Abstract: The micromechanism of fatigue crack propagation of ductile iron is investigated by the action of constant amplitude and tensile cycle load. The effect of graphite morphology on initial crack and propagation behavior of fatigue crack is analysed. The results show that graphite morphology of cast irons had great effects on initial fatigue crack and propagation behavior. Because of the interconnect of graphite flakes in the grey iron's eutectic cells, fatigue crack always propagated along graphite flake-matrix interface, which is perpendicular to outer stress. The graphite flakes constitute the majorities on the fracture surface, so fatigue crack propagation for grey iron was mainly restricted by graphite flakes. Fatigue crack propagation for ductile iron is influenced by matrix structures and graphite morphology, but the initial fatigue crack and the whole propagation process are determined by plastic zone, crack tip passivation zone, and slip band produced on the spherodial graphite-matrix interface. The regular pattern of fatigue crack propagation of ductile iron is as follows: cracks' continuous propagation in the matrixes-rapid propagation-crack tip blunting-bringing out cracks-continuous propagation of the cracks.

Key words: Ductile iron, Fatigue crack, Micromechanism