Description of the thermo-mechanical fatigue of thick-walled pressure vessels

The paper discusses the issue of the modelling of strains and stresses resulting from heating and cooling processes of the components in power plants. The main purpose of this work is to determine the mechanical behaviour of power plant components operating under mechanical and thermal loading. The heat-transfer coefficients as time-dependent variables have been introduced. Computer FEM modelling of the temperature strain and stress fields with temperature measurements have been used as the methods of research. It is readily apparent from the stress versus time graphs that under unsteady operating conditions, components analysed in the paper, especially in the case of boiler restarts, may operate with transient thermal stresses which sometimes reach values higher than a yield point. Consequently, a thermo-mechanical fatigue process takes place in the materials of the components in question. The stress-strain diagrams for the selected points of the plant components have been determined. The paper will be a contribution to the justification of the necessity of the application of design methods of highly reliable pressure vessels, which take into consideration an influence of TMF processes on these vessels durability. (C) 2014 Elsevier Ltd.