Surface Crack Growth Characteristics on Two Kinds of Specimen Diameters in Rotating Bending Fatigue

Surface crack growth characteristics and influence of the specimen diameter in rotating bending fatigue tests were investigated for quenched and tempered SCM 435 and annealed S45C steels. The simple relation (a/D)=0.86 (2b/πD)0.84 (a: crack depth, b: half crack length at the surface, D: specimen diameter) was experimentally affirmed independently of the stress level, the kind of steels and the specimen diameter over a wide range of crack length. The surface crack shapes in the specimens of 4 mm and 12 mm diameter, which was approximated to parts of ellipses, were similar to each other if the crack size ratio was equal to the diameter ratio. The crack growth rates in the direction of depth and along the surface were both proportional to (ΔK/2)m: the proportional constants C depended on the diameter for SCM 435, and on the stress level for S45C, consequently the relations db/dN∝(Δσ/2)nbp and da/dN∝(Δσ/2)nap held for S 45 C. The influence of diameter on the surface crack growth rate was discussed based on these two relations.