High‐cycle rotating bending fatigue property in very long‐life regime of high‐strength steels

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In order to investigate the fatigue properties of high‐strength steels in the very long‐life regime up to over 109 cycles, cantilever‐type rotating bending fatigue tests were carried out for two kinds of high‐strength steels, SUJ2 and SNCM439, which were machined by grinding and finished by electropolishing after grinding. And also, the residual stress on the specimen surface of the ground specimen was examined by X‐ray diffractometer in order to investigate effects of the residual stress on the fatigue properties. From the investigations, the S–N curves clearly have a tendency to decrease again in the longer‐life range over 107 cycles for both types of specimen and for both steels. From observations of fracture surfaces, it was found that fatigue crack origins could be grouped into two types: (i) ‘surface crack origin type’ in the shorter‐life regime and (ii) ‘internal crack origin type’ in the longer‐life regime.