In order to clarify the effect of defect orientation on the fatigue limit of two types of steels, JIS‐S15C and JIS‐S45C, a small semi‐circular slit was introduced onto the surface of a round specimen. The slit was tilted at 0 °, 30 ° or 60 ° with respect to the plane normal to the loading axis, but all of them had the same defect size, area = 188 µm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. In all the combinations of the materials and tilt angles, a non‐propagating crack was found at or just below the fatigue limit, that is, the fatigue limit was determined by the non‐propagation condition of a crack initiated from the defect. In both steels, the fatigue limit was found to be nearly independent of the tilt angle for the same value of projected size area, which was in good agreement with the prediction by the area parameter model. In this paper, a mechanistic explanation for the insensitivity of the fatigue limit to the tilt angle is proposed.

Structural steels contain various material irregularities and natural defects which cause local stress concentrations from which fatigue cracks tend to initiate. Two defects in close proximity to each other may affect local stress distributions, and thus, begin to interact. In this paper, the effect of interacting small cracks on the fatigue limit is systematically investigated in a medium carbon steel. The growth of interacting cracks, as well as the characteristics of non‐propagating cracks and microstructural aspects, was closely examined via the plastic replica method. It was found that although the fatigue limit is essentially controlled by the mechanics of interacting cracks, based on their configuration, the local microstructure comprised ferrite and pearlite has a statistical scatter effect on the behaviour of interacting cracks and non‐propagating thresholds. With respect to the fatigue limit, when two defects were in close proximity, they behaved as a larger single defect. However, with greater spacing between defects, rather than mechanical factors, it is the local microstructure which determines the location and characteristics of non‐propagating cracks.

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