these soft porous foam materials is designed and used to perform impact tests with energy ranging from 0.38 to 1.90 J, corresponding to strain rates from 64/s to 143/s. The thermoformed foams only show auxeticity when loaded along the transverse direction to the uniaxial thermoforming compression, with Poisson's ratio V21 reaching as low as −1.5 at 20% strain and then close to 0 at large strains. The samples deform with large shear band deformations, also due to the auxeticity. The negative Poisson's ratio foams along that transverse direction also show enhanced impact energy absorption performance, with normalized peak force reduction as high as ∼40% against the pristine foam; the reduction is however 20% along the thermoforming direction. A constitutive model based on Nagy's approach is applied to describe the enhancement of dynamic stress during the impact tests compared with the quasi-static one, due to strain rate effects.

Mechanical Properties Analysis on a Novel Negative Poisson's Ratio Voronoi Foam‐Filled Corrugated Tube Under Impact Loading

A novel negative Poisson's ratio (NPR) foam‐filled corrugated tube, made up of a corrugated tube and NPR isotropic foam core, is established by the finite element method. The 3D NPR isotropic foam core is achieved by using the triaxial compression method on a cubic Voronoi model, which is created based on the Voronoi tessellation technique. The triaxial compression ratio is defined as η. The Poisson's ratio of Voronoi foam decreases sharply and then increases gradually with an increasing η. A minimum Poisson's ratio, −0.076, is obtained when η = 0.2. Moreover, the Voronoi foam‐filled corrugated tube with minimum Poisson's ratio foam core exhibits the highest energy absorption capacity, which is 10.8% higher than that of the Voronoi foam‐filled corrugated tubes with uncompressed foam core.

Shear modulus of conventional and auxetic open-cell foam

This work analyses shear moduli of conventional and auxetic open-cell polymer foams. Shear moduli are i) measured directly and ii) calculated by applying elasticity theory for isotropic solid materials, using Young's moduli and Poisson's ratios from compression tests. Zero and negative Poisson's ratio foams are fabricated from conventional foams using a thermo-mechanical process. Fabricated and conventional foams are compression tested in three orthogonal directions, up to densification at ~60% compression, with full-field strain measurements obtained using Digital Image Correlation. Compression testing is followed by shear testing. The measured shear moduli vary from 16±7 kPa for negative Poisson's ratio foams to 38±2 kPa for zero Poisson's ratio foams, with conventional foams in between with a mean value of 32±8 kPa. The calculated shear moduli are typically lower than the measured values. The results suggest that the application of elasticity theory to calculate the low strain shear modulus of open-cell foam from Young's modulus and Poisson's ratio measured in compression tests is appropriate if the foam is isotropic.

Novel Method for Preparing a High-Performance Auxetic Foam Directly from Polymer Resin by a One-Pot CO2 Foaming Process.

So far, negative Poisson's ratio foam (auxetic foam) is usually prepared by the transformation of conventional polymer foam, and it is a challenge to prepare auxetic foam directly from polymer resin. In this work, a novel method based on the synergism between the phase transition of water and the permeability rate difference of the CO2 blowing agent from air in the foaming process is put forward to prepare auxetic foam directly from the polymer resin. Using this method, herein, nylon elastomer (NE) foam with auxetic behavior is successfully prepared from NE resin by a one-pot CO2 foaming method inside an autoclave aided by water; the obtained auxetic foam has a Poisson's ratio value of -1.29 and has excellent tensile cycle stability and energy absorption performance. This method breaks through the bottleneck for preparing auxetic foams from the polymer resin directly, which is also green and environment-friendly. It is believed that the abovementioned method also applies to other thermoplastic polymers, which paves a way for designing and preparing multifunctional auxetic foam materials in the future via a one-pot CO2 foaming process. Furthermore, this work first demonstrates that the transformation between auxetic foam and positive Poisson's ratio foam is reversible due to the closed cell structure, which provides an opportunity to reversibly adjust the performance and the shape of polymer foam materials.

