TourTour Fatigue strength of a SHAFT in BENDING
 Shaft diameter dk 10-3 m
 Tensile strength Rp0.2 MPa
 Dynamic to static tensile strength ratio σe/Rp0.2
 Stress concentration factor Kt or Kf
 
 Section modulus for bending S=I/(dk/2)=(π/32)dk3 10-6 m3
 Maximum bending moment statically loaded Mb=SRp0.2 Nm
 Maximum bending moment cyclically loaded Mb=Sσe/Kt Nm
 
Shaft diameter dk: Local diameter at the critical area (grooved section).

Endurance strength σe: The ratio σe/Rp0.2=CfCe/Cm where Cf represent the influence factors listed in eq(3.1),  Ce is the ratio of the endurance limit with the ultimate tensile strength as listed in Table 3.1 and Cm is the ratio of the elastic limit and the ultimate tensile strength.

Stress concentration factors: For high strength steel grades and brittle materials the notch sensitivity in not far from unity so Kt=Kf. For ductile materials the lower value of the fatigue stress concentration factor Kf can be applied. It is always save to use the geometrical stress concentration factor Kt which based on (numerical) calculation. Examples of how to limit the stress concentration factor see fig 3.7 and 3.8.

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