LOCAL FRACTURE BEHAVIOUR AND CRACK TIP DEFORMATION IN METAL FOAMS

The renewed interest in metal foams, which has been induced by the transport industry in the last few years, leads to a need of detailed information about mechanical properties of this new material class. The aim of this study is to provide information on fracture toughness and fracture processes of ductile metal foams. Investigations were carried out on ALPORAS and ALULIGHT aluminium foams with different specimen sizes and different relative densities. Standard fracture mechanic tests were performed. The fracture toughness values are in the range of 0,1 to 1,5 MPa.m1/2 for KQ and 0,1 to 1,0 kN/m for J0,2, mainly depending on the relative density. In-situ tests in the scanning electron microscope and in the optical stereomicroscope were performed also to observe the crack growth processes and determine the local deformation in the cell walls in front of the crack tip. The crack follows apparently the weakest path through the foam structure and is accompanied by building of crack bridges and by initiating of secondary cracks in front of the crack tip, so the crack resistance can decrease with increasing crack propagation. The tensile deformation of the foam structure is very localized in few cells and is further concentrated to the weakest part of the cell walls