Microstructural characterisation and experimental determination of a multiaxial yield surface for open-cell aluminium foams

Metal foams are bio-inspired microheterogeneous materials, which exhibit a strong structure-property relationship. Their global mechanical properties depend strongly on the local micromechanical properties of the struts and on the pore geometry of the foams. A solid micromechanical and macromechanical understanding of the yield behaviour of the foams under realistic complex stress states is essential in order to be able to design components made of foams. However, up to now, experimental yield surface data for foams are very limited. The present contribution deals with the structural characterisation of open-cell aluminium foams of different pore sizes by X-ray computed tomography (CT). The strut geometry e.g. regarding cross-sectional shape and the mass distribution along the struts is evaluated from the CT data. Yield surfaces for 10, 20 and 30ppi foams are experimentally probed by performing uniaxial tensile and compression tests, pure torsion as well as combined compression-torsion and tension-torsion tests. This results in one of the most comprehensive experimental data sets in the literature ever reported for open-cell aluminium foams. The shape of the yield surface and its degree of asymmetry were connected to the geometric data from the structural characterisation. It provides a deeper understanding of aluminium foams under complex multiaxial stress states.