Factors influencing the mechanism of superlong fatigue failure in steels

When the fatigue life Ni of a specimen of 10 mm in thickness is longer than 10(8) cycles, the average fatigue crack growth rate is much less than the lattice spacing (similar to 0.1 Angstrom or 0.01 nm) that is 10(-11) to 10(-12)m/cycle. In the early stage of the fatigue process, the crack growth rate should be much less than the average growth rate, and accordingly we cannot assume that crack growth occurs cycle by cycle. In this paper, possible mechanisms for extremely high cycle fatigue are discussed. Of some possible mechanisms, a special focus was put on a newly found particular fatigue fracture morphology in the vicinity of the fracture origin (non-metallic inclusions) of a heat-treated alloy steel, SCM435, which was tested to N greater than or equal to 108. The particular morphology observed by SEM and AFM was presumed to be influenced by the hydrogen around inclusions. The predictions of the fatigue limit by the root area parameter model are similar to 10% unconservative for a fatigue life of N-f = similar to 10(8), though it successfully predicts the conventional fatigue limit defined for N = 10(7). Thus, the fatigue failure for N greater than or equal to 10(8) is presumed to be caused by a mechanism which induces breaking or releasing of the fatigue crack closure phenomenon in small cracks. In the vicinity of a non-metallic inclusion at the fracture origin, a dark area was always observed inside the fish-eye mark for those specimens with a long fatigue life. Specimens with a short fatigue life of N-f = similar to 10(5) do not have such a dark area in the fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from the usual fatigue fracture surface in a martensite lath structure. Considering the high sensitivity of high-strength steels to a hydrogen environment and the high hydrogen content around inclusions, it may be concluded that the extremely high cycle fatigue failure of high-strength steels from non-metallic inclusions is caused by environmental effects, e.g. hydrogen embrittlement coupled with fatigue.