PSI - Issue 46

8 6

L. Frank et al. / Procedia Structural Integrity 46 (2023) 3–9 L. Frank and S. Weihe / Structural Integrity Procedia 00 (2019) 000–000

Notch

Bearing flank

EoC Notch

Bearing flank

EoC

Fig. 6. Calculated distribution of FDP FS as well as predicted number of cycles obtained for five different bending amplitudes.

4. Conclusion This paper presents results from experimental and numerical fatigue assessments on the influence of multiaxial stress state caused by high centrifugal forces superimposed with bending loads due to blade vibrations on the lifetime of end stage blades from steam turbines. For the experimental investigations, a test rig was developed by the MPA University of Stuttgart. Based on the experimental investigations on original-sized end stage blades as well as on the fractographic post-examinations using magnetic particle inspection, light and scanning electron microscopes, two competing damage mechanisms were identified, which occurred at different locations. As a result of the experimental investigations, it can be stated that the occurrence of the damage location and the damage mechanism depends on the level of the bending stress. In addition, numerical investigations were carried out based on a developed concept. The multi-axial stress state in the blade root was evaluated using the critical plane approach in conjunction with the Fatemi and Socie fatigue hypothesis. The lifetime of the end stage blades were calculated for five different bending amplitudes and compared to the experimental results. In summary, with the parameter FDP FS both the mechanism change as a function of the bending amplitude can be described as well as a very good prediction of the expected cycles is obtained.

Acknowledgements This research work was founded by the Federal Ministry for Economic Affairs and Energy (BMWi) under contract No. 03ET7021A. For their support in the project, our great thanks also belong to Siemens Energy Global GmbH & Co. KG

References

EPRI. 2008. Steam Turbine Blade Failure Root Cause Analysis Guide. EPRI, Palo Alto, CA: 2008. 1014137. Fatemi, A., Socie, D. F., 1988. A critical plane approach to multiaxial fatigue damage including out-of-phase loading, Fatigue & Fracture of Engineering Materials & Structures 11 (1988) 3, pp. 149-165, DOI:10.1111/j.1460-2695.1988.tb01169.x Fesich, T. M. 2013. Festigkeitsnachweis und Lebensdauerberechnung bei komplex mehrachsiger Schwingbeanspruchung, VDI Fortschr.-Ber. Reihe 5, Nr. 748, VDI Verlag Frank, L. et al. 2018. Endstufenschaufeln für hochflexible Fahrweisen und hohe Startzahlen, Fachlicher Abschlussbericht innerhalb des BMWI PTJ-Verbundvorhabens COOREFLEX-turbo, Förderkennzeichen 03ET7021A und 03ET7020E, MPA Universität Stuttgart / Siemens AG Gupta, S. et al. 2011. A critical plane based model for fatigue assessment under fixed and rotating principal direction loading, in SMiRT 21, Volume: Div-II: Paper ID# 624, New Delhi, India