FEA of the effects of initial tube-tubesheet clearance, wall reduction and material strain hardening on rolled joint strength

The tube-to-tubesheet joint strength is measured in terms of residual contact pressure between the tube's outer surface and tubesheet hole surfaces. The joint integrity is affected by several design parameters, including the type of tube and-tubesheet materials, level of expansion and the initial radial clearance between the tube and tubesheet. In the present work, an axisymmetric finite element model based on the sleeve diameter and rigid roller concepts is developed. The model has been used to evaluate the combined effects of clearance, wall reduction level and the-strain hardening of tube and tubesheet materials on the interfacial pressure between tube and tubesheet. The FE results show that the initial clearance effect is dependent on the strain hardening capability of the tube material. For low strain hardening tube material the interfacial pressure remains almost constant well above the TEMA (Tubular Exchanger Manufacturing Association) maximum radial over tolerance of 0.0254 mm. These results are validated by the experimental data developed during the research program. As expected, a drastic reduction in joint strength is observed at high values of radial clearances. The cut-off clearance (clearance at which the interfacial pressure starts to drop) is found to vary linearly with the level tube material hardening level. The residual pressure is found to increase slightly for moderate strain hardening tube materials but shows lower cut-off clearances. Wall reductions ranging from 1% to 10% were utilized in calculating the contact pressure as a function of radial clearance. The results show that for low strain hardening materials the optimum value of residual contact stress is obtained for the industry recommended value of 5%. Finally, because of the absence plastic deformation in the ligament, the level of tubesheet material strain hardening does not have any noticeable effect to the joint strength.