Von-mises stresses in Hydrotest

Hi,
First time useing this forum.I have one confusion. Analytically,Von-mises stresses should be same along the circumference of a pressure vessel while under constant internal pressure but during hydrotest I observered a difference in values of around 150 to 200 MPa.Pl. note that stain gauges were on 4 points at 0,90,180,270 degrees on same line of circumference.
Hope experts will comment.If possible refer any informative site regarding Pressure vessel Hydrotest.
cheers,

Experimental stress analysis is outside the scope of the forum. However shear strain energy (Von Mises), although generally considered to be the more accurate method of assigning stresses in a three dimensional distribution, is not used in practical problems. Our software uses the less onerous Tresca method. (ASME Section VIII, Division 2). The simple maximum shear stress is considered accurate enough for just about all practical applications. For ASME Section VIII, Division 1, we only consider direct stress. The method has been used for many years with success.

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