PSI - Issue 2_A

Jenni Herrmann et al. / Procedia Structural Integrity 2 (2016) 2951–2958 Jenni Herrmann et al./ Structural Integrity Procedia 00 (2016) 000–000

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Semi-elliptical surface cracks up to an initial depth of 4 mm are not growing more than 1 mm in these 20 years in any of the considered materials. Except for the GJS-1000-5-shaft, after 2E+08 cycles (around 6.7 years) an initial crack in the shaft hotspot with a depth of 10 mm is propagating less than 5 mm. And only in austempered ductile iron unstable crack growth occurs before the end of the lifetime is reached, when a 10 mm crack is located in the hotspot area since first day of operation. If the initial crack in the notch region has a depth of 20 mm, in several shafts unstable crack growth starts before completion of service life (see Fig. 12). After 1E+07 load cycles (one third of a year) no crack has grown more than 5 mm. But after 2.2E+07 load cycles (less than three quarters of a year), first of all the GJS-1000-5-rotor shaft fails, thereon the GJS-800-10-shaft (nearly 2.6E+07 load cycles), followed by the shaft made of GJS-400-18-LT (nearly 2.7E+07 load cycles) and subsequently the GJS-600-3-shaft (at 2E+08 cycles, around 7 years). In the shafts made of GJSF-SiNi30-5 and of forged steel there is no unstable crack growth till the end of turbine life. Once again, it appears that considering the realistic stress level with regard to fatigue and remaining life in the material selection process is highly important. In this manner maintenance intervals can be deduced easily for shafts made of different materials. It should be noted, however, that these statements apply for climatically uncritical conditions. For negative temperatures a different order may arise. 5. Conclusion and outlook In conclusion the paper contains a comprehensive comparison of the rotor shafts made of different materials under test conditions and under realistic loads in a wind turbine. It is demonstrated, why it is important to take account of other materials for the rotor shaft of a wind turbine in regard to the fatigue and the fracture mechanical behaviour. While rotor shafts made of cast iron have better qualities concerning fatigue at low loading, forged steel is superior at high load levels. Fracture mechanical investigations show a higher resistance of the forged shaft against fatigue crack propagation. Though, at stresses below the endurance limit cast iron is also persevering. In order to realistically evaluate the risk of undetected cracks or premature initiated cracks in the rotor shaft while turbine operation, the crack propagation of different potential imperfections will be considered more closely. Acknowledgements The research project BeBen XXL is done in collaboration with Fraunhofer IWES and Suzlon Energy. It is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi). References ASTM E 647-13a, 2013. Standard Test Method for Measurement of Fatigue Crack Growth Rates. ASTM International. AWEA, 2011. Wind Energy Industry Manufacturing Supplier Handbook. American Wind Energy Association. Fraunhofer, 2016. http://www.windenergie.iwes.fraunhofer.de/en/research_projects/anlagen--und-systemtechnik/beben_xxl.html, 28.03.2016. Germanischer Lloyd, 2010. Guideline for the Certification of Wind Turbines. Germanischer Lloyd WindEnergie GmbH. Gudehus, H., Zenner, H., 2007. Leitfaden für eine Betriebsfestigkeitsrechnung – Empfehlung zur Lebensdauerabschätzung von Maschinenbau teilen. 4th Edition, Verlag Stahleisen GmbH, Düsseldorf. Hau, E., 2008. Windkraftanlagen. 4th Edition. Springer-Verlag Berlin Heidelberg. Henkel, S., Hübner, P., Pusch, G., 2008. Zyklisches Risswachstumsverhalten von Gusseisenwerkstoffen – Analytische und statistische Aufbereitung für die Nutzung mit dem Berechnungsprogramm ESACRACK. 40. DVM-Tagung, Arbeitskreis Bruchvorgänge. Mikoleizik, P., Geier, G., 2014. SiWind – Werkstoffentwicklung für Offshore-Windenergieanlagen im Multimegawatt-Bereich. GIESSEREI 101. Pollicino, F., 2006. Bruchmechanische Fragestellung bei der Lebensdauerberechnung von Windenergieanlagen. Deutscher Verband für Materialforschung und –prüfung e.V. Sander, M., 2008. Sicherheit und Betriebsfestigkeit von Maschinen und Anlagen. Springer-Verlag Berlin Heidelberg. Shirani, M., Härkegard, G., 2011. Fatigue life distribution and size effect in ductile cast iron for wind turbine components. Engineering Failure Analysis 18. Herfurth, K., 2003. Austenitisch-ferritisches Gusseisen mit Kugelgraphit, Teil 1/ Teil 2. Giesserei-Praxis. Kyling, H., 2014. BEBEN XXL Test Bench - Final Design Review. Project-internal Document.