The Influence of the Initial Ovality Tolerance on the Nonlinear Cycling Analysis of Piping Bends

This paper presents the nonlinear shakedown analysis using finite element method for piping bends taking into account the maximum ovality tolerance. The resulting strain state of the stabilized cycle for the pipe bend including the maximum ovality tolerance is compared with the strain state for the perfectly round pipe bend. The boundary between shakedown and ratcheting is presented through global and local criteria in terms of displacements, and the Bree diagram (1989, “Plastic Deformation of a Closed Tube Due to Interaction of Pressure Stresses and Cyclic Thermal Stress,” Int. J. Mech. Sci., 31(11/12), pp. 865–892) converted into the interaction diagram is used to correlate the results of the nonlinear cyclic analyses and ASME Code limits for primary and secondary loads. This paper also discusses the choice of the global shakedown linked with the axial strain and the local through-thickness shakedown associated with the hoop strain. Furthermore, a simple fatigue calculation is conducted to illustrate the difference between pipe bends with perfectly round cross section and pipe bends with initial ovality under similar boundary conditions.