Fatigue behavior of GCr15 steel in ultra-high life region

For the sake of revealing the ultra-high cycle fatigue behavior, GCr15 bearing steel was used as testing material and rotating bending fatigue test was adopted. The fracture part of the sample was observed by using SEM. Test results show that fatigue crack initiation has two different mechanisms. One is called the surface crack initiation mechanism. It is governed by a crystal slip or nonmetallic inclusion on the specimen surface, which occurs in the region of short fatigue life and high stress amplitude level. The other is called the internal crack initiation mechanism. It is governed by nonmetallic inclusion at subsurface which occurs in the region of long fatigue life and low stress amplitude. Through the analysis and disposal of the experimental results, the S-N curve was depicted. Through the calculation and evaluation of crack initial size parameter, mechanism for internal fracture mode where fatigue crack occurs at specimen subsurface was discussed and fatigue life prediction procedures were indicated based on crack initial size parameter.