Foams as 3D perforated systems: An analysis of their Poisson's ratios under compression

Using experimental data, it is shown that the Poisson's ratios of standard (untreated) open cell polyurethane foam vary when loading in different loading directions, since foam is inherently asymmetric and anisotropic. The loading direction which is in plane orthogonal to the rise direction of the foam constantly exhibited the most negative Poisson's ratio across foam samples of different types, even at moderate compressive strains. It was also shown that although the foam may become auxetic at very high values of compressive strain, it shows an incremental negative Poisson's ratio at moderate compressive strains. The results obtained are discussed in light of the deformation mechanisms, which have been proposed in literature to explain negative Poisson's ratios in foams. The effects of changing loading direction on the mechanical response of the foam are highlighted. An image of the symmetrical face of a foam cube, taken using a confocal microscope at 4× magnification.