Effect of anisotropy on the mechanical properties of polyurethane foams: An experimental and numerical study

The mechanical behaviour of anisotropic, closed-cell PU foams has been studied from the experimental and simulation viewpoints. Anisotropic PU foams with different densities were tested in compression along the rising and the perpendicular directions to determine the mechanical properties (elastic modulus and plateau stress). In addition, the microstructure of the foams (cell size distribution and anisotropy, cell wall thickness, etc.) was carefully measured and the mechanical properties of the solid PU within the foam were determined by means of nanoindentation. This information was used to build realistic representative volume elements of the microstructure of the foams by means of Laguerre tessellations and the mechanical behaviour was obtained by means of the finite element simulation of these representative volume elements. The numerical simulations corroborated the large increase in the stiffness, in the stress at the onset of plastic instability and in the plateau stress of the foam along the rising direction, in agreement with the experimental results. In addition, the analysis of the elastic response in the representative volume elements showed that deformation along the rising direction (parallel to the longer axis of the cells) was dominated by the axial deformation of the struts while bending controlled the deformation perpendicular to the rising direction. The stress at the onset of plastic instability was controlled by the buckling of the struts, which was triggered at lower stresses in the direction perpendicular to the rising direction because of bending.