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This work describes the identification of the shear modulus of open cell polyurethane thermoformed auxetic foams from 3- and 4-point bending tests. The foams are incorporated in sandwich beams with carbon fibre/epoxy face skins, and benchmarked against similar sandwich structures made with the conventional counterpart open cell foam. Three types of beams are tested: one with auxetic foams, another type related to a conventional foam core with the same thickness of the auxetic porous materials, and a third type of beam consisting in conventional foam with a thickness corresponding to an iso-weight configuration to the auxetic specimen. The auxetic foam has a shear modulus 7% lower than the one of the bulk conventional specimens, but higher shear stresses at large deformations and a smoother strain stiffening response compared to the beams with the conventional thinner core. The paper also highlights the low shear wave speed of these auxetic foams compared to other porous polymers used in helmet and head protection applications, as well as potential uses of the quasi-zero-stiffness behavior here observed for the auxetic foam sandwich beam. [ABSTRACT FROM AUTHOR]

This paper reports quasi-static and low-kinetic energy impact testing of auxetic and conventional open-cell polyurethane foams. The auxetic foams were fabricated using the established thermo-mechanical process originally developed by Lakes. Converted foams were subject to compression along each dimension to 85% and 70% of the unconverted dimension during the conversion process, corresponding to linear compression ratios of 0.85 and 0.7, respectively. The 0.7 linear compression ratio foams were confirmed to have a re-entrant foam cell structure and to be auxetic. Impact tests were performed for kinetic energies up to 4 J using an instrumented drop rig and high speed video. A flat dropper was employed on isolated foams, and a hemispherical-shaped dropper on foams covered with a rigid polypropylene outer shell layer. The flat dropper tests provide data on the rate dependency of the Poisson's ratio in these foam test specimens. The foam Poisson's ratios were found to be unaffected by the strain rate for the impact energies considered here. Acceleration-time data are reported along with deformation images from the video footage. The auxetic samples displayed a six times reduction in peak acceleration, showing potential in impact protector devices such as shin or thigh protectors in sports equipment applications. [ABSTRACT FROM AUTHOR]

This work describes a series of investigations carried out on sets of pristine and auxetic (negative Poisson's ratio (NPR)) open‐cell polyurethane foams subjected to relative humidity (RH) conditioning ranging from 9% to 92% at room temperature. The foams have been produced using a uniaxially thermoforming process. Pristine and auxetic foams have then been subjected to quasi‐static compressive cyclic loading (with maximum strains of 10% and 80%, respectively), as well as vibration transmissibility tests with base accelerations up to 2.29 g. Increasing levels of RH do not seem to statistically affect the moduli and PRs of foams subjected to lower maximum strains, however, especially the auxetic foams at higher compressive deformations show a decrease in the stiffness with the increase of the RH. The vibration tests show an increasing trend of the dynamic modulus of the foams with the increase of the RH. The results indicate the complexity of the interaction between foam architecture and absorption/desorption mechanisms occurring inside these porous auxetic materials. [ABSTRACT FROM AUTHOR]

The utility of auxetic materials in applications such as foams and composites is contingent upon the performance of these materials under cyclic loading. Recently, several textile materials (knitted, woven, and nonwoven fabrics) have been found to exhibit auxetic properties, either through design and/or the use of a postprocessing treatment. In the wake of these advancements, this research examines the reversibility of the out‐of‐plane auxetic response in previously studied polyester fiber needle‐punched nonwovens. Herein, a heat‐compressed nonwoven is strained cyclically to different levels, and the thickness is measured before and after straining. While axial extension is found to be elastic for up to 3% strain, thickness increases (auxetic responses) are almost completely reversible for 1% axial strain and highly reversible for 2% and 3% axial strain. Irreversible changes can be clearly observed starting at strains of 2% and larger, especially for a higher number of cycles. Combining these results with the previous understanding from microscopic and tomographic imaging, possible reversible (straightening of bent fibers and fiber bundles) and irreversible (slipping or breaking of fibers) structural deformations leading to thickness changes are identified. These processes influence the magnitude of the auxetic response within and beyond the elastic strain regime, respectively. [ABSTRACT FROM AUTHOR]

