Yield surface of polyurethane and aluminium replicated foam

It has been proposed in the literature, based on theoretical considerations and on finite-element calculations, that all three stress tensor invariants govern the yield surface of cellular materials. Recent experiments on 75 gm pore size aluminium replicated foams (Combaz E, Bacciarini C, Charvet R, Dufour W, Mortensen A. Multiaxial yield behaviour of AI replicated foam, submitted for publication) showed such a dependence of the yield surface in axisymmetric tests. This study explores the yield behaviour of 400 gm pore size aluminium replicated foams: experiments confirm the influence of the third invariant on the yield surface shape, together with the observations that (i) the yield surface shape does not depend on relative density and (ii) measured flow vectors conform with normality. A simple parabolic model fitting data in the previous study also captures well the present data under all tested stress states (biaxial, axisymmetric and Pi-planes in stress space). Biaxial and axisymmetric tests are also performed on 400 pm pore size polyurethane (PU) replicated foams with a similar mesostructure. Results show yield to occur at a value lower than predicted by micromechanical models for both matrix materials (aluminium and PU). This suggests that the "knock-down" factor usually observed between predicted and observed stress values probably cannot be explained by a lowered yield stress in the material making the foam. The data also suggest an influence of the matrix nature on the yield surface geometry. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.