Question about the safety factor for a shaft diameter

[Arnsol]

Homework Statement

w

Relevant Equations

See attepmt

Hello,

So we are designing a turbine. We have a shaft that receives 710W / 13.6NM / 500RPM from the runner and we are going to find a diameter with safety factor of 3.
Take the following information regarding metal:
Aluminium 6061

Tensile strength	115 MPa
Yield strength	48 MPa
Shear strength	83 MPa
Fatigue strength	62 MPa

i know F.O.S = Max load / Load applied.

Still i can't wrap my mind around which of those i should use. First thought is going with the 48 MPA yield since aluminium is ductile?

Any thoughts? I am so confused...

 

[rudy]

I would agree with you on that. Once you surpass the yield strength you enter the "plastic region" of the stress-strain curve. This means that once the load is released the metal will not return to its original dimensions, and therefore no longer function as designed.

I think what the chart is referring to as "tensile strength" is also called "ultimate strength". This value is way into the plastic region and you would not want the shaft to reach this value.

 

[Arnsol]

Thank you.Yes, its the ultimate strength... But it makes sense to me to chose the yield value.
Regards