Quasi-static characterisation and impact testing of auxetic foam for sports safety applications

Abstract

This study compared low strain rate material properties and impact force attenuation of auxetic foam and the conventional open-cell polyurethane counterpart. This furthers our knowledge with regards to how best to apply these highly conformable and breathable auxetic foams to protective sports equipment. Cubes of auxetic foam measuring 150 x 150 x 150 mm were fabricated using a thermo-mechanical conversion process. Quasi-static compression confirmed the converted foam to be auxetic, prior to being sliced into 20 mm thick cuboid samples for further testing. Density, Poisson's ratio and the stress-strain curve were all found to be dependent on the position of each cuboid from within the cube. Impact tests with a hemispherical drop hammer were performed for energies up to 6 J, on foams covered with a polypropylene sheet between 1 and 2 mm thick. Auxetic samples reduced peak force by ~10 times in comparison to the conventional foam. This work has shown further potential for auxetic foam to be applied to protective equipment, while identifying that improved fabrication methods are required.

Auxetic Foam for Snow-Sport Safety Devices

“…32,38], while recent work has produced larger samples as the research moves closer to commercial applications [23-25, 40, 41]. Producing large samples is challenging as the thermo-mechanical process can result in inhomogeneous auxetic foam due to non-uniform temperature and compression gradients present during fabrication [25,33,40,42]. A reliable method of fabricating large volumes of auxetic foam is therefore required to facilitate production and testing for snow-sport safety applications, including monoliths for crash pads and sheets for body armour.…”

“…Poisson's ratio is often measured during low-speed stretching or compression, by tracking the position of markers on a sample followed by linear regression of lateralstrain vs. axial-strain data in the low axial-strain region [e.g. 25,34]. Negative Poisson's ratios have been measured for auxetic foams subject to high-speed compression [24,43].…”

“…This lateral contraction can cause auxetic foam to deform less in the direction of the applied load under impact [23] and reduce peak force in comparison to conventional open-cell foam [23,24]. Auxetic foams have been shown to be particularly effective at limiting forces from concentrated impact loads when combined with a semi-rigid shell [24,25], which should lend them well to padding in body armour.…”

“…In this work, Poisson's ratio was measured in tension to avoid issues with, (1) contact surface friction when compression testing thin sheets [24][25][26] and, (2) positioning and tracking pins in a small sample (cut from the sheet) under compression. After testing the pads, a sample of auxetic foam of each thickness and porosity was cut into three equal strips (resulting in six measuring 10 × 30 × 90 mm and six measuring 20 × 30 × 90 mm in total) and cardboard was glued to the ends so they could be gripped (Fig.…”

“…[22][23][24][25][26], and there are patents for sportswear with auxetic components [27][28][29]. Auxetic foams are characterised by a negative Poisson's ratio; when compressed in one direction these materials contract in one or more perpendicular directions.…”