Initiation of fatigue cracks in commercial aluminium alloys and the subsequent propagation of very short cracks

Experiments have been made on two commercial aluminium alloys (BS L65, Al-Cu-Mg-Si-Mn; DTD 5050, Al-Zn-Mg-Cu-Cr) to observe the initiation of fatigue cracks at a plane polished surface and the subsequent growth of very short cracks [0.006 mm (0.00025 in.)-0.5 mm (0.02 in.) deep]. It was found that cracks initiated at surface inclusions, either from the interface between an inclusion and the matrix or from a crack in an inclusion. In both cases the crack ran into the material in directions approximately perpendicular to the applied tensile stress. The growth rates of the short surface were compared, using linear elastic fracture mechanics, with the rates far long [>0.25 mm (0.01 in.)] through-section cracks. The growth rate for the short cracks tended towards that predicted from long cracks for creek depths greater than about 0.127 mm (0.005 in.) but the average crack rate in the early stages of growth was about 1.27 × 10^?6 mm/cycle (5 ×10^?8in./cycle) which is much faster than would be predicted from the long crack data. For cracks about 0.025 mm (0.001 in.) deep the rate varied approximately as the square root of the crack depth. The effect of stress on the proportion of the total life occupied by initiation and by propagation of the crack is discussed.