Low-Cycle Fatigue Life Prediction of LP Steam Turbine Blade for Various Blade-Rotor Fixity Conditions

In the present paper, the damage and life cycle of the last-stage blades (LSB) of a 250-MW low-pressure steam turbine are predicted under low-cycle fatigue conditions. A finite element-based analysis is carried out to compute fatigue life and damage for a blade with a fir-tree root region under the loading condition of centrifugal stresses and using eight possible cases of contact between blade fir-tree root region flank and rotor flank. An experimental modal analysis (EMA) is performed for the identical modeled blade with a fir-tree root in a stationary test rig to find resonant frequencies and mode shapes, and these mode shapes show agreement with the experimental data. Dynamic response analysis of cracked and healthy blades for a case is also done. Further, the minimum number of life cycles, fatigue damage, and safety factor are computed employing mean stress correction approaches such as strain-based life, Morrow and Smith Watson Topper (SWT) to initiate a crack in blades for the same. The results for the lowest minimum life and serious damage among all eight blade-rotor fixity conditions are identified at the leading edge of lower serration when the pressure side lower flank is in contact with the rotor.