Toughness of aluminium alloy foams

The fracture behaviour of closed cell aluminium-based foams (trade-name “Alulight”) 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 testing procedures are critically analysed, and the origins of the observed R-curve behaviour for metal foams are explored. A major contribution to the observed increasing crack growth resistance with crack advance is in the development of a crack bridging zone behind the crack tip. The crack bridging response is quantified in terms of a crack traction vs extra displacement curve by performing independent tests on deep notch specimens. The area under the bridging traction vs extra displacement curve from the deep notch tests is approximately equal to the measured initiation toughness J_IC, in support of the crack bridging concept. A line spring model is then used to interpret the fracture response. The effect of material composition and relative density upon the initiation toughness is measured, and the accuracy of an existing micromechanical model for the fracture toughness of a brittle foam 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 Alulight foams are notch-insensitive, with the net section strength equal to the unnotched strength.