Deformation and fracture of aluminium foams

The tensile and compressive properties and the fracture resistance of two a luminium alloy foams have been measured. The yield strength, unloading modu lus and toughness increase with relative density in such a manner that the closed cell foams of this study behave as open cell foams. These relationsh ips can be described adequately by power law fits. Experimental results, wh en compared with theoretical models based on idealised foam structures, rev eal unexpected discrepancies. We conclude that they are caused by morpholog ical 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 obse rved to be the development of crack bridging ligaments behind the crack tip . The compact tension specimens employed were sufficiently small for the un cracked ligaments to suffer plastic yielding during the fracture tests. The crack bridging response was quantified in terms of the normal traction ver sus 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 fractu re toughness of brittle open cell foams is assessed, and a new toughness mo del for ductile foams is derived. (C) 2000 Elsevier Science S.A. All rights reserved.