Toughness of aluminium alloy foams

The fracture behaviour of closed cell aluminium-based foams (trade-name "Al ulight") is characterized for the compositions Al-Mg1-Si0.6 and Al-Mg1-Si10 (wt%), and for a relative density in the range 0.1-0.4. The toughness test ing procedures are critically analysed, and the origins of the observed R-c urve behaviour for metal foams are explored. A major contribution to the ob served increasing crack growth resistance with crack advance is in the deve lopment of a crack bridging zone behind the crack tip. The crack bridging r esponse is quantified in terms of a crack traction vs extra displacement cu rve by performing independent tests on deep notch specimens. The area under the bridging traction vs extra displacement curve from the deep notch test s is approximately equal to the measured initiation toughness J(IC), in sup port of the crack bridging concept. A line spring model is then used to int erpret the fracture response. The effect of material composition and relati ve density upon the initiation toughness is measured, and the accuracy of a n existing micromechanical model for the fracture toughness of a brittle fo am is assessed. Finally, the reduction in tensile and compressive strengths due to the presence of an open hole is determined; it is found that the Al ulight foams are notch-insensitive, with the net section strength equal to the unnotched strength. (C) 1999 Acta Metallurgica Inc. Published by Elsevi er Science Ltd. All rights reserved.