On the mechanism of fatigue failure in the superlong life regime (<i>N</i>&gt;10⁷ cycles). Part II: influence of hydrogen trapped by inclusions

High cycle fatigue fracture surfaces of specimens in which failure was initiated at a subsurface inclusion were investigated by atomic force microscopy and by scanning electron microscopy. The surface roughness R a increased with radial distance from the fracture origin (inclusion) under constant amplitude tension–compression fatigue, and the approximate relationship: R a ≅ C Δ K 2 I holds. At the border of a fish‐eye there is a stretched zone. Dimple patterns and intergranular fracture morphologies are present outside the border of the fish‐eye. The height of the stretch zone is approximately a constant value around the periphery of the fish‐eye. If we assume that a fatigue crack grows cycle‐by‐cycle from the edge of the optically dark area (ODA) outside the inclusion at the fracture origin to the border of the fish‐eye, we can correlate the crack growth rate d a/ d N , stress intensity factor range Δ K I and R a for SCM435 steel by the equation and by d a/ d N proportional to the parameter R a . Integrating the crack growth rate equation, the crack propagation period N p2 consumed from the edge of the ODA to the border of the fish‐eye can be estimated for the specimens which failed at N f &gt; 10 7 . Values of N p2 were estimated to be ∼1.0 × 10 6 for the specimens which failed at N f ≅ 5 × 10 8 . It follows that the fatigue life in the regime of N f &gt;10 7 is mostly spent in crack initiation and discrete crack growth inside the ODA.