Can you use the Goodman relation

[bill nye scienceguy!]

if your stress cycles between some stress xMPa and 0? My fatigue limit is coming out smaller than my stress amplitude and consequently my component is falling outside of the bounded region.

After looking a little closer, obviously the Goodman relation is still valid. So a subsequent question is: can I asume the fatigue (endurance) limit to be roughly half the ultimate tensile strength for steel? I think this may stop me getting the nonsensical answers my diagram is producing.

 

[FredGarvin]

For plain carbon steels, the relation of the endurance = .5(UTS) is a valid rule of thumb. Just remember that, in practice, you usually have to include derating factors since that is derived from lab samples, not real parts. I have also seen the relation that fatigue limit is also approximately 250(Brinnel Hardness) up to a Bh = 400.

 

[bill nye scienceguy!]

thanks, FredGarvin. I hadn't heard about the Brinell relationship. I found a reference that said Goodman used 1/3 UTS to approximate fatigue limit so I used that to be conservative.