Low‐kinetic energy impact response of auxetic and conventional open‐cell polyurethane foams

Abstract

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.

Auxetic Foam for Snow-Sport Safety Devices

“…24, 32-35] although a chemical bath can also be used ("chemical-mechanical" or "mechanical-chemical-thermal softening") [36,37]. Increasing volumetric compression ratio (VCR-ratio of uncompressed to compressed volume), up to a limit of approximately five, generally enhances auxetic behaviour and lowers Poisson's ratio [24,33]. The conversion process can be applied to a range of materials [e.g.…”

“…25,34]. Negative Poisson's ratios have been measured for auxetic foams subject to high-speed compression [24,43]. This is especially relevant to snow-sport safety devices, which are typically required to absorb energy through compression at relatively high speed.…”

“…Auxetic foams have higher resilience [24,32] and absorb more energy [44] under compression compared to their conventional open-cell counterparts. Their stressstrain curves have an extended quasi-linear region and the foam densifies earlier as a result of lateral contraction [34].…”

“…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.…”

“…[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.…”