In order to clarify effects of stress ratio (R= sigma_(min)/sigma_(max)) on the interior-originating fatigue properties, uniaxial fatigue tests of high strength steel (SNCM439) were carried out under three different stress ratios, R=0.3, 0.1 and -1. The results are as follows: (1) The number of cycles to occurrence of interior-originating fractures depended on stress ratio, and decreased with decreasing stress ratio. (2) It is inappropriate and risky to apply modified Goodman diagram to high strength steel which main fracture mode was the interior-originating fatigue fracture. (3) Effects of Stress ratio on the fracture surface of the interior-originating fatigue fracture were stronger than that of the surface-originating fatigue fracture.
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