13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- inclusion is assumed as most likely explanation for the formation of FGAs in AISI304 at VHCF-relevant numbers of cycles. Hence, crack initiation and FGA formation is the consequence of agglomerated hydrogen in the martensitic phase around the inclusion increasing the amount of localized plastic deformation in this particular microstructural region. However, this assumption has to be further discussed as in the study presented no final evidence for hydrogen assisted crack initiation was found. Fatigue crack growth tests simulating the microstructural and testing conditions discussed will be necessary for further clarification. a) b) c) Fig. 7: Examples for the formation of fisheye fracture morphology with different sizes (depending on the distance between inclusion and specimen surface) a) /2 = 515 MPa, NF = 1.49 x 10 8, b) /2 = 535 MPa, NF = 9.09 x 10 6 and c) /2 = 470 MPa, NF = 6.84 x 10 8. 4. Conclusion The study presented discusses the change in failure mechanisms of a metastable austenitic steel – an alloy that would typically be classified as material type II with respect to its VHCF behavior – as a function of its volume fraction of deformation-induced ’ martensite. In the HCF region AISI304 always failed at inclusions at or very near to the surface, irrespective of its volume fraction of ’ martensite. Only surface flaws of a certain size led to earlier failure for a martensite content > 30%. No failure was observed in the VHCF regime for the fully austenitic condition. The VHCF behavior can be explained by the local work hardening of the soft austenitic phase and a very localized phase transformation, both resulting in a decrease of strain amplitude. Hence, the load controlled fatigue tests no longer causes a strain amplitude exceeding a threshold value for crack initiation or microcrack propagation. In case of an ’ martensite volume fraction of 27% a true durability can still be confirmed together with an increase of cyclic strength from ~250 MPa to ~480 MPa compared to the fully austenitic condition. A surface roughening could be observed for the 27% specimens but did not result in crack initiation. However, failure mechanism changes completely with a higher amount of ’ martensite volume fraction. A volume fraction of 54-56% leads to a formation of connected
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