fatigue-sizing a shaft subject to torsion

[gio1]

Hello

Which equivalent stress should I use for fatigue calculations on a shaft subject to pure torsion?

Von Mises and Maximum principal differ significantly in this case and give hugely different fatigue lifes.

Thanks

Gio1

 

[MotorVib]

I generial use Von Mises. Its is also what most FEA show. What type of load do you have with pure torsion?

If you want to be on the safe side go with the shorter life.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[rb1957]

this is the 2nd kick at the cat for this topic ?

i'd use principal, but then Petersen bases his Kt on shear stress = T*16/(pi*d^3), refer fig 79, pg 105. but then in this case principal = shear, no?

 

[gio1]

rb1957

Yes this topic has moved to the FEA section, sorry.

Your point is interesting, if Petersen used max principal for Kt then it must apply for fatigue
(Kt is the concentration factor for fatigue, right?)

Thanks

Gio1

 

[gio1]

Sorry, I think the concentration factor in fatigue is Kf...

 

[rb1957]

Kt is general usage (in my mind at least) for stress concentration. some folks make a distinction between stress concentration for static loads and stress concentration for fatigue