13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- martensite clusters. The localized plastic deformation can no longer be compensated by the softer austenite phase nor will the strain amplitude be reduced due to a local phase transformation. Moreover, it is assumed that hydrogen trapped in the vicinity of inclusions in the specimen’s interior increases the dislocation mobility in this region and at the same time promotes the brittleness of the martensitic phase. Inclusions surrounded by such brittle martensite act as local stress raisers and lead to crack initiation in the form of a fine granular area. Fisheye morphology was observed for all specimens that failed in the VHCF regime and the typical smooth fracture surface surrounding the FGA can be explained by crack growth under vacuum-like conditions. A schematic summary of the different failure mechanisms of AISI304 determined for the VHCF behavior is presented in figure 8. Fig. 8: Change in failure mechanism for VHCF crack initiation in correlation with the volume fraction of ’ martensite. Acknowledgements The authors gratefully acknowledge the financial support of this study by Deutsche Forschungsgemeinschaft (DFG). References [1] M. Zimmermann, Diversity of damage evolution during cyclic loading at very high numbers of cycles – An overview, Int. Mater. Rev., 57 (2012) 73-91. [2] S. X. Li, Effects of inclusions on very high cycle fatigue properties of high strength steels, Int. Mater. Rev., 57 (2012) 92-114. [3] H. Mughrabi, Specific features and mechanisms of fatigue in the ultrahigh-cycle regime, Int. J. Fatigue, 28 (2006) 1501-1508. [4] H. Mughrabi, On ‘multi-stage’ fatigue life diagrams and the relevant life-controlling mechanisms in ultrahigh-cycle fatigue, Fatigue Fract. Eng. Mater. Struct., 25 (2002)755-764. [5] S. K. Jha, K. S. R. Chandran, An unusual fatigue phenomenon: duality of the S-N fatigue curve in the -titanium alloy Ti-10V-2Fe-3Al, Scr. Mater., 48 (2003) 1207-1212.
RkJQdWJsaXNoZXIy MjM0NDE=