S-N Curve - not even the manufacturer knew this one!

alexisonsmith

Hey,

So I am currently trying to workout the fatigue characteristics of my Formula Student Space frame design, I am using Cold Drawn Seamless mild steel.

I have the following mechanical properties:
Re - 360 MPa
Rm - 450 MPa

I think these are the effective stress intensity range and the max or mean I am not sure stress.

How would I go about creating an S-N curve from this data? If you know you will be a lifesaver!

Thanks,

Alexisonsmith

minger

In most literature that I've seen, R represents the cyclic load kind (e.g. -1 is fully reversed, etc). Perhaps one is the mean stress and one is the maximum?

Also, the is important, as the loading will have an effect on fatigue life. Exactly what type of steel is it? You should be able to directly find fatigue curves.

alexisonsmith

Cold Drawn Seamless Steel - CFS BKRLS BS6323 Part 4 CFS 3-Bk

I tried to find the S-N Curve by I can't find it at all!

minger

That's not really an exact kind of steel, just a type and what looks to be a part number.

alexisonsmith

It is a mild steel with the following characteristics:
0.2% - C
0.35% - Si
0.9% - Mn
0.05% - P
0.05% - S

Studiot

Don't know about this steel, but many iron / steels, especially the ductile ones, do not have a fatigue limit.

alexisonsmith

I thought was aluminium I thought Steel's did have a fatigue limit?

xxChrisxx

Pretty much all steels I have ever worked with have a fatigue limit. As a rule of thumb steels have a fatigue strength of 0.4UTS, it's very rough and doesn't hold up for all steels so you've got to be careful.

If the manufacturer doesn't know then, they fail. It's more likely they just aren't letting you know. Just badger them to give you a value for fatigue strength. To this you need to apply some 'fiddle factors' for real life fatigue strength.

And why on earth would a ductile steel have no fatigue limit? They cope with crack propgation better than hard/brittle steels. Unles you mean carburized steels, that have no fatigue lmiit.

Studiot

Oh dear, I didn't say that very well did I, managed to get it well and truly _-_ backwards.

I meant to say that most steels are employed at well below their endurance limit so the fatigue issue never arises, especially if you are designing to codes. Don't forget that the S-N fatigue only works if you are considering whole cycles of stress. Otherwise you need a Soderberg or similar diagram analysis.

http://www.engrasp.com/doc/etb/mod/f...life_help.html

R_m is the ultimate strength

R_e is the yield stength of

http://www.corusgroup.com/file_sourc...EN10025-04.pdf

xxChrisxx

I do that all the time , fortunately this place is filled with picky buggers that will weed out even the tinyest wording error.

EDIT: It's generally why you'll never find one of my posts that hasn't been edited. I always think of a better way to put stuff afterwards.