Study on distributions of fatigue crack-nucleating particle sizes and cracks formation in aluminum alloy

The fatigue test for LY12 aluminum alloys specimens at various stresses and stress ratios were performed. The distributions of the crack-nucleating particle sizes and ratios were determined using scanning electron microscopy (SEM) measured from specimens fracture surfaces. The results show that the correlation between particles sizes and ratios versus fatigue lives. And with the increase of stress ratio, fatigue crack originated from deeper subsurface at the same maximal stress. With the increase of maximal stress, fatigue crack originated from shallower subsurface or surface at same stress ratio. It has been also observed that the incubated fatigue micro cracks source localizes at the triple junction adjacent particle boundaries. The calculated results show that microcracks could nucleate and propagate from a smaller particle under a higher stress ratio, which agree reasonably with experimental observation.