PSI - Issue 7

M. Nesládek et al. / Procedia Structural Integrity 7 (2017) 190–197 M. Nesládek et al. / Structural Integrity Procedia 00 (2017) 000 – 000 where the exponent can be estimated by using the following empiric equation (Papuga et al. (2012)): = 1 − (2 −1 0 ) log(2) . −1 and 0 are the fatigue limit strength in fully reversed and repeated tension, respectively. 4.2. The Nagode approach The Nagode approach to thermo-mechanical fatigue (TMF) (Nagode (2014), Nagode et al. (2009)) is intended to analyse the variable amplitude stress, strain and temperature signals. It allows for continuous damage parameter calculation, thus avoiding a cycle closure problem that may appear due to variable temperature. The procedure requires as inputs time history of signed local equivalent stress and strain and temperature obtained by elastic-plastic FEA. Particular attention must be paid to signing so that physically correct inputs are used for prediction. Regarding the material inputs, a set of isothermal Manson-Coffin and Basquin curves for a range of temperatures spanning the values within the analysed time history need to be provided. The fatigue curves for intermediate temperatures were interpolated with 1 °C increment. The method is based on the Smith-Watson-Topper (SWT) parameter. The Masing and the memory rules are applied for determining the instantaneous stress and strain amplitudes and means. With the amplitude signed according to the equivalent strain, the instantaneous SWT parameter is then evaluated and the value of fatigue damage using the Prandtl hysteresis operator is found. 195 (7)

Fig. 3. The basic conception of the developed software codes for fatigue prediction of steam turbine shafts.