Influence of Bauschinger effect on the residual contact pressure of hydraulically expanded tube-to-tubesheet joints

The level of the contact pressure and the stresses induced during the hydraulically expanded tube–to-tubesheet process are the key factors for the integrity of a leak free expanded joint. The modeling of this type of joint requires an adequate representation of the material behavior in order to accurately evaluate important joint parameters such as the residual contact pressure and induced residual stresses. Maintaining a lower bound safe limit of the initial residual contact pressure over the lifetime of the expanded joint insures its durability. An analytical model that simulates the proper material-geometry behavior during the process of hydraulic expansion of a tube-to-tubesheet is developed. The proposed model is based on strain hardening material behavior of the tube and the tubesheet and takes into account reverse yielding (Bauschinger effect) during unloading. The interaction of the tube and the tubesheet components is considered at the different stages of the applied expansion pressure. The results show that the residual contact stress is overestimated by more than 100% when reverse yielding is not accounted for. The findings from the analytical model are supported by those of the numerical FEA counterparts. Two joints of different geometries and materials are treated to demonstrate the importance of considering the real material behavior in the analysis of such an expansion process.