Abstract: The behavior of part-through and through short cmcks in single edge blunt notched specimens of a medium carbon steel was investigated by a replication method. It is found that the fatigue failure of these notched specitnensis caused mainly by the growth of short surface cracks originating from the sudece of notch root. More than 70% of the fatigue lofe is spent in the regimes in which short surface cracks initiate and propagate as part-through cracks before joining up to form a single through-thickness cmck. The effect of original crack profile which is forsned through the coalescence of multiple part-through cracks is the main reason causing the "anomalous" propagation behavior of the through-thickness cruck in its early stage.

Key words: medium carbon steel, fatigue, short fatigue crack, crack initiation, crack propagation