PSI - Issue 7

A. Giertler et al. / Procedia Structural Integrity 7 (2017) 321–326

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A. Giertler et Al./ Structural Integrity Procedia 00 (2017) 000–000

4. Conclusions The fatigue behavior in the VHCF regime of the steel 50CrMo4 was examined as a function of both, the strength condition as well as the test frequency in the fatigue experiments. It was found that the VHCF damage mechanisms depend strongly on the strength of the material. In case of the material hardness of 37HRC, the crack initiation on the surface dominates the material failure. Slip planes caused by irreversible plastic deformation within the martensitic microstructure show small microcracks below the surface. The propagation of these microcracks is prevented by the efficient barrier effect of the surrounding microstructure. Thus, taking into account a constant stress amplitude, the existence of a real fatigue limit for this material condition is supported. In contrast, the material state of 57HRC shows a higher fatigue strength, but these values decrease continuously with increasing load cycles. Furthermore, in this case, the internal crack initiation on non-metallic inclusions dominates the fatigue life. With decreasing stress amplitude and therefore extended fatigue life, the formation of an FGA on the fracture surface was observed. By applying the model according to Murakami, a fracture mechanical assessment of critical inclusion sizes becomes possible to predict the fatigue life. In addition, it was shown that the fatigue strength must be determined taking account of the test frequency, the critical volume size and the material strength. Acknowledgements The German Ministry of Education and Research (BMBF) and the Robert BOSCH GmbH is gratefully acknowledged for financial support of this work. References Cayron, C. 2006. ARPGE: A computer program to automatically reconstruct the parent grains from electron backscatter diffraction data. In: Journalof Applied Crystallography 40(6), 1183–1188. Grad P, Reuscher B, Brodyanski A, Kopnarski M, Kerscher E. 2012. Mechanism of fatigue crack initiation and propagation in the very high cycle fatigue regime of high-strength steels. In: Scripta Materialia, 67, 838–41. Kitahara, H., Ueji, R., Tsuji, N., Minamino, Y. 2006. Crystallographic features of lath martensite in low-carbon steel. In: Acta Materialia 54(5), 1279–1288. Morito S., Huang, X., Furuhara, T., Maki, T., Hansen, N., 2006. The morphology and crystallography of lath martensite in alloy steels. In: Acta Materialia 2006;54:5323–31. Mughrabi, H. 2006. Specific features and mechanisms of fatigue in the ultrahigh-cycle regime. In: International Journal of Fatigue 28:1501–8. Mughrabi, H. 2002. On 'multi-stage' fatigue life diagrams and the relevant life-controlling mechanisms in ultrahigh-cycle fatigue. In: Fatigue & Fracture of Engineering Materials & Structures 25, 755–64. Murakami Y, Kodama S, Konuma S. 1989. Quantitative evaluation of effects of nonmetallic inclusions on fatigue strength of high strength steels. I: Basic fatigue mechanism and evaluation of correlation between the fatigue fracture and the size and location of non-metallic inclussions. In: Int International Journal of Fatigue 11, 291–8. Kunio, T., Shimizu, M., Yamada, K., Sakura, K., Yamamoto, T., 1981.The early stage of fatigue crack growth in martensitic steel. In: International Journal of Fracture 17, 111–119. Kurdjumow, G., Sachs, G., 1930. Über den Mechanismus der Stahlhärtung. In: Zeitschrift für Physik, 64(5-6): 325–343. Sakai T., 2009. Review and Prospects for Current Studies on Very High Cycle Fatigue of Metallic Materials for Machine Structural Use. In: JMMP 3, 425–39. Seeger, A. 1954. The temperature dependence of the critical shear stress and of work-hardening of metal crystals. In: The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science: Series 7, 45:366, 771-773. Zhai, T., Jiang, X., Li, J., Garratt, M., Bray, G., 2005. The grain boundary geometry for optimum resistance to growth of short fatigue cracks in high strength Al-alloys. In: International Journal of Fatigue 27, 1202–1209.