DAMAGE MODELS AND CYCLE COUNTING METHODS FOR FATIGUE ASSESSMENT OF POWER PLANTS COMPONENTS
Felippe M. S. Costa1; Jurgen Rudolph2; José Luiz F. Freire1; José Eduardo Maneschy3
1 PUC-Rio; 2 Areva GmbH; 3 JEM Consultoria
doi:10.20906/CPS/COB-2015-2661
Resumo
Structural components of power plants are subjected to thermal transients during their operational life. These transients generate unequal temperature distributions across the thickness of the components causing high thermal stresses. Repetition of transients and consequently repetition of stress and strain variations are responsible for fatigue at critical points. For these cases, fatigue is assessed by calculating the cumulative usage factor CUF. The CUF calculations are based on the stresses and strains histories, on fatigue curves and fatigue damage models, and on algorithms used to determine the number of cycles a given stress or strain range occurs during the life period. This paper presents and discusses fatigue damage models and their association with cycle counting methods that are possible to be applied to power plant components. A selection of combinations of damage and cycle counting models was used in two examples. The damage models addressed are the ASME fatigue curve associated to elastic stress analysis, the Coffin Manson, the Smith-Watson-Topper, the Brown-Miller, the Fatemi-Socie and the Gupta-Fesich. The cycle-counting algorithms used were the Extreme Value, Rainflow, Simplified Rainflow, Bannatine and Socie, Wang-Brown, Modified Wang-Brown and Path-Dependent Maximum Range Models. In order to account for stress-strain relations generated in non-proportional load histories, the Modified Armstrong-Frederick using the Tanaka's non-proportionality model was also applied. A software was developed to assist the evaluation of the fatigue analysis using the procedures proposed. In the first example, an actual measured load history of pressure and temperature was applied to a thick-walled piping. Two transient analytical solutions earlier proposed by Albrecht were implemented and their results compared with results determined with a commercial finite element software. The main goal of implementing the analytic stress solution was to optimize the time consumed to obtain the stresses that occur along
Palavras-chave: Power plant components; design against fatigue; cycle counting methods; multiaxial loading