This paper discusses the issue of the modeling of strains and stresses resulting from the heating and cooling processes of components in power plants. The main purpose of this work was to determine the mechanical behavior of power plant components operating under mechanical and thermal loading. The finite element method has been used to evaluate the temperature and stress changes in components as a function of time. The temperature fields in the components of power plants are dependent, apart from other relevant parameters, on variable heat-transfer conditions between these components and the fluid medium (which may change its state) flowing inside them. For this reason an evaluation of the temperature field and the consequent stress fields requires the consideration of heat-transfer coefficients as time-dependent variables, which in turn calls for suitable techniques for the determination of appropriate values for these coefficients. Methodology that combines computer modeling of the temperature fields with temperature measurements performed at selected points of the pipelines may be used in this case. It is readily apparent from the stress-versus-time graphs that under unsteady operating conditions the components analyzed in this study, especially in the case of boiler restarts, may operate with transient thermal stresses that sometimes reach values higher than a yield point. Consequently, a thermo-mechanical fatigue process takes place in the materials of the components in question. Local stress-strain diagrams for the selected points of the plant components describe this kind of fatigue. These diagrams characterize the intensity of the process and are necessary when the fatigue life is predicted. Such diagrams are part of thermo-mechanical fatigue life prediction methods. The problem of their description is important in the development of a new design methodology for highly reliable pressure vessels.
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