A Study on Rotating Bending Fatigue Property
　　　　　　　　for Giga-Cycle of Surface Transaction Material
　　　　　　　　　and Different Strength Level Steel Material



　　This laboratory's theme is "Giga-Cycle fatigue". It means that "Giga = 10⁹" and "Cycle = a number of related events that happen again and again in the same order". In short, we research about metal fatigue property in adding the cyclic loading which exceeds 1 billion times.

　　We had cut off the fatigue testing when the number of cycles got 10⁷, because it had been considered that steel is not failed even if repeated loading is added degree of the what, if it is less than fatigue limit. Indeed this concept applies to many steel materials, but it is not necessarily true by being developed the high strength steel which was not existed before. When one executes the fatigue testing of high strength steel, it is clarified that two step bending phenomenon of S-N curve, which S-N curve goes down as from 10⁷ after it gives the horizon, arises. We consider that "two step bending phenomenon of S-N curve" is "double S-N curve", in short, "the surface origin type failure", the fracture is generated from the test piece surface, and "the interior origin type failure", the fracture is generated from the test piece inside, appear in the different place. The existence of "Double S-N property" means the hazard of machinery which is designed under the premise that it is not failed eternally.


To insure the security of the steel materials, we need to solve the following items.

1, Is this "double S-N property" occurred from the steel material with the strength of which degree?
2, What is the cause of producing " double S-N property"?
3, Does it really not occur "double S-N property" in the low strength steel?


　　In this laboratory, in order to clarify these subjects experimentally, we execute the rotating bending fatigue testing about several strength steel materials over giga-cycle field, analysis the data, observe the fracture surface by SEM, inspect the test piece which was failed by the interior origin type failure and study about the internal fracture generation mechanism.