Fatigue properties of Al-1 Mg-0.6 Si foam at low and ultrasonic frequencies

The fatigue properties of Al-1wt%Mg-0.6wt%Si aluminium foam were investigated at 1-10 Hz with a servohydraulic fatigue testing machine and at 20 kHz using ultrasonic equipment. Foamed rods were cycled at constant amplitude under fully reversed loading conditions. Within the ranges of scatter, lifetimes measured at both frequencies are similar. The foam shows an endurance limit, and specimens either fail below approximately 10 ⁷ cycles or they survive for 10 ⁹ cycles or more. A probability of 50% for fracture (mean endurance limit) was found at Δε/2=0.32×10 ^-3 (1.31 MPa). Fatigue cracks initiate in the interior closed cell structure at holes or pre-existing cracks in the cell walls or in areas where the cell walls are thin, and the surface layer cracks in the following. At the endurance limit, fatigue cracks may initiate in cell walls, however, they are trapped at nodes of cells.