can any body explain to me 'SIF' in realtion to a say 2" - weldolet/ pipe stub placed over 12" Header pipe ?

Actually I was trying to apply the lessons taught to us in our curriculum texts . There we are briefed about " stress Concentration". The stress concentration is pictured as a slab with a hole in it and force lines getting clustered closer and closer near the hole !! Well that picture was enough to explain the term "concentration".

But now when I apply same on the pipe say first the header Pipe, the same concept provides the "visualisation" (please correct if my line of thought is wrong). And we say that SIF must be inserted in Header Pipe at this point where weldolet is getting attached. I mean any stress ( bending, etc but excluding Torsion) will be in "concentrated' form at that point (hole).


But, the same logic doesnt apply when we say SIF is to be applied in the branch !!

Now , either the way I am trying to VISUALISE SIF is wrong or there is some other additional thing which I am not taking in to account!

Can any body suggest either some good reference ( priority 1) or simply explain ( priority 2 ) ??