Notable size effects on very high cycle fatigue properties of high-strength steel

► Notable size effect was found in fatigue of steel exhibiting internal fracture. ► The size effect on internal fracture was much larger than that on surface fracture. ► Oxide-type inclusions were what caused the large-size effect. ► This result urged the use of large specimens in fatigue tests on high-strength steel. ► Evaluation of very high cycle fatigue considering the inclusion size were discussed. Very high cycle fatigue (VHCF) properties were compared between two types of specimens: enlarged specimens and our standard specimens. Fatigue tests were conducted by ultrasonic fatigue testing; the material used was commercial spring steel. All tests ended in internal fracture, with large-size effects observed, i.e., the enlarged specimens showed lower VHCF strength than the standard specimens. Most of the internal fracture origins were oxide-type inclusions that were larger in the enlarged specimens than in the standard specimens, indicating the size effect to be caused by the difference in oxide-type inclusion sizes at the origins of internal fractures. The large-size effect strongly urges the use of large specimens when conducting VHCF tests on high-strength steel. Moreover, the large-size effect implies that fatigue strength cannot in this case be determined using the conventional S– N curve approach, since the S– N curve depends on the specimen size. The evaluation of the VHCF strength thus needs two steps: an estimation of the maximal inclusion size, followed by an estimation of the VHCF strength based on the maximal inclusion size.