Initial yield behaviour of closed-cell aluminium foams in biaxial loading

This study investigates the initial yield characteristics of closed-cell aluminium foam (Alporas) in multiaxial stress states using experimental and numerical approaches. The mechanical responses of foams with two relative densities (11.7% and 17.2%) are presented in uniaxial compression, uniaxial tension, biaxial compression, biaxial tension and combined tension-compression loading conditions. Cubic and dog-bone specimens are fabricated for uniaxial compression and tension tests respectively. A Modified Maltese Cross Specimen (MMCS) geometry is custom designed for testing closed-cell foams in biaxial stress states. The experimental investigations reveal a consistently stiffer response of closed-cell foams in tensile dominated stress states. This is attributed to the stretching mode of deformation observed in the cell walls in tensile loading conditions, in contrast to the bending/buckling dominant deformation in compressive loading conditions. To complement the experimental data, a wide range of multiaxial stress states is simulated using irregular 3D Voronoi models. An initial yield surface normalized with respect to the uniaxial compressive yield strengths is constructed in the mean-effective stress space using both experimental and numerical data. The normalized initial yield surfaces appear considerably shifted towards the positive mean stress axis. An yield criterion for closed-cell aluminium foams that adequately captures this asymmetric nature of the yield surface about the effective stress axis is proposed, which requires only uniaxial compressive yield strength as a parameter. An empirical relation function estimating the uniaxial compressive yield strength based on the density of the foam material is proposed to facilitate direct application of the yield criterion in early stages of the design.