Fracture behaviour and fracture toughness of ductile closed-cell metallic foams

Standard fracture mechanics tests were carried out on two different types of aluminium foam, ALPORAS® foams and ALULIGHT® foams, with a variety of densities. Standard fracture toughness tests on compact tension (CT) specimens with widths from 50 mm to 300 mm and in situ tests in the scanning electron microscope were performed. Fracture toughness values in terms of the critical stress intensity factor, KIC, the critical J-integral, JIC, and the critical crack-tip opening displacement, COD5,i, were determined. To identify the fracture process, local deformation measurements were performed on the foam surfaces with a digital image processing system.From the deformation measurements it is evident that the deformation is strongly localised on different length scales. A relatively large fracture process zone, 6–8 cells in height, is developed, where only few of them are heavily deformed. On the cell wall level the deformation is again strongly localised to the thinnest parts of the cell wall, where cracks initiate and propagate. The crack propagates through the foam, building many secondary cracks and crack bridges. The comparison of K vs. Δa (crack extension), J vs. Δa and COD vs. Δa with the current fracture processes at the crack tip and the load–displacement response reveals that COD gives the most reliable measured values to characterise the fracture toughness.