Indentation and impact response of conventional, auxetic, and shear thickening gel infused auxetic closed cell foam

Abstract

Auxetic closed cell foams, and highly viscoelastic foams, both show potential to improve impact protection. Specifically, auxetics adapt to the shape of impacting bodies, while highly viscoelastic foams stiffen during severe impacts. So, we made auxetic closed cell foam sheets, including those that were infused with (highly viscoelastic) shear thickening gel (STG). We then undertook comparative quasistatic and impact (drop) tests. Quasistatic tests included compression, tension and indentation. Impact tests were with a flat faced impactor at energies of 1, 3 and 5 J, and a 50 mm diameter hemisphere at 1 and 3 J. Poisson’s ratios of the foams were obtained by optical full field strain measurement. An analytical model was used to separate the contribution of the various measured orthotropic properties during the hemispherical impact and indentation tests. The Poisson’s ratios of the converted foams (both with and without STG) were close to zero or marginally negative when measured through thickness. Planar values of Poisson’s ratio (measured in tension) were as low as −0.6. Through thickness Young’s moduli of the converted foams were 0.5 MPa, and planar moduli were ∼12 times higher. The auxetic foams outperformed the unconverted ones during the more severe impacts, exhibiting about half the peak force during the 3 J hemispherical impacts (2.5 vs. 5 kN). The reduction in peak force was related to a measured doubling in indentation resistance for the auxetic foam. The analytical model suggests that 7%–15% of the measured doubling in indentation resistance was due to (negative) Poisson’s ratio. Infusing the auxetic foams with STG caused, at best, a marginal reduction in peak impact force, attributed to low and non-uniform levels of infusion.