Auxetic Polyurethane Research Articles

Experimental studies on the impact properties of auxetic materials

This paper reports an experimental investigation on the effect of auxeticity towards impact loads. Both conventional and auxetic polyurethane (PU) foams were subjected to impact drop tests, and the damaged surfaces were visually inspected. Although it is known that auxetic materials resist indentation due to densification along the load line, experimental impact tests on conventional and auxetic PU foams reveal that auxetic foams are not necessarily advantageous over conventional ones for very high impact. This paper proposes a geometrical model that resembles the microstructure of auxetic foams, as well as elucidates both the auxetic behavior and the limiting strength at high impact.

Tomographic examination of auxetic polyurethane foam structures

Polyurethane foams processed according to a hybrid mechanical–chemical–thermal (M–Ch–T) process developed at Air Force Institute of Technology (AFIT) have been tested to verify the inner structure deformation after the process. The X-ray microtomography of both the original EUROFOAM S28280 foam and the processed specimens allowed the creation of three-dimensional virtual models of tested specimens and enabled evaluation of homogeneity of foam structures. In order to examine variations in the processed material's inner structure, samples from different locations of the specimen were scanned. Moreover, a foam specimen was divided into nine subspecimens to perform a set of static tests on each of them separately and to check the gradient of properties throughout the sample volume. Presented methods allowed to visualize the inner structure of the processed foams as well as to estimate inhomogeneity. Obtained results will be used for optimization of the manufacturing process.

Comparison of the Mechanical Behaviour of Standard and Auxetic Foams by X‐ray Computed Tomography and Digital Volume Correlation

ABSTRACTThe tensile behaviour of standard and auxetic polyurethane foams are contrasted by digital volume correlation of 3D images collected by in situ X‐ray computed tomography (CT). It was found that subset sizes of 32 and 64 voxels for the auxetic and standard foams were optimal for strain resolutions in the order of 0.1%. For the standard foam, good uniformity of strain was observed at low strains giving a tangent Poisson's ratio of 0.5. Some heterogeneity of strain was observed at higher strains, which may be related to the fixtures. The behaviour of the auxetic foam was totally different, with strain being spatially heterogeneous with transverse strains both positive and negative but giving a negative Poisson's ratio on average. This suggests that the unfolding tendency of some groups of cells was higher than others because of the complex frozen starting microstructure. Further different methods of deriving Poisson's ratio gave different results. Besides revealing interesting microstuctural mechanisms of transverse straining, the study also shows digital volume correlation of tomography sequences to be the perfect tool to study complex mechanical behaviour of cellular materials.

Tests of polyurethane foams with negative Poisson's ratio

Auxetic materials exhibit counterintuitive and interesting properties, which can be utilized to develop new families of products, which can meet sophisticated needs. Study presented within this paper is focused on manufacturing and testing auxetic polyurethane foams for potential introduction for seats in military helicopters. Firstly, a manufacturing process was developed which benefits from the well‐known methods of processing conventional foams and allows preparing necessary test specimens for further tests. Secondly, a series of static and dynamic tests was performed during which the market available conventional foams used in transport are compared with processed auxetic foams.

The Effect of Negative Poisson’s Ratio Polyurethane Scaffolds for Articular Cartilage Tissue Engineering Applications

An auxetic polyurethane (PU) scaffold was prepared to investigate chondrocyte proliferation under compressive stimulation for cartilage regeneration. To give a negative Poisson’s ratio to the PU scaffold, volumetric compression with a 3 : 1 ratio was applied during heat treatment. For the control PU scaffold, the Poisson’s ratio was 0.9 ± 0.25 with elongation at 20% of the strain range. Poisson’s ratio for experimental specimens was approximately −0.4 ± 0.12 under the same conditions. In cell proliferation tests, cells were cultivated within the prepared scaffold under compression with a 20% strain range. With a 20% strain range elongation, the compressive load was approximately 0.3 N. The experimental group showed a 1.3 times higher cellular proliferation rate than that of the control group after 3 days in culture. At day 5 of culture, however, the rate of proliferation of the control group increased so that there was no significant difference between groups. However, collagen content (produced by the cells) in the cell-proliferated medium was 1.5 times higher in the experimental group after 5 days in culture. This may have been due to the effectiveness of the auxetic structure of the scaffold. An isotropic compressive load was transmitted to the cells due to the negative Poisson ratio of the scaffold.

On the effect of heat and solvent exposure on the microstructure properties of auxetic foams: A preliminary study

AbstractThe effect of elevated temperature and solvent exposure on the microstructure of auxetic polyurethane foams is investigated. It is shown that such effects result in an expansion of the auxetic foams accompanied by the removal of the highly convoluted features in the microstructure of auxetic foams with the result that such foams lose their auxetic characteristics. It is also shown that such changes do not occur if the foams are subjected to the high temperature or solvent exposure in a contained state which does not permit the expansion. This means that auxetic foams can still be used in such environments provided that the right precautions are taken (e.g. putting them in a cover thereby constraining them to retain their volume).

In situ three-dimensional X-ray microtomography of an auxetic foam under tension

X-ray microtomography has the potential to unambiguously identify the predominant deformation mechanisms responsible for the auxetic response of polymeric foams. It has been performed in situ on an auxetic polyurethane foam subjected to incremental uniaxial tensile loading. A Poisson’s ratio of −0.20 measured from localized microstructural changes observed during the tomographic sequence compares well with the bulk value of −0.21 obtained by videoextensometry. Evidence obtained by digital image correlation for straightening of bent ribs and rotation of junctions connecting ribs during straining is presented.

Mechanical Performance of Auxetic Polyurethane Foam for Antivibration Glove Applications

In this study the static and dynamic characteristics of conventional open cell polyurethane (PU), of auxetic (negative Poisson's ratio) and of iso-density foams were analysed. The specimens were produced from conventional gray open-cells polyurethane foam with 30–35 pores/inch and 0.0027 g/cm3density, by means of process which has been previously defined by the authors. Poisson's ratio measurements were performed under quasi-static conditions using an MTS 858 servo-hydraulic test machine and a video image acquisition system. For the auxetic foams the results suggested similar behaviour to that previously reported in the literature, with significant increases in stiffness during compressive loading, and a significant dependence of the Poisson's ratio on the applied strain. Transmissibility tests, performed in accordance with the ISO 13753 procedure for antivibration glove materials, suggested a strong dependence of the transmissibility on the foam manufacturing parameters. Within the frequency range from 10 to 31.5 Hz the transmissibility was found to be greater than 1, while it was less than 1 at all frequencies greater than 31.5 Hz. The transmissibility results were similar to the mean values for 80 resilient materials tested by Koton et. al., but were higher than the five best materials (not all polymeric) identified by the same researchers. In this study it has been suggested that the resilient behaviour of glove isolation materials should also be evaluated in terms of the indentation characteristics. A simple, linear elastic, Finite Element simulation was therefore performed, and the indentation results suggested that auxetic foams offer a significant decrease in compressive stresses with respect to conventional PU foams.

Trends in acoustic properties of iron particle seeded auxetic polyurethane foam

The sound absorption characteristic of a clean, open-cell configuration, typical polyurethane flexible foam is shown against that of an auxetic foam made from it, and this same foam after it has been seeded with a magnetorheological fluid (then dried), having 2–5 μm carbonyl iron particles and subjected to zero, weak and concentrated magnetic fields in an acoustic impedance tube facility. The resultant foam indicates the capability of shifting the peak acoustic absorption coefficient within a given frequency bandwidth when constant intensity magnetic fields are applied.