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The dependence of ĢKth on crack size and material properties under stress ratio R=-1 was studied on various materials and microstructures. The values of ĢKth for all the materials investigated were standardized with one geometrical and one material parameter.

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In order to elucidate the effect of small defects on the torsional fatigue strength of steels, reversed torsion tests were carried out on the 0.46 percent C steel specimens containing a small hole. The hole diameter ranges from 40 to 500μm. The fatigue behaviors on the specimen surface and near the hole were observed with the aid of a microscope. The torsional fatigue strength of steels containing small holes can be predicted from the results of rotating bending fatigue test by considering the threshold condition for crack propagation.

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