Creep analysis of boiler tubes by fem

Abstract

In this paper an analysis is developed for the determination of creep deformation of an axisymmetric boiler tubes subjected to axisymmetric loads. The stresses and the permanent strains at a particular time and at the steady state condition, resulting from loading of the tube under constant internal pressure and elevated temperature are evaluated taking into account the secondary creep characteristics of a given material. Finite element solution is presented and discussed for a class of problems in which no prior analytical solution may exist. The method of solution developed is an extension of the direct stiffness method. The body is replaced by a system of discrete triangular cross-section ring elements interconnected along circumferential nodal circles. The equations of equilibrium for the element are derived from the principle of minimum potential energy. The creep behaviour of the body is formulated in terms of creep laws in current use.

Starting with the elastic solution of the problem, creep strains are treated as initial strains to determine the new stress distributions at the end of the pre-set time interval. The procedure is repeated until the final time is reached and/or until the stress distribution reaches a steady stated condition. Numerical results obtained for a tube material 12CrMoV are presented at the end.