Development of Rolled Joint Numerical Models

Abstract

Six different finite element models have been developed to analyse rolled joints between pressure tubes and end fittings. A brief summary of each is presented here. Five of them axe axisymmemc and one is a plane strain model. Each model simulates one or more aspects of rolled joint behaviour. Some of them also include the effects of operating conditions (elevated temperature and pressure). It is hoped that by combining the models/results, a more complete description of the behaviour of rolled joints can be obtained. The axisymmetric models are presented here in the order in which they were developed. The first is the interference fit model, where contact pressure is simulated by shrink fitting an initially larger tube into a smaller end fitting. The pullout process is also analysed in this model. Two forced displacement models are presented, one for the expanded zone and one for the transition zone, in which tube wall thinning is simulated by specifying radial displacements for the insi& and outside surfaces of the tube. In the axial rolling model, rolling is simulated by imposing sequential radial displacements along the length of the tube and forcing the tube material into the grooves of the end fitting, which is considered rigid. Finally, a boundary condition model is presented, which uses shell elements and imposed boundary conditions to calculate stresses in the transition zone only. The circumferential rolling model is a plane strain model which simulates the rolling of a tube into a grooveless end fitting. Radial displacements are imposed on the tube, applying them sequentially in the circumferential direction at five discrete locations. This simulates the local loading and unloading of the tube material that occurs at the rollers. Results are also presented from a series of tests to validate the interference fit model. The tests involve shrink fitting a pressure tube into an end fitting, applying additional external pressure on the outside to increase the contact pressure between the two pieces, and then performing a pullout test to separate the tube from the end fitting. These tests provide important information regarding the state of friction between the tube and end fitting. In addition, simple friction tests involving small samples of pressure tube and end fitting material are also being carried out. The results from both series of tests indicate that the coefficient of friction between the tube and end fitting is relatively large (could be as high as 0.65 under some conditions) and is influenced by the magnitude of the contact pressure and the condition of the tube surface (presence of oxide, surface roughness, and hardness).