Browsing by Author "Taye, Chanyalew"

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    Thermal Stresses and Creep Analysis of Boiler Tubes
    (Addis Ababa University, 2004-09) Taye, Chanyalew; Bazezew, Alem (PhD)
    An analysis is developed for the calculation of creep deformation of an axisymmetric boiler tube subjected to axisymmetric load. 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 were evaluated when accounts is taken to the secondary creep characteristics of a given material. In this thesis first the formulation of an analytical theory of creep for tubes according to the Bailey creep theory [41] was discussed. Bailey theory was proposed for an idealized homogeneous material loaded uniaxially. The theory takes into account the initial elastic strain, the transient creep strain, and the minimum creep rate strain. Next, more general solution by finite element method are presented and discussed for a class of problems in which no prior analytical solution may exists; like the cases of cracked and/or pitted boiler tubes. The method of solution 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 body are derived from the principle of minimum potential energy. The creep behavior 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 time interval. The procedure is repeated until either the final time is reached or until the stress distribution is not changed i.e. when a steady stated condition is reached. Calculated results according to arbitrarily selected boiler tube data are shown at the end.