The effect of cross bore geometry on the strength of pressure vessels

Abstract

The stress distribution and fatigue behaviour of a thick walled closed erided pressure vessel containing a transverse hole or cross bore with various blending geometries at its intersection with the main cylinder bore has been investigated using a two and three dimensional finite element approach supplemented by experimental observations. Initially, four simple models were investigated to consider their potential for approximately simulating the stress on the highly stressed region of the cross bore and a curved beam model specimen was found to most closely simulate the stress distribution found in a thick walled cylinder. This curved beam model was subsequently used in finite element investigations and in fatigue tests to evaluate the relative differences in stress and fatigue life when a blending radius or chamfer is introduced at the cross bore - main bore intersection of a cylinder. The static finite element results showed that no blending geometry produced a low local stress field although in fatigue tests curved beam models with a chamfered blending geometry were found to have ,a marginally improved fatigue life. The longitudinal stress existing in a pressure vessel was not taken into account in the curved beam model. It became possible later in this work to analyse, using finite elements, actual cross bore cylinders containing various cross bore blending geometries. The results for cylinders having outer to inner diameters ratios of 2, 1.8 and 1.4 are presented in this thesis. Fatigue tests were also carried out on similar specimens made from 826M40 steel using a specially designed fatigue testing facility. The experimental results are presented and show the plain cross bore to have marginally superior fatigue behaviour for a cylinder thickness ratio of 2. For a thickness ratio of 1.4, fatigue life curves were found to be very similar. From the results of this work, it has been found that a power form relationship exists between fatigue life and the principal strain range, the shear strain range and the applied maximum internal pressure