Malik,

At a particulat point, in hydrotest , the stresses in three directions are :

Longitudinal pressure stress, circumferential or hoop pressure stress, and radial pressure stress which is -p on the inside and 0 on the outside. Now if we use these pressure stresses for the von mises criteria, they will be invariant at the locations you have mentioned.

But what about the longitudinal bending streses? They will not be the same at the four points you have mentioned.Also what about the shear stress, which is ( from simple theory of strength of materials) max at neutral axis and zero at free surface?Also there will be circumferential component of bending stress also which will also change with location.

So keeping in view all these the von mises stress should vary at the four points.

Best regards