Fracture toughness and fatigue crack propagation behavior of aluminium foams

Structural efficiency and cost requirements have created enormous interest in cellular metals. Developments in manufacturing processes permit to produce such materials in huge quantities and that can also be integrally bonded to dense face material. From the mechanical point of view the potential applications include energy-absorber and structural components, for example light-weight cores for panels, shells and tubes. For the application as structural material it is essential to know the deformation behaviour under tensile loading, the fatigue data, S-N-curves, endurance limits, the fatigue crack propagation behaviour, the threshold of the stress intensity range and the fracture toughness. No scaling relations exist for closed cell systems of ductile materials between the above mentioned properties and density, cell morphology and the cell wall properties, and defect size. In this part of the joint project we (the Erich-Schmid-Institut, ESI, and the Institut für Leichtbau und Flugzeugbau of the TU Wien ILFB as sub-contractor) will try to find and test such scaling relations for the fatigue crack propagation and the fracture toughness. Four alloys with different density and different rnicrostructures (with varying cell size irregularity of cell distribution, etc.) are used as model foams. As a first step we will adopt the existing fracture mechanics test procedures to this special class of materials. In the main part of the project we will investigate the relevant properties by experimental and theoretical methods. In the experimental studies we will investigate the global fracture mechanics parameters, toughness, fatigue crack propagation behaviour including the threshold of the tress intensity range, as well as the local fracture processes. The methods are in-situ experiments in the scanning electron microscopy, optical stereo microscope and quantitative fractography. The modelling includes analytical (at the ESI) and finite-element analyses at the ILFB. The aim is to establish a correlation between the local processes and the global properties. Finally, the other partners and we will check whether the fracture mechanics parameters (local and global) can describe the tensile and fatigue properties of these materials, or not. First, we will measure these fracture mechanics parameters which are important for the application of these materials. The main purpose of the proposed project is to develop relations between the structure and the fracture mechanical properties of a closed foam. Such fundamental relations are essential for the structural design of metals foams.