Hello
I am new and i have a

I have some tubular components with steel material 20MnB. This component diameter (100 thickness /thickness/3.8mm /lenght 600mm) is transformed (with stamping tools)to a "U" shape.This component basically works under torsional loads applied on each extremity.
I want to characterize this material under fatigue situation. I need the curve e-N, / cyclic curve for material / strain-life curve

But, I do not know followings:

1. How i can extract / obtain a good specimen test from torsion beam, and therefore have a realistic material behavior?
it is difficul to extract  this specimen and others with same geometry, basically because it is formed in a U shape as i said before, an the exterior the surfaces could be not totally regular

2. Do you know any standard / instruction that could define how i have to proced to charaterize this type of tubular components?

3. How many specimens i need to test under fatigue loads, to obtain a real e-N curve

Please, could be one one help me?. I am not an expert