Unconventional Defect‐Shape Dependence of Fatigue Limit in Martensitic Steel

The influence of defect shape on the fatigue limit of tempered martensitic steel was investigated using specimens containing either a drilled hole or an electrical discharge–machined (EDM) notch. Fatigue tests confirmed an unconventional defect‐shape dependence. At higher stress amplitudes, the EDM‐notched specimens exhibited lower fatigue strength, whereas at lower stress amplitudes, the difference between the two defect types diminished, and the drilled‐hole specimens ultimately showed a lower fatigue limit. Detailed observations near the defects showed discontinuous crack propagation involving temporary crack arrest and coalescence of sub‐cracks, which is characteristic of a damage accumulation mode. Fractographic and crystallographic analyses indicated that the damage accumulation may be distributed more widely around the drilled hole than the EDM notch. These findings suggest that the unconventional defect‐shape dependence may emerge at the fatigue limit due to a transition in crack propagation mode from conventional continuous propagation to a discontinuous mode governed by damage accumulation.