1991 Volume 40 Issue 458 Pages 1447-1452
High-cycle fatigue behavior of JIS SUP spring steels was discussed from a point of view of effects of non-metallic inclusions on fatigue strength and on behavior of fatigue crack initiation and propagation. Hardness of the materials ranged from HV 420 to HV 520. Fatigue tests were carried out in air environment up to 108 cycles under conditions of rotating bending and reversed torsion.
The results obtained are as follows:
(1) Relationship between hardness (HV) and rotating bending fatigue strength (σwb: MPa) for steels where fatigue cracks initiated from the specimen surface independently of inclusions is described by σwb=1.71HV. This relationship is considered as that for inclusion-free high strength steels.
(2) Stage I cracks were observed to start in multiple directions from a fatigue crack initiation site near an inculsion under a rotating bending condition. The size of the State I crack is larger than that of the inclusion itself.
(3) Under reversed torsion, fatigue fracture occurred from Stage I cracks initiated at a parallel plane to the specimen axis, where no inclusion could be observed.
(4) For both types of fatigue tests, Stage I cracks initiated under a shear stress larger than τa =0.85HV, which is a half of the rotating bending fatigue strength of inclusion free steels. As it was not confirmed whether non-propagating cracks existed on the specimen surface endured 108 cycles or not, endurance limit for crack initiation was not determined.
(5) The value of stress intensity factor at a tip of Stage I crack is an important factor or Stage I crack to start propagating as Stage II crack.