Crashworthiness analysis of cylindrical tubes filled with conventional and negative Poisson's ratio foams

The influence of the Poisson's ratio of the foam on the energy absorption of the foam-filled tube is investigated in this paper. Firstly, the relationship between the mechanical properties and the structural parameters of the 3D lattice structure is calculated by using the theory of material mechanics. The finite element analysis software ABAQUS is used to simulate the axial dynamic response of cylindrical tubes filled with conventional and negative Poisson's ratio foams. Based on this, the interaction between the tube wall and foam is analyzed in detail. Combining the structural properties of both the positive and negative Poisson's ratio foams, a new foam model with mixed Poisson's ratio (MPR) is proposed, and the crashworthiness of three different types of tubes, namely the sandwich double tubes filled with positive, negative and mixed Poisson's ratio foams, is analyzed. The results show that the interaction between the foam and the tube wall has a significant effect on the energy absorption performance. The sandwich double tube filled with MPR foam has better specific energy absorption (SEA) and stronger interaction than those filled with pure positive or negative Poisson's ratio foam. This study provides a new insight into crashworthiness design of foam-filled structures.

Fabrication and characterization of auxetic shape memory composite foams

Shape memory polymers , as a kind of smart materials, play an important role in more and more fields, such as aerospace, biomedicine and intelligent clothing field and so on. Comparing with traditional materials, negative Poisson's ratio foam has excellent mechanical properties, such as double curvature, light weight, high shear resistance, auxetic etc., and it has a great potential application in the field of aerospace. Therefore, in this project, shape memory composite foam was fabricated based on the commercial soft polyurethane foam material as matrix and shape memory epoxy resin as functional phase. Negative Poisson's ratio of foam was fabricated based on its shape memory feature through a process of triaxial compression with heat treatment. Microstructure deformation was characterized after the transformation of auxetic foam. By adjusting the processing parameters, the auxetic shape memory foam with different “re-entrant” structure was obtained. Such fabricated shape memory composite foams display variable stiffness with auxetic behavior. Effective compressive and tensile modulus was obtained by compression and tension tests . The effect of processing parameters on foam Poisson's ratio was analyzed, which provided certain guiding significance for the further shape memory foam preparation.

Three‐Dimensional Stiff Cellular Structures With Negative Poisson's Ratio

In this paper, a novel three‐dimensional (3D) cellular structure with negative Poisson's ratio was designed by alternating cuboid surface indents on the vertical ribs of the unit cells. The Poisson's ratio and Young's modulus of structures with different geometric parameters were determined using the finite element method (FEM) as a function of these parameters. Samples with identical geometric variables were fabricated via 3D printing, and their through‐thickness direction Poisson's ratios were measured and compared with simulation results. Results showed that the Poisson's ratio of the 3D cellular structures can be tuned from positive to negative and can reach a minimal value of −0.958. Good agreement was found between the experimental results and the simulation. This lattice structure is considerably stiffer than re‐entrant negative Poisson's ratio foam with the same solid phase. The design concept developed here can be optimized for specific applications via geometric parameters manipulation.

Compressive uniaxial properties of auxetic open cell PU based foams

AbstractThe mechanical properties of negative Poisson's ratio foams are described under compressive loading both from a numerical and experimental point of view. Different samples of open cell PU (polyurethane) based auxetic foams (i.e. with negative Poisson's ratio) were manufactured following a dedicated manufacturing process, using different compression ratios and temperature profiles. High strain rate and quasi static compressive loading tests were performed in order to benchmark the numerical model. A numerical technique based on loading simulations of statistical assemblies of cells has been adapted to represent the uniaxial compressive behaviour of the auxetic foams. The results can be applied to predict the auxetic foam's behaviour and solving problems related to protection and packaging.

