The mechanism of failure of tube-to-tubesheet joints is related to the level of stresses produced in the tube expansion and transition zones during the expansion process. Maintaining a lower bound limit of the initial residual contact pressure over the lifetime of the expanded joint is a key solution to a leak free joint. An accurate model that estimates these stresses can be a useful tool to the design engineer to select the proper material geometry combination in conjunction with the required expansion pressure. Most existing design calculations are based on an elastic perfectly plastic behavior of the expansion joint materials. The proposed model is based on a strain hardening with a bilinear material behavior of the tube and the tubesheet. The interaction of these two components is simulated during the whole process of the application of the expansion pressure. The effects of the gap and the material strain hardening are to be emphasized. The model results are validated and confronted against the more accurate numerical finite element analysis models. Additional comparisons have been made to existing methods.

SS316L finned tubes are becoming very popular in high-pressure gas exchangers and particularly in CO2 cooler applications. Due to the high-pressure requirement during operation, these tubes require an accurate residual stress evaluation during the die expansion process. . Die expansion of gas coolers finned tubes creates not only high stresses that can surpass the UTS when combined with operation stresses but also micro-cracks during expansion when the process is not very well controlled. This research work aims to study the elastics-plastic behavior and estimate the residual stress state of tubes subjected to the die expansion process. The stresses and deformations of the expanded SS316L tube are analyzed numerically using the finite element method. The expansion and contraction process is modeled considering elastic-plastic material behavior for different die sizes. The maximum longitudinal, tangential, and contact stresses are evaluated to verify the critical stress state of the joint during the expansion process. The importance of the material behavior in evaluating the residual stresses using kinematic and isotropic hardening is addressed. Finally, an experiment was conducted to assess the tangential and longitudinal strains of a 3/8 stainless steel subjected to expansion with an oval shape die.

The loss of the initial tightness during service is one of the major causes of failure of tube-to-tubesheet joints. The initial residual contact pressure and its variation during the lifetime of the joint is among the parameters to blame. A reliable assessment of the initial contact pressure value requires accurate and rigorous modeling of the elasto-plastic behavior of the tube and the tubesheet during the expansion process. This paper deals with the development of a new analytical model used to accurately predict the residual contact pressure resulting from a hydraulic expansion process. It is based on the elastic perfectly plastic material behavior of the tube and the tubesheet and the interaction between them. The model results have been compared and validated with those of the more accurate numerical FEA models. Additional comparisons have been made with existing methods.

Steam generators are the subject of major concern in nuclear power plant safety. Within these generators, the tightness barrier, which separates the primary and secondary circuits, is ensured by the existence of a residual contact pressure at the tube-to-tubesheet joint interface. Any leakage is unacceptable, and its consequences are very heavy in terms of the human and environmental safety as well as maintenance cost. Some studies have been conducted to understand the main reasons for such failure. However, no analytical model able to predict the attenuation of the residual contact pressure under the effect of material creep relaxation behavior. The development of a simple analytical model able to predict the change in the residual contact pressure as a function of time is laid out in this paper. The results from the analytical model are checked and compared with those of finite elements. A simulation conducted on a high temperature tube-to-tubesheet joint has shown that relaxation of the contact stress due to creep can be significant and should be considered in the design.

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