Am designing a pressure connection, to the following nominal dimensions outside dia = 30mm inside dia = 20.7 (wall thickness 4.65mm) internal test pressure = 172N/mm^2 working pressure = 120N/mm^2 As it is a thick walled cylinder, using Lames equation I get the following stresses: Stress in axial direction = 76.4 N/mm^2 Stress in circumfrential direction = 324.4 N/mm^2 Stress in radial direction = -171.6 N/mm^2 (@r=internal radius) I need to find out what material to specify (thinking of using St. St. 1.4005 [416 s21] approx proof stress = 280 N/mm^2). There are no other variables involeved (e.g. temperature/vibration/cyclic loading conditions). I think I should be using Mohrs circle or Tresca criterion, but have never used these before, and only seen examples for thin-walled theory. Can anyone please point me in the right direction Cheers Dave

Hi Dave, To determine what theory you should apply, you need to see what are the limitations and boundaries of each theory, when and how they should be applied. I, particularly, know more about mohr's cycle. This part is a very tricky part. The selection of material to be used has to do also of what type of product you are going to use, such as whether it's corrosive, very hot or cold product, etc.

The part will be used to contain hydraulic mineral oil under pressure at temperature between 10-30C. There is no shock load (unless a part fails and pressure then goes to zero), once at pressure it will remain constant. The part will be mounted in a vertical orientation, with no transverse loads applied to it. Cheers Dave

For calculations of thick cylinder generally different methods are used. Maximum Principal Stress Theory – Lame’s Equation – For homogeneous, isotropic material which obeys hooks law – cylinder open at both ends – You can have maximum stress in cylinder equal to maximum permissible stress in material. Maximum Strain Energy Theory of Failure – Clavarino’s and Birnie’s Equations – Used for Low carbon steel, brass, bronze, aluminium alloys etc. Ductile Material. From whatever given it appears that you should choose different material with higher permissible stress.

Thanks for the reply/information. I have never come across Clavarinoâ€™s and Birnieâ€™s Equations but the full assembly also has aluminium and bronze components in it. Have done a quick search on the web, but most information is regarding gun barrels - anybody know of some simple examples of these equations, which I may be best starting out looking at. Cheers Dave

use simulation I think you should has simulation at your pressure vessel... than after you got the result. You could choose what kind of material suittable for 172 mpa... I mean for internal pressure...