PSI - Issue 14

Takashi Nakamura et al. / Procedia Structural Integrity 14 (2019) 978–985 Author name / Structural Integrity Procedia 00 (2018) 000–000

984

7

image corresponds to the internal fatigue crack, and it propagated radially outward from the internal origin (Fig. 10(a) – (c)) and eventually reached the specimen surface (Fig. 10(d)). And then, this crack propagated as a surface crack leading to final fracture. Fig. 11 plots the relationship between d a /d N and the stress intensity factor range Δ K . Here, Δ K was calculated by using √ method proposed by Murakami et al, 1989. Triangles and diamonds in the figure represent the results for the internal and surface cracks, respectively. The data in air and in vacuum shown in Fig. 6 were also replotted in this graph without distinction of specimens in each environment. As shown in Fig. 11, the propagation rate of the internal crack was very small, less than 10 -10 m/cycle, and was significantly lower than that of the surface crack in air. In contrast, it agreed very well with that of the surface crack in vacuum. On the other hand, the propagation rate of the internal crack once it reached the specimen surface was of the same order as that of the surface crack in air. Namely, the internal crack upon reaching the specimen surface changes its propagation behavior to that of a surface crack initiated at the surface. These findings suggest that the reason for the great difference between the propagation rates of internal and surface cracks is the difference in environments around them. The good agreement between the propagation rate of the internal crack and that of the surface crack in vacuum indicates that the effect of the environment inside an internal crack is almost the same as that of a high vacuum. Therefore, the propagation rate of the internal crack is significantly retarded through the effect as discussed in Section 4.1. Consequently, the vacuum-like environment at the tip of internal crack is considered to be a reasonable cause for the long fatigue life of internal fractures through its retarding effect. To investigate the reason for internal fatigue fractures in the VHCF regime, this study focused on the idea that the environment around internal crack is vacuum-like. Small crack growth tests were conducted in air and in high vacuum, and the crack growth behaviors and fracture surfaces were closely examined. In addition, the crack growth rate in vacuum was compared with that of internal crack measured by synchrotron radiation μCT imaging. Considering the results obtained, effects of vacuum like environment around an internal crack were investigated. The major results obtained are as follows: 1) The crack propagation rate in high vacuum was smaller than that in air. The pronounced effect of vacuum was shown in the small crack regime, and the crack propagation rates were less than 10 -10 m/cycle in this regime. 2) Fracture surfaces in high vacuum were extremely different from those in air. Especially in high vacuum, a unique granular feature with a few micrometer convexo-concave pattern was frequently observed. This feature resembled the fracture surface of internal crack. 3) The propagation rate of the internal crack was very small, less than 10 -10 m/cycle, and was significantly lower than that of the surface crack in air. In contrast, it agreed very well with that of the surface crack in high vacuum. 4) The effect of the environment inside an internal crack is considered same as that of a high vacuum. This special environment is a reasonable cause for the long fatigue life of internal fractures through its retarding effect. 5. Conclusion

Acknowledgements

The synchrotron radiation experiments were performed at the BL20XU in SPring-8 with the approval of JASRI (Proposal No. 2013A1218 and 2013B1470). The authors are profoundly grateful to Dr. Akihisa Takeuchi, Dr. Kentaro Uesugi and Dr. Masayuki Uesugi at JASRI for their generous technical support to the experiment. The authors acknowledge the support of a Grant-in-Aid for Scientific Research (A, 2018, 18H03748) from JSPS, Japan.

References

Atrens, A., Hoffelner, W., Duerig, T. W. and Allison, J. E., 1983. Subsurface crack initiation in high cycle fatigue in Ti-6Al-4V and in a typical stainless steel, Scripta Metallurgica, 17, 5, 601–606. Duquette, DJ., Gell M., 1971. The effect of environment on the mechanism of Stage I fatigue fracture, Metall Trans, 2, 1325–31. Hong, Y., Liu, X., Lei, Z. and Sun, C., 2016. The formation mechanism of characteristic region at crack initiation for very-high-cycle fatigue of high-strength steels. Int. J. Fatigue, 89, 108-118. McEvily, AJ., Gonzalez, Velazquez JL., 1992. Fatigue crack tip deformation processes as influenced by the environment, Metall Trans A, 23, 2211–21. Murakami, Y., Kodama, S., Konuma, S., 1989. Quantitative evaluation of effects of nonmetallic inclusions on fatigue strength of high strength steels. I: basic fatigue mechanism and evaluation of correlation between the fatigue fracture stress and the size and location of non-metallic inclusions. Int J Fatigue, 11, 291–8. Murakami, T., Nomoto, T., Ueda, T., 1999. Factors influencing the mechanism of superlong fatigue failure in steels, Fatigue Fract Eng Mater Struct, 22, 581-590. Nakamura, T., Oguma, H. and Shiina, T., 2003. Influential factors on interior-originating fatigue fractures of Ti-6Al-4V in gigacycle region: focusing on stress ratios and internal environment of material, Ti-2003, Science and Technology, 1775–1782. Nakamura, T., Oguma, H., Shiina, T., 2010. The effect of vacuum-like environment inside sub-surface fatigue crack on the formation of ODA fracture surface in high strength steel, Procedia Engineering, 2, 2121-2129. Neal, D. F. and Blenkinsop, P. A., 1976. Internal fatigue origins in α - β titanium alloys, Acta Metall urgica, 24, 1, 59–63. Nishijima, S., Kanazawa, K., 1999. Stepwise S–N curve and fish-eye failure in gigacycle fatigue. Fatigue Fract Eng Mater, 22, 601–7. Nishitani, H., Chen, D., 1984. Stress intensity factor for a semi-elliptic surface crack in a shaft under tension. Trans JSME A, 50, 1077–82. Oguma, H. and Nakamura, T., 2010. The effect of microstructure on very high cycle fatigue properties in Ti-6Al-4V, Scripta Materialia, 63, 1, 32–34. Oguma, H., Nakamura, T., 2013. Fatigue crack propagation properties of Ti-6Al-4V in vacuum environments, Int J Fatigue, 50, 89-93.