Issue 35

Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01 10 C ONCLUSIONS he effects of hydrogen on the fatigue crack growth rates of torsional prestrained ferritic–pearlitic low-carbon steels (JIS-S10C and JIS-S25C steels) were investigated. The following conclusions were obtained. 1. Hydrogen content increased with the torsional prestrain for both S10C and S25C steels. The hydrogen content of hydrogen-precharged torsional prestrained S10C steel was lower than that of S25C steel at the same torsional prestrain. No clear difference in the maximum hydrogen contents between the hydrogen-precharged torsional fractured specimens of S10C and S25C steels existed. 2. With respect to crack initiation, there was no obvious difference between the uncharged and hydrogen-precharged specimens in spite of the large amount of C H induced by torsional prestrain. The acceleration of fatigue crack growth by hydrogen was the main cause of the decreased fatigue life. 3. For the virgin material, no obvious effect of hydrogen on the fatigue crack growth rate was observed. In contrast, for torsional prestrained materials, the acceleration ratios, {(d a /d N ) H /(d a /d N ) U }, increased with the torsional prestrain and hydrogen content. However, an upper bound of {(d a /d N ) H /(d a /d N ) U } of approximately 30 was observed, even when large amounts of hydrogen were charged (10.0 ≤ C H ≤ 30.3 mass ppm). 4. A hydrogen content threshold was found; hydrogen content above this limit enhances the growth of the non- propagated crack, even for metals with lower hardness (HV < 200). A CKNOWLEDGMENTS his research has been supported by the NEDO project “Fundamental Research Project on Advanced Hydrogen Science (2006e2012)”. R EFERENCES [1] Saburo, M., Noriko, T., Yukitaka, M., Effects of Hydrogen on Fatigue Crack Growth and Stretch Zone of 0.08 mass%C Low Carbon Steel Pipe, Trans. Jpn. Soc. Mech. Eng. A, 74 (2008) 1528–1537. [2] Michio, Y., Takashi, M., Noriko, T., Hisao, M., Saburo, M., Effects of hydrogen gas pressure and test frequency on fatigue crack growth properties of low carbon steel in 0.1–90 MPa, Trans. Jpn. Soc. Mech. Eng. A, 80 (2014) SMM0254–SMM0254. [3] Yoshiyuki, K., Masanobu, K., Keiko, S., Jun-ichiro, Y., Effect of Absorbed Hydrogen on the Near Threshold Fatigue Crack Growth Behavior of Short Crack: Examination on Low Alloy Steel, Carbon Steel and Heat Resistant Alloy A286, Trans. Jpn. Soc. Mech. Eng. A, 74 (2008) 1366–1372. [4] Hiroshi, N., Ryota, K., Takayuki, F., Effects of Hydrogen on Tensile and Torsional Strength Properties of Torsional Prestrained Ferritic-Pearlitic Carbon Steel, Proceedings of International Hydrogen Conference (IHC 2012). [5] Yukitaka, M., Stress Intensity Factors Handbook (In 2 Volumes), Committee on Fracture Mechanics, Pergamon Press, Japan, (1987) 659–662. [6] Takai, K., Watanuki, R., Hydrogen in Trapping States Innocuous to Environmental Degradation of High-strength Steels, ISIJ international, 43 (2003) 520–526. [7] Tanaka, H., Homma, N., Matsuoka, S., Murakami, Y., Effect of Hydrogen and Frequency on Fatigue Behavior of SCM435 Steel for Storage Cylinder of Hydrogen Station (<Special Issue> Strength Problems of Materials Used for Hydrogen Energy System), Trans. Jpn. Soc. Mech. Eng. A, 73 (2007) 1358–1365. T T