How the fatigue curve are defined?

Nicolò Lodini_21627

Hi, I don't understand the SN and EN curve.

SN case

if i put a Yield strenght of 290 and i click estimate the curve from UTS, at 10^3 cycles the curve shows me a value of 800MPA, How it is possible that has a value greater than the static value?

 

Thanks

Gildas_GUILLY

Hello Nicolò,
Unless specified otherwise (there is a parameter for that), curves for fatigue material are defined in stress range, not stress amplitude.
And Stress range = 2 x stress amplitude.
On your picture we see a stress range of about 520MPa (the 800MPa is the extrapolation at 1 cycle).

Gildas

Nicolò Lodini_21627

thanks, i didn't notice that and i was going crazy, and what about the EN curve?

I'm not understanding why the stress range has a negative exponent.

Thanks for helping.

Gildas_GUILLY

Nicolò Lodini_21627 said:

thanks, i didn't notice that and i was going crazy, and what about the EN curve?

I'm not understanding why the stress range has a negative exponent.

Thanks for helping.

I do not understand your question : What "stress range" are you talking about for EN ?
Gildas

Nicolò Lodini_21627

I'm not understanding why the stress range is about 0.1 or 0.01, I suppose that it has to be a value like the ultimate strenght value (290).

Thanks.

Gildas_GUILLY

Nicolò Lodini_21627 said:

I'm not understanding why the stress range is about 0.1 or 0.01, I suppose that it has to be a value like the ultimate strenght value (290).

Thanks.

In EN, the law is defined in strain, not stress (as you can see on the curve title)
Gildas