The rotating bending and the plane bending fatigue tests were carried out on specimens having a small blind hole (diameter: 0.3mm) and smooth specimens of an annealed 0.42%C steel. The difference in fatigue strength between these two types of loading was investigated by the successive observations of crack initiation and crack propagation processes. The results obtained are summarized as follows: (1) The number of cycles to failure in the plane bending test was larger than that in the rotaing

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... nding test under the same stress. (2) There was little difference in fatigue limit and in length of non-propagating cracks between two types of loading. (3) The crack growth rate in the rotating bending test was faster than that in the plane bending test under the same stress and crack length. On the other hand, the crack initiation lives were roughly equal each other. Therefore, the difference in fatigue strength between two types of loading results from the difference of their crack propagation processes.