There are forumulas for stresses in bellows:

Meridional Membrane Stress due to Deflection
SMMDU = ( EAMB1*T*T*E )/(2.0*W^3*Cf )
Meridional Bending Stress due to Deflection
SMBDU = 5.0*EAMB1*T*E/(3.0*W*W*CD)

They are taken from PVELITE manual...but can U give me any
origin, source of this formulas
HOw they had been written and why?

THKS