The deflections and the stresses in U-tubes due to differential thermal expansion between two legs are nonlinear functions of the thermal loading even with linear elastic materials. The nonlinear behavior is due to the fact that the induced axial force in U-tubes, tension in cold leg and compression in hot leg, have some effects on the deflection of the tubes. In the present work, the nonlinear analysis method is fully described and applied. Numerical results are shown in comparison with those given by simplified linear analysis in which the effects of axial force on deflections of tubes are neglected. In general, the effects of the nonlinear behavior are negligible for the maximum stress; however, they may be significant for some secondary details such as the tube rotation and the stress at apex of the tube. The present work is intended to justify the validity of some earlier formulas, developed by the author in a previous work, for calculation of secondary stresses in heat exchanger U-tubes.

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