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Fatigue strength under alternating torsion stresses
The greatest variable stress component oscillating about the mean stress zero that a specimen can resist an unlimited number of times without fracture or inadmissible deformation (see DIN 50 100).
This value is influenced by shape and surface finish.
The ratio between the fatigue strength of the unnotched and polished specimen and the fatigue strength of the test specimen is referred to as notch factor βk.
βk varies from material to material and decreases in value as the static tensile strength value increases. With RINGFEDER® locking connections, shafts and hubs remain unnotched.
Consequently, the stress conditions at the connection point are virtually identical with those of a smooth shaft, i.e. the material is utilized by almost 100%.

http://www.ringfeder.de/php/index_az.php3?area=0&lang=en

cf. Fatigue strength under alternating torsion stresses
The greatest variable stress component oscillating about the mean stress zero that a specimen can resist an unlimited number of times without fracture or inadmissible deformation (see DIN 50 100).
This value is influenced by shape and surface finish.
The ratio between the fatigue strength of the unnotched and polished specimen and the fatigue strength of the test specimen is referred to as notch factor βk.
βk varies from material to material and decreases in value as the static tensile strength value increases. With RINGFEDER® locking connections, shafts and hubs remain unnotched.
Consequently, the stress conditions at the connection point are virtually identical with those of a smooth shaft, i.e. the material is utilized by almost 100%.

http://www.ringfeder.de/php/index_az.php3?area=0&lang=de