Deformation and fracture of aluminium foams

The tensile and compressive properties and the fracture resistance of two aluminium alloy foams have been measured. The yield strength, unloading modulus and toughness increase with relative density in such a manner that the closed cell foams of this study behave as open cell foams. These relationships can be described adequately by power law fits. Experimental results, when compared with theoretical models based on idealised foam structures, reveal unexpected discrepancies. We conclude that they are caused by morphological defects in the microstructures of the foams, the effects of which were not included in the models. Tests on samples with deep sharp notches show that the tensile and compressive strengths are notch-insensitive. Fracture toughness measurements show an R-curve behaviour. This is analysed in terms of the underlying microstructure — the major cause of the R-curve was observed to be the development of crack bridging ligaments behind the crack tip. The compact tension specimens employed were sufficiently small for the uncracked ligaments to suffer plastic yielding during the fracture tests. The crack bridging response was quantified in terms of the normal traction versus plastic displacement curve; the area under this curve for a deep double edge-notched specimen is approximately equal to the measured steady state toughness. The accuracy of an existing micromechanical model for the fracture toughness of brittle open cell foams is assessed, and a new toughness model for ductile foams is derived.