Calculation of contact pressure in a tube-tubesheet joint according to Yokel

[KKaammiieell]

Hi all,

I found a good formula to calculate the residual interface pressure after tube expanding.
I found this formula in this document: (I also uploaded the document as an appendix)

https://www.researchgate.net/public...ce_effect_on_tube-to-tubesheet_joint_strength

The formula is:
Pcon=Pe[1-(ri/ro)^2 ]-(2/3^0,5 )*σy*[ln⁡(ro/ri) ]
Pe=σy*(1,945-1,384*(ri/ro))

With this formula you only need to know the yield strength of the pipe material, and the inner/outer radius of the pipe to determine the resiudal interface pressure. In the example in the paper it gives an answer of 30,6 Mpa. But if I calculate it by myself I get only 14,5 Mpa. My calculation is as follows:
These are numbers given in the paper:
σy =248 Mpa
Ro=9,51 mm
Ri=7,27 mm

Pe=σy*(1,945-1,384*(ri/ro))=248*(1,945-1,384*(7,27/9,51)=220 MPa

Pcon=Pe[1-(ri/ro)^2 ]-(2/3^0,5 )*σy*[ln⁡(ro/ri) ]=220[1-(7,27/9,51)^2 ]-(2/3^0,5 )*248*[ln⁡(9,51/7,27) ]=91,43-76,9=14,5 MPa

Does anyone have the same problem? am I overseeing something? and does anyone use other formulas/references to determine the residual interface pressure?

You would help me a lot,

Kamiel