An attempt was made to correlate bending and direct stress fatigue strengths by using the cyclic stress-strain relation of the material.
The cyclic stress-strain relation was shown to be representable with the ultimate tensile strength: σB as follows
By using this relation, the nominal stress σn in bending was calculated and related with the true stress σ at the specimen surface. Then, by replacing the nominal stress by the surface stress, S-N curves obtained from the bending and direct stress fatigue tests on S15 CK steel were compared each other and shown to be in good agreement. Therefore, the bending fatigue strength was found to be correlatable with the reversed direct stress fatigue strength through σn-σ diagrams.