PSI - Issue 2_A

Andrei Grigorescu et al. / Procedia Structural Integrity 2 (2016) 1093–1100 Author name / Structural Integrity Procedia 00 (2016) 000–000

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resulting in a decrease of strain amplitude.  At higher amounts of α’ martensite the damage mechanisms change and failure occurs after very high number of cycles due to internal crack initiation at inclusions accompanied by the formation of a fine granular area.  A direct correlation between the FGA-area and the number of cycles to failure could be found for RD samples (with small area proj ) whereas no such relationship could be confirmed for the TD-samples (with larger area proj ). This confirms the fatigue life controlling role of the crack initiating inclusions if they exceed a certain projected area. This statement is supported by the correlation between the stress intensity factor at inclusions and the fatigue life, which was confirmed only for the TD-samples. In the case of RD samples the lack of correlation is explained by the dependence of the FGA-growth on the surrounding microstructure for the small area proj of the crack initiating inclusions.  The notch sensitivity increases at 30 vol-% α’-martensite, whereas higher martensite contents do not show any further significant increase. In this respect, a martensite content lower than 30 vol-% is recommended for optimized HCF and VHCF properties. Acknowledgements The authors gratefully acknowledge the financial support of this study by Deutsche Forschungsgemeinschaft in the framework of the priority programme Life  (DFG-SPP1466). Bowe, K., H., Hammerschmidt, W., Hornbogen, E., Hühner, M., 1988. Bruchmechanische Eigenschaften von metastabilen Austeniten. Materialwissenschaft und Werkstofftechnik 19, 193-201. Grad, P., Reuscher, B., Brodyanski, A., Kopnarski, M., Kerscher, E., 2012. Mechanism of fatigue crack initiation and propagation in the very high cycle regime of high-strength steels. Scripta Materialia 67, 838-841. Li, S., X., 2012. Effects of inclusions on very high cycle fatigue properties of high strength steels. International Materials Reviews 57, 92-114. Lukas, P., Kunz, L., 2002. Specific features of high-cycle and ultra-high-cycle fatigue. Fatigue&Fracture Enginnering Materials & Structures 25, 747-753 Maier, H., Donth, B., Bayerlein, M., Mughrabi, H., Maier, B., Kesten, M., 1993. Optimierte Festigkeitssteigerung eines metastabilen austenitischen Stahles durch verformungsinduzierte Martensitumwandlung bei tiefen Temperaturen. Zeitschrifft für Metallkunde 84, 820-826. Mughrabi, H., 2002. On ‘Multi-stage’ fatigue life diagrams and the relevant life-controlling mechanisms in ultrahigh-cycle fatigue. Fatigue&Fracture Enginnering Materials & Structures 25, 755-764 Murakami, Y., 2002. Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions, Elsevier, Oxford. Murakami, Y., Nomoto, T., Ueda, T., 1999. Factors influencing the mechanism of superlong fatigue failure in steels, Fatigue & Fracture of Engineering Materials & Structures 22, 581-590. Murakami, Y., Matsunaga, H., 2006. The effect of hydrogen on fatigue properties of steels used for fuel cell systems. International Journal of Fatigue 28, 1509-1520 Müller-Bollenhagen, C., Zimmermann, M., Christ, H.-J., 2010. Adjusting the very high cycle fatigue properties of a metastable austenitic stainless steel by means of the martensite content. Procedia Engineering 2, 1663-1672. Myeon, T., H., Yamabayashi, Y., Shimojo, Y., Higo, Y., 1997. A new life extension method for high cycle fatigue using micro martensitic transformation in austenitic stainless steel. International Journal of Fatigue 19, 69-73. Sakai, T., 2009. Review and prospects for current studies on very high cycle fatigue of metallic materials for machine structural use. Journal of Solid Mechanics and Materials Engineering 3, 425-439. References