13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- at the surface of the material, but also increased the surface roughness. The local stress concentration created due to the sharp edges of PSBs act as the crack initiation and propagation [8, 14-15]. The slip markings developed due to the dislocation accumulation, transformed into the PSBs in the form of extrusions and intrusions. The deepest intrusion PSBs act as the potential fatigue crack initiation and propagation region [11, 15-17]. The fatigue failure with and without PSBs for specimens failed above and below 106 cycles showed that the fatigue crack initiation was highly dependent on the applied stress. At higher stresses, PSBs were not observed, yet the crack initiation took place from the surface of the specimen. The carbide precipitates were found as the crack initiation sites. However, when the stress levels were reduced, the PSBs played pivotal role in limiting the fatigue life of the material from 106 cycles and above. At further lower stress levels, at or below the fatigue limit, the PSBs were developed but found as non-damaging and the crack initiation was not observed. These PSBs were formed due to the large number of fatigue cycles and the stress concentration due to the lower stress was not sufficient to initiate the crack. 4. Conclusions Fatigue behavior of AISI 310 was investigated up to very high cycles. S-N curve with very small decreasing slope was obtained without any step. It was found that the fatigue fracture can occur beyond 107 cycles in this material. It was observed that all cracks were initiated from the surface of the specimen irrespective of the stress levels and the cycles before fatigue failure. No fish-eye or subsurface crack initiation was observed. It was found that at higher stress levels, the specimen exhibited lower fatigue life and cracks initiated from carbide precipitates on the surface of specimen. However, above 106 cycles at lower stress levels, PSBs were found at the surface of the specimen and acted as the fatigue crack initiation site. Arrays of intrusions and extrusions around the whole periphery of the specimen were observed. The sharp edges of PSBs served as the fatigue crack initiation and propagation. The lower slope in S-N curve showed that little decrease in stress level increased the failure cycles significantly. The higher fatigue cycles were used in development of PSBs all over the surface of the specimen as compared to the materials exhibit fish-eye failure, which required localized plasticity in subsurface region for initiation of crack. Acknowledgments This work was supported by the National Natural Science Foundation of China (10925211 and 11150110139). 6. References [1] C. Bathias and P. C. Paris. Gigacycle fatigue in mechanical practice. Taylor & Francis, 2004. [2] I. Marines, X. Bin, C. Bathias. An understanding of very high cycle fatigue of metals. Int. J. Fat., 25 (2003), 1101–1107. [3] C. Bathias, L. Drouillac, P. L. Francois. How and why the fatigue S–N curve does not approach a horizontal asymptote. Int. J. Fat., 23 ( 2001), S143–S151. [4] H. Mughrabi. Specific Features and mechanisms of fatigue in the ultrahigh-cycle regime. Int. J. Fat., 28, 2006, 1501-1508. [5] C. Stocker, M. Zimmermann, H. Christ. J. Effect of precipitation condition, prestrain and temperature on the fatigue behaviour of wrought nickel-based superalloys in the VHCF range. Acta Mater., 59 (2011), 5288-5304. [6] S. T Stanzl, H. Mughrabi, B. Schoenbauer. Int. J. Fat., 29 (2007), 2050–2059.
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