Hi Rakesh,
Of course your point is quite valid. The 45 degree elbow is NOT as flexible as the 90 degree fitting. I am sure you also know that flanges on the ends of elbows will also "steal away" some of the flexibility.
It is interesting that the flexibility factors and stress intensification factors are "turned on" at the "entering" weld line and "turned off" at the "leaving" weld line. Actually, the flexibility (and associated stress intensification) is the result of the cross sectional shape of the fitting being forced into a slight ovalization, thereby giving the fitting (pipe) a "strong axis" and a "weak axis". But the ovalization does not stop at the weld line and the flexibility carries on into the matching straight pipe (unless of course there is a flange there). But, in the long run, in most cases it work out and the Code gives you guidance to design a reliable piping system.
I wonder, when engineers develop SIF's by using an FEA model, do they carry the straight pipe out beyond the "weld lines".....about 4 or 5 diameters past the weld line would be good.
Regards, John.
_________________________
John Breen