Effects of friction on roller expanded tube-tubesheet joint strength

Roller expansion of new tubes in enlarged heat exchanger tubesheet holes requires higher rolling torques which may result in over-thinning of the tube wall. This affects adversely the tube-tubesheet joint strength, which is measured in terms of residual contact pressure between the expanded tube and tubesheet. The axial force required to cause the mechanical failure of the tube-tubesheet joint has therefore been considered as an indication of the integrity of the joint. This force is influenced by several parameters such as the type of tube and tubesheet materials, the initial clearance and coefficient of friction. In the present work, an axisymmetric finite element (FE) model is used to evaluate the combined effects of friction between tube and tubesheet, initial clearance and tube material strain hardening on the strength of the tube-tubesheet joint. The FE results show that the increase in friction between tube and tubesheet results in higher residual contact stress and lower cutoff clearances. The residual contact stress also increased linearly with increasing tube material strain hardening level for all friction coefficients.