Effect of Poisson's ratio on the indentation of open cell foam

We tested whether conventional and auxetic (negative Poisson's ratio) foam indentation response fits with classical indentation theory. We first made foam cubes with 20–25 mm sides, and Poisson's ratios spanning negative and positive values (−0.35 to 0.3). These foam cubes were compression tested, and indented by the curved face of two cylinders (10 and 50 mm diameters) and a stud (12 mm diameter), to 20% of their thickness. Full-field true strains were measured by digital image correlation, to obtain Poisson's ratios and to study foam deformation during indentation. Indentation force vs. displacement was measured and calculated using incremental Poisson's ratios and tangent moduli. Normalised root mean square errors between measured and calculated indentation forces were ∼5% of measured values. Foam densification during indentation, and compression towards sample edges, increased with the magnitude of negative Poisson's ratio; these may both increase indentation resistance beyond predictions from indentation theory.