13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- results in a decrease of the strain amplitude. Hence, the global loading situation declines well below a threshold value for irreversible slip activities so that the fatigue limit can be regarded as a true infinite durability for the fully austenitic condition. However, the martensite formation during cyclic loading in the VHCF regime is quite surprising as the plastic strain amplitude (3 x 10-4) is well below the threshold value for martensite formation of 3 x 10-3 according to literature. The fatigue behavior of specimens in the one-step predeformed condition displays a steady increase of cyclic strength in the HCF regime with increasing volume fraction of deformation induced martensite (Figure 1a). At the same time the disposition to a transient behavior during cyclic deformation fades. For the HCF regime the fatigue limit increases with increasing martensite volume fraction. However, this is only true for specimens with a well preserved surface without flaws and defects. In case of artificial micro notches (with a depth of 12 up to 40 m) at the surface a peak cyclic strength could be identified for a volume fraction of ’ martensite ≤ 30% . A comparison of actively cooled and uncooled fatigue tests demonstrate the influence of ’ martensite formation on the cyclic strength during testing. The lack of active cooling prevents the formation of ’ martensite and as a consequence lowers the cyclic strength drastically (figure 1b). For the VHCF regime a change in the fatigue behavior can be observed with increasing volume fraction of alpha prime martensite. For a volume fraction of 54% failure occurs even beyond the classical fatigue limit resulting in a two-step S-N-curve with crack initiation up to a number of loading cycles N > 108. Therefore it stands to reason that a change in damage mechanism due to the degree of phase transformation is inherent for AISI304. This phenomenon can be explained by the metallographic analysis presented in the subsequent chapter. a) b) Figure 1: Fatigue behavior in the HCF and VHCF regime (a) and comparison of fatigue limit for the HCF region related to different testing conditions and samples with artificial micro notches (b). 3.2 Damage mechanisms The fatigue behavior of AISI304 in the fully austenitic condition is characterized by two major microstructural processes: inhomogeneously distributed strain localization in narrow slip bands and formation of ’ martensite at intersections of slip bands as well as formation of martensite needles (figure 2). The phase transformation leads to a decrease of dislocation mobility and hence to an increase in cyclic strength and a true durability in the VHCF regime. Fractographic analysis of the specimens tested and failed in the HCF region (here: failure only for N < 106) reveal the importance of microstructural flaws and defects at or near the specimens surface. All failures in the HCF regime can be attributed to crack initiation at nonmetallic inclusions in the near-surface region (figure 3a). 200 250 300 350 400 450 500 550 600 650 0 1020304050607080 martensite [vol-%] fatigue limit [MPa] cooled, electropolished uncooled, notched uncooled, electropolished cooled, staircase tests
RkJQdWJsaXNoZXIy MjM0NDE=