Analytical parametric analysis of the contact problem of human buttocks and negative Poisson's ratio foam cushions

Analytical investigations on the contact problems between two homogeneous and isotropic soft bodies were performed to simulate the contact of human buttocks and seat cushions. The cushion materials' Poisson's ratio were allowed to be negative. The human buttocks were modeled as an ideal sphere with radius 15 cm, and assumed to have a low Young's modulus and a Poisson's ratio close to 0.5. These parameters were held constant during our analysis. Peak contact pressure was reduced by adjusting the contour curvature of cushions according to Hertz theory, as expected. Moreover, analysis by both the Hertz model and a finite thickness 3D elasticity model showed that using negative Poisson's ratio cushions could further reduce the pressure. Negative Poisson's ratio cushions may be beneficial in the prevention of pressure sores or ulcers in the sick and in reduction of pressure-induced discomfort in seated people.

Influence of Cell Size on Re-Entrant Transformation of Negative Poisson's Ratio Reticulated Polyurethane Foams

Several foams of different cell-size, including Scott Industrial polyurethane foam with large cells (20 pores per inch, ppi, or 1.2 mm per pore, black), medium cells (65 ppi, or 0.4 mm per pore, green), and near-microcellular (100 ppi, 0.25 mm per pore, white), were processed over various time and temperature regimes to ascertain the role of cell size in transformation to negative Poisson's ratio materials. These foams were transformed successfully, and exhibited negative Poisson's ratio behavior. Poisson's ratio was measured using a new laser based setup. For all as-received (unprocessed) foams with different cell sizes, Poisson's ratio decreased with compressive axial strain and increased with tensile strain up to a maximum. The maximum Poisson's ratio in tension decreased as cell size increases. The strain at which maximum Poisson's ratio occurs, increased with cell size. In negative Poisson's ratio foams, minimum Poisson's ratios of −0.8, −0.5, and −0.4 for 20 ppi, 65 ppi, and 100 ppi foams, respectively were observed. Furthermore, the cell size effects on transformation parameters were also found.

Scale-up of Transformation of Negative Poisson's Ratio foam: Slabs

This paper presents an experimental study performed on the scale-up of processing of negative Poisson 's ratio foam. These materials become thinner under compression and fatter under tension, opposite to ordinary materials. The transformation process consists of triaxial compression combined with heat treatment. Transformed foam cells have a concave or "re-entrant" shape. Scaleup is achieved by use of a modified mould assembled around the foam slab and longer processing times. Foam with a distribution of cell sizes was transformed in small block and large slab forms. These blocks and slabs exhibited a negative Poisson's ratio.

Hysteresis Behaviour and Specific Damping Capacity of Negative Poisson's Ratio Foams

Previously open cell polyurethane foams have been made to be re-entrant, in that their cell ribs are inwardly bent, which results in the foam exhibiting a negative Poisson's ratio(1). Untransformed control and transformed negative Poisson’s ratio foams were cyclically loaded at various levels of pre-strain to determine their hysteresis behaviour, and from this, to elucidate their specific damping characteristics. Both untransformed and transformed foams exhibited non-linear hysteresis behaviour at high cyclic strains, regardless of their initial levels of pre-strain. The specific damping capacity of the transformed foam with extreme compressive pre-strain was higher than that of the untransformed foam. This effect was attributed to the rubbing of the inwardly buckled cell ribs of the transformed foam. The specific damping capacity varied with both the level of pre-strain and with the amplitude of cyclic strain about that level.

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.

Indentability of Conventional and Negative Poisson's Ratio Foams

The indentation resistance of foams, both of conventional structure and of a novel re-entrant structure giving rise to negative Poisson's ratio, was studied using holo graphic interferometry. In holographic indentation tests, re-entrant foams had higher yield strengths σy and lower stiffness E than conventional foams of the same original relative density. Damage in both kinds of foam occurred primarily directly under the indenter. Calculated energy absorption for dynamic impact is considerably higher for re-entrant foam than conventional foam

Design of a Fastener Based on Negative Poisson's Ratio Foam

In this article we make use of the negative Poisson's ratio of recently developed cellular solids in the design of a press-fit fastener. Insertion of the fastener is facilitated by the lateral contraction which negative Poisson's ratio materials exhibit under compression. Removal of the fastener is resisted by the corre¬sponding elastic expansion under tension.