Life‐changing illnesses and fatalities caused by inefficient personal protective clothing and equipment are increasing day by day. The discrepancy between the test standards, environmental conditions, and infield collision is the reason why personal protection equipment and clothing have not yet decreased the number of injuries in various fields such as military and sports. To further increase the efficiency of personal protection materials, auxetic materials are suggested because of their high energy absorption, good permeability, form‐fitting ability, and high indentation resistance. Auxetic materials are nonconventional materials with a negative Poisson's ratio and can shrink under compression and expand when subjected to stretching. In comparison to nonauxetic conventional materials, auxetic materials have improved properties that can be useful for personal protection applications. This review focuses on the importance of auxetic materials for personal protection, focusing on reducing the chances of injury and what possible ideas can further benefit the protection products. The first part of the review discusses the auxetic structures and their protection applications followed by the fabrication of auxetic structures. The review will conclude with limitations, economic prospects, and what possible work can further improve the potential use of auxetic materials in personal protection. [ABSTRACT FROM AUTHOR]

This paper describes the fabrication of polymeric auxetic foam using a quasi tri‐axial compression method. In this method, the conversion process is executed by applying a quasi tri‐axial compression on conventional closed‐cell polyurethane foams. In order to achieve optimum auxetic characteristics, the densification points in various directions are evaluated. In addition, the heating time of the compressed foam is also determined and applied to minimize the residual stress due to the volumetric compression. It leads to the fabrication of auxetic foams with achievable maximum re‐entrancy and minimum collapsed cells without surface creasing. The results also show that the optimized auxetic foam exhibits isotropic behavior compared to its pre‐cursor foam as verified by the morphological analysis. The produced foam has all the desired properties of auxetic foams. The auxetic foam with a negative Poisson's ratio may be have great potential as the supplementary materials in energy absorption and structural applications. [ABSTRACT FROM AUTHOR]

The article describes the design, manufacturing, modelling and testing of a hybrid composite support made from auxetic open cell foam liners and curved thermoplastic plates with rhomboidal perforations for human body support. Both the foam and the curved perforated plate have in-plane negative Poisson's ratio behaviour. The static bending stiffness of the hybrid auxetic composite support is modelled using an analytical and Finite Element approach benchmarked against experimental results from three-point bending tests. The benchmarked Finite Element models are used to develop a map of optimised static stiffness versus the geometry of the rhomboidal perforations. A Design of Experiment testing campaign is also carried out on 16 hybrid auxetic composite plates to understand the interaction and correlation of the static bending versus the geometry of the perforations and the contribution given by the auxetic foam liner. [ABSTRACT FROM AUTHOR]

Auxetic foams have been widely studied in view of their superior properties and many useful applications and various models have been developed to help explain the auxetic behavior in such foams. One such model involves the description of auxetic foams in terms of rotating units (e.g. the joints where different cell walls meet), a mechanism, which has also been observed experimentally. In the models, the rotating units are taken, to a first approximation, to be representable through rotating rigid triangles, which correspond to the 2D projection of these rotating units and although this model has been improved significantly since it was first proposed, current models still do not fully capture all the deformations that may occur in real foams. In this work, we propose an extended model which not only allows for relative rotation of the units (joints), represented by non-equilateral triangular units, but also for differing amount of material at the joints as well as deformation of the joints themselves, a scenario that is more representative of real auxetic foams. This model shows that, by permitting deformation mechanisms other than rotation of the triangles, the predicted extent of auxeticity decreases when compared to the equivalent idealized rotating rigid triangles model, thus resulting in more plausible predictions of the Poisson's ratios. Furthermore, it is shown that in the manufacturing process, a minimum compression factor, which is dependent on the amount of materials at the joints, is required to obtain an auxetic foam from a conventional foam, as one normally observed in experimental work on foams. [ABSTRACT FROM AUTHOR]