Abstract
Steam turbine last stage blades can experience a wide range of load spectra during their lifetime, which involves both Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) phenomena.In the paper a numerical model is proposed for considering this specific load spectrum in the fatigue life assessment of turbine blade components made of high strength precipitation-hardening steels.The method is based on cyclic isothermal stress and strain controlled fatigue test results of notched and un-notch specimen up to the HCF-regime. Moreover, stress controlled tests on notched specimens with different stress ratios were performed in order to evaluate the influence of mean stress and to derive a local Haigh diagram. The Highly Stressed Volume (HSV) has been used to transfer characteristic fatigue properties from specimens to a critical area (notched part) of a turbine blade. Furthermore, the real operating conditions of the component were used to allow the application of Palmgren-Miner damage rule.An ad-hoc MATLAB® software has been developed for the post-process of Finite Element Analysis (FEA) results, in particular for the automatic calculation of the HSV on the critical areas of the component. The proposed fatigue model has been finally applied on a real case study (blade dovetail) using appropriate safety margins for reliable design assessment.
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