Analytical and finite element study of residual stresses in the transition zone of hydraulically expanded tube-to-tubesheet joints

Abstract

The process of tube expansion has been the subject of much research throughout the years. The first study conducted by Oppenheimer (1927) was dedicated to the mechanical rolling technique of expanding the tube. In fact, initial investigations were mainly concentrated on the manufacturing process of tube expansion, and no attention was paid to tube failure due to the other parameters arising in this process. In 1966, Toba A. alerted researchers to the fact that the highest residual stresses at the tube transition zone are the origin of tube failure, because they cause stress corrosion cracking. Later, in 1976, Krips and Podhorsky modeled the new hydraulic expansion method to expand the tube, which has several advantages in comparison with mechanical rolling. These improvements are the accurate determination of expansion pressure and the longitudinal uniform expansion. This new method reduces the level of tensile residual stresses at the transition zone which, under a corrosive environment, are the main contributors to crack propagation and, ultimately, tube degradation. Therefore, the analytical and finite element analysis of this zone is the subject of this study. The evaluation of the residual stresses, taking into account as many factors as possible in tube expansion, is the objective of the study. It is worthy to note that influence factors such as tube strain hardening and reverse yielding in the transition zone are not part of this work. The study begins with the development of an analytical model in which the Von Mises yield condition for a rigid-plastic circular cylindrical shell subjected to axially symmetric loading is considered. The expansion pressure level is limited in order to avoid the tubesheet plastic deformation, although its effect is not significant on the transition zone stresses. The material behavior is assumed to be elastic perfectly plastic (EPP). The results disclosed that three different regions in the transition zone of the tube should be considered: full plastic, partial plastic and elastic regions. The elastic beam foundation theory and the plasticity theory are used to determine the residual stresses in these regions. The validation of the analytical model is conducted by comparing the results obtained with those of the finite element analysis using ANSYS Workbench 16.2. The results show a good agreement between the two models. Nonetheless, some limitations should be considered in order to obtain reliable analytical residual stresses at the transition zone of an expanded tube.

Date	29 Mar 2018
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Hakim A. Bouzid (Supervisor)