Fatigue properties of aluminium foams at high numbers of cycles

Endurance fatigue experiments have been performed with two kinds of Al-Mg-Si foams and one Al-Si foam under fully reversed loading conditions using the ultrasound fatigue testing method. Young's modulus of the foams is 3.9 GPa. Constant amplitude fatigue data show endurance limits on the basis of 10 ⁹ cycles between 1.1 and 1.4 MPa which is 16-23% of the plateau stress. Lifetimes exhibit a pronounced scatter, which is caused by the inhomogeneous structure of the foams. Fatigue damage is governed by the formation of cracks, which preferentially initiate in the interior sections of cell walls at initial defects, like precracks or holes. No strain localization and formation of deformation bands was found. Fatigue crack growth preferentially follows areas of cell walls with a minimum wall thickness, and eventually may stop near cell-nodes. The cyclic properties of foams can be improved, if initial defects are small, if the mean cell sizes are reduced, and if a more homogeneous foam is obtained. (C) 2000 Elsevier Science S.A. All rights reserved.