PSI - Issue 7

Ning Wang et al. / Procedia Structural Integrity 7 (2017) 376–382

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N. Wang et al. / Structural Integrity Procedia 00 (2017) 000–000

Fig. 5 Fatigue crack initiation morphologies from micro-defects for hydrogen charged specimens: (a) σ a =510 MPa, N f =8.8 × 10

5 cycles, (c) σ

a =500

MPa, N f =2 × 10

5 cycles, (e) σ

a =500 MPa, N f =1.8 × 10

5 cycles, (g) σ

a =510 MPa, N f =6.8 × 10

5 cycles; (b), (d), (f) and (h) are the magnified views of

the crack initiation area.

4. Conclusions In this work, axially push-pull fatigue tests up to the VHCF regime were performed in a low strength Cr-Ni-Mo V steel at room temperature under ultrasonic frequency. The influence of hydrogen and micro-defects on the long term fatigue behavior was discussed. The main conclusions are listed as follows: (1) For the low strength Cr-Ni-Mo-V steel investigated herein, S - N curves showed a continuously decreasing mode. Compared with as-received specimens, the hydrogen charged specimens have obviously lower fatigue strength and shorter fatigue lifetime with crack mainly initiated from interior or subsurface inclusions. (2) Hydrogen trapped around micro-defects activated several crack initiation sites and had a close link to the easiness of early interior cracking process resulting in weaker fatigue behavior. Acknowledgements The authors are grateful for the financial supports provided by the National Natural Science Foundation of China (Nos. 51575182 and 51405159) and the Program of Shanghai Subject Chief Scientist (14XD1401300). References Abe, T., Kanazawa. K., 1996. Influences of nonmetallic inclusion and carbide on high cycle fatigue strength of tool steels. Journal of the Society of Materials Science, Japan, 45(1): 9-15. Kanazawa K, Nishijima S., 1997. Fatigue fracture of low alloy steel at ultra-high-cycle region under elevated temperature condition. Journal of the Society of Materials Science, Japan, 46(12): 1396-1401. Katano, G., Ueyama, K., Mori, M., 2001. Observation of hydrogen distribution in high-strength steel. Journal of Materials Science, 36(9): 2277 2286. Kuroshima, Y., Shimizu, M., Kawasaki, K., 1993. Fracture mode transition in high cycle fatigue of high strength steel. Transactions of the Japan Society of Mechanical Engineers, Part A, 59(560): 119-124. Li, Y., Yang, Z., Li, S., et al., 2009. Effect of Hydrogen on Fatigue Strength of High - Strength Steels in the VHCF Regime. Advanced Engineering Materials, 11(7): 561-567. Murakami, Y., Endo, M., 1994. Effects of defects, inclusions and inhomogeneities on fatigue strength. International journal of fatigue, 16(3): 163-182. Murakami, Y., Takada, M., Toriyama, T., 1998. Super-long life tension–compression fatigue properties of quenched and tempered 0.46% carbon steel. International Journal of Fatigue, 20(9): 661-667. Murakami, Y., Nomoto, T., Ueda, T., 2000. On the mechanism of fatigue failure in the superlong life regime (N> 10 7 cycles). Part 1: influence of hydrogen trapped by inclusions. Fatigue & Fracture of Engineering Materials & Structures, 23(11): 893-902. Murakami, Y., 2002. Metal fatigue: effects of small defects and nonmetallic inclusions. Elsevier.