Large variability in fatigue life is often found for ultra-high strength steels. In this study the fatigue crack initiation and propagation mechanisms were investigated to understand the origins of the different fatigue lives. From quantitative SEM fractography observations, sizes of inclusions, fine granular area (FGA) and fish-eye were measured for internal fatigue crack initiation. Each stage of the micro-propagation of the crack was evaluated with threshold values of stress intensity factor range calculated from Murakami's equations. The objective of this research is to study crack initiation mechanisms in order to understand the competition between surface and internal crack initiation which are leading to different fatigue lifetime. Another aim is to make the link between microstructure, tests conditions, environment, temperature and fatigue lifetime for different crack initiation mechanisms.
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