13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- by the findings of Sohar et al. for a high Cr alloyed cold worked tool steel, where internal as well as near surface fisheye fracture was observed in the VHCF regime and attributed to the formation and coalescence of microcracks around carbide clusters [11]. Despite these findings not all steels containing nonmetallic inclusions can be classified in one of the given models. Results for the HCF and VHCF behavior of a metastable austenitic stainless steel presented in this paper demonstrate that both the actual cyclic strength of a material, the course of the S–N curve and the underlying damage mechanisms are highly sensitive to microstructural changes. 2. Material and Experimental Methods The material chosen for this study was a solution annealed metastable austenitic stainless steel sheet AISI304 (1.4301) with a thickness of 2 mm. Its chemical composition determined by means of spectral analysis is given in table 1. A relatively low stability of the austenite phase is caused by the low amount of nickel in the overall alloy composition. The deformation induced ’ martensite content was regulated by a one-step predeformation carried out by means of a servohydraulic test system equipped with a liquid nitrogen-cooled chamber, allowing a deformation start temperature of -100°C. Before fatigue testing, all specimens were mechanically and subsequently electro-chemically polished but for several reference specimens with artificial micronotches at the surface. For the fatigue experiments two testing systems were used: a resonance pulsation test stand operating at ~ 90 Hz and an ultrasonic fatigue testing system working at a frequency of ~ 20 kHz. All fatigue experiments were executed in a fully reversed mode under load control. Specimens’ heating was averted by cooling with compressed air and in addition by a pulse-pause testing mode for the ultrasonic system. The alpha prime martensite volume fraction was measured by means of a magneto-inductive testing device (Fischer feritscope). The characterization of microstructure and the fractographic analysis of the fatigued samples were accomplished by means of a scanning electron microscope (FEI, Helios) equipped with an ion source which permits a selective removal of lamellae for transmission electron microscopy (Hitachi). Table 1. Chemical composition of the alloying elements for AISI304 analysed by means of spectral analysis. C Si Mn P S Cu Cr Mo Ni V N 0,024 0,43 1,43 0,021 0,007 0,14 18,3 0,038 8,11 0,1 0,067 3. Results 3.1 Fatigue behavior Fatigue results are given in figure 1a, open symbols indicate samples that were solely tested with the resonance system up to 108 cycles while crossed symbols represent samples fatigued firstly with the resonance pulsation system up to cyclic saturation and subsequently with the ultrasonic system. In the fully austenitic condition the metastable austenitic steel studied exhibits a constant fatigue limit of 250 Pa in the HCF and the VHCF region. No failure happened even up to a number of loading cycles N > 108. A true durability for N > 106 cycles for AISI304 in the fully austenitic condition is confirmed by various authors [12-15] except for the work by Bathias [16], who observed failure in the VHCF regime for this steel type. In the fully austenitic condition the material’s fatigue behavior is dominated by a strong transient behavior with a pronounced cyclic softening up to N ~ 105 followed by cyclic hardening up to 2 x 107 cycles. More details of the correlation between the cyclic deformation behavior of the material presented and its resonance behavior is given in [17]. The cyclic hardening can be explained by a martensite formation which
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