Yasuhiro Ohtake

Residual Stress Evaluation of Hydraulically Expanded Tube-to-Tubesheet Joint

An evaluation of a residual stress of hydraulically expanded tube-to-tubesheet joints is important in strength evaluation of certain devices such as heat exchangers, steam generators, etc. In this study, we carried out the analytical and experimental investigation in order to develop a model for the prediction of residual stress.The experiment of the tube expansion was carried out with nickel-base alloy tubes and a low-alloy steel tubesheet. The residual stress of each tube was evaluated by sectioning method where the released strain was measured after each tube was cut into thin strips. As a result, the tensile residual stress was detected at the transition zone between the expanded zone and the non-expanded zone on each tube. A finite element analysis was also performed with a three dimensional model for the tube-to-tubesheet joint. The accuracy of the analysis, however, needed to be improved. Therefore FEM models were developed where the following factors were considered: (i) work-hardened layer on the tube before expansion, (ii) friction between the inner surface of the tube and the mandrel used for hydraulic expansion. The accuracy improvement of the analysis was confirmed through the comparison of the numerical and experimental results.Copyright