How the fatigue curve are defined?

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

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.

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

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.

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