What wall thickness caesar use for SIF calculation if I don't enter "Fitting thk" (left the text box blank)?
What is matching pipe? Does it means that caesar take wall thickness of the pipe (bend WT = pipe WT)?

If you leave the "Fitting Thickness" blank, the SIF computation uses the thickness off of the Spreadsheet input field (right below "Diameter"). If you do specify a "Fitting Thickness", this value is used in the computation of the SIF and Flexibility factor.

Stresses are always computed using the section modulus of the matching pipe. The reference to "matching pipe" means the thickness off of the Spreadsheet input field (right below "Diameter").

Thank you Richard for your answer!

I don't understand why Caesar use section module of matching pipe instead of section module of the bend...
I think SIF is:
SIF = 0.5*(peak stress in bend)/(nominal stress in bend)
and not:
SIF = 0.5*(peak stress in bend)/(nominal stress in matching pipe)

The article "Fatigue tests of piping components" by A.R.C. Markl said:
"the section modulus Z used in equation [2] is that of pipe of the size and schedule or weight with which the fitting is intended to be used, wich is not necessary that of the fitting itself. the difference may be considerable as in the case of forged tees where excess metal is distributed variously to provide reinforcement for the branch opening."

This phrase is impossible to understand in respect to bend "section modulus Z used in equation [2] is that of pipe of the size and schedule or weight with which the fitting is intended to be used". We could use bends with various wall thickness. It's not the same as the pipe. And It could be different at various manufacturers!
The SIF for bends i=0.9/h^(2/3) is not obtained from experiment, It obtained from theory (Clark&Reisner, Hovard, Wigness, Kostovetsky). Markl's experiments just confirmed the theory. So I think It's better to use section module Z for bend, not Z for matching pipe...
But there's no theoretical data for tees, so we have no other choose except to use Z for matching pipe...
In Russian codes we use Z for bend itself, not for matching pipe

CAESAR II uses the "fitting thickness" for the SIF computation. The matching pipe thickness is used to determine the section modulus for the stress computation (M/Z). That's what the Codes say.

If you don't want to do it that way, then set the bend up as it's own element (as opposed to a straight run with a bend on the end) and change the thickness.

Thank you Richard