Cellular Metals and Metal Foaming Technology (Eds. J.Banhart, M.F.Ashby, N.A.Fleck), pp. 299-304, 2001


C. Motz¹, R. Pippan¹, A. Ableidinger², H.J. Böhm² and F.G. Rammerstorfer²

¹Erich Schmid Institute, Austrian Academy of Sciences,
Leoben, Austria
²Institute of Lightweight Structures and Aerospace Engineering,
TU Wien, Vienna, Austria

Abstract - The tension and fracture behaviour of a commercially available aluminium foam, an ALPORAS foam with a density of about 0.25 g/cm³, was studied. The investigations were carried out on notched and unnotched dog-bone specimens. Besides the common stress–strain measurements, also local deformation, notch opening displacement and crack propagation were measured. The deformation characteristics and the final failure mechanisms deviate from those observed at aluminium foams in compression. No deformation bands or plastic instabilities could be observed in tension, which are very frequent in compression of metallic foams. The fracture strain was found to be only a few percent and the plastic Poisson's ratio was about 0.35.

In addition, finite element simulations were performed on an idealised closed-cell foam. Since no failure mechanisms, e.g. crack initiation, were implemented into the FEM code the tensile stress–strain curves could be calculated up to a macroscopic deformation of about 30%. Instabilities (folding and buckling of cell walls) can only be observed at large strains, which cannot be reached in real foams. Despite the differences of the idealised structure and the real foam the initial part of the stress–strain curves agrees well.

Four stages in the stress–strain curves and the deformation maps are evident: the linear elastic stage, the plastic stage with no significant crack initiation and propagation, the stage of formation of a fracture process zone and at last, the stage of fracture, where a main crack propagates through the specimen, which leads to failure. The notched specimens showed increasing fracture strengths in terms of the net section stress with increasing notch depth. It is suggested that a change in stress state in front of the notch tip causes an increase of the fracture strength similar to that in ductile metals, which is supported by surface strain measurements and finite element calculations.