PSI - Issue 19

Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 312–319 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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the specimen did not fail at N = 10 7 cycles, were not conservative due to hydrogen-induced Δ Κ th degradation, although the Δ Κ th values of the non-charged specimen were in good agreement with the prediction using Eq. (2). Kondo et al. (2008) and Ogawa et al. (2018) also reported the hydrogen-induced degradation of the Δ Κ th of small cracks in other materials with HV in excess of 300, but not in softer materials, suggesting that hardness is one of the determining factors in the hydrogen degradation of Δ Κ th . However, no such degradation was observed in the Δ Κ th of the large crack present in the 800-µm EDM-notched specimen. As a result, it can be inferred that the Δ Κ th degradation factor may depend on the fulfilment of the small-scale yielding condition which also determines the difference between small and large cracks. Yet, assuming that the circumferential notch was categorized as a large crack, the circumferentially-notched specimen proved to be an exception, since the Δ Κ th was degraded in that case. Furthermore, a conclusive explanation in support of the hypothesis has yet to be advanced. Nevertheless, this study produced some important experimental facts regarding the effect of hydrogen on the Δ Κ th of various sizes of defects. 4. Conclusions In this study, tension-compression fatigue tests were carried out on both non-charged and hydrogen-charged Alloy 718 with defects of various sizes, in order to investigate the influence of defects and hydrogen on the fatigue limit of the alloy. Based on the experimental results, the following conclusions were reached: 1. The fatigue limit of Alloy 718 with defects was classified into three stages, depending on the defect size: (i) harmless defect regime, (ii) small-crack regime and (iii) large-crack regime. The classification enabled a comprehensive evaluation of the fatigue limit of Alloy 718 in a wide range of defect sizes and diverse matrix characteristics, i.e. , grain size and hardness. 2. The fatigue limit was defined as the largest stress amplitude at which specimens did not fail when the number of cycles, N , reached 10 7 cycles. In Alloy 718 specimens with small defects, the fatigue limit was degraded by 25% ~ 30% as a result of solute hydrogen, depending on the individual defect size and the hydrogen content. 3. Based on the crack-growth behavior of cracks progressing from the small-crack regime to the large-crack regime, it was inferred that the hydrogen-induced degradation of the threshold stress intensity factor range, Δ Κ th , was determined by whether or not the small-scale yielding condition had been fulfilled. This study was performed as part of a collaborative research project between the Japan Aerospace Exploration Agency (JAXA) and Kyushu University, Japan. References Yadollahi, A., Mahtabi, M. J., Khalili, A., Doude, H. R., Newman, J. C. Jr, 2018. Fatigue life prediction of additively manufactured material: Effects of surface roughness, defect size, and shape, Fatigue and Fracture of Engineering Material and Structure 41, 1602 – 1614. Yamashita, Y., Murakami, T., Mihara, R., Okada, M., Murakami, Y., 2018. Defect analysis and fatigue design basis for Ni-based superalloy 718 manufactured by selective laser melting, International Journal of Fatigue 117, 485 – 495. Matsuoka, S., Yamabe, J., Matsunaga., H., 2016. Criteria for determining hydrogen compatibility and the mechanisms for hydrogen-assisted, surface crack growth in austenitic stainless steels, Engineering Fracture Mechanics 153, 103 – 127. Ogawa, Y., Matsunaga, H., Yamabe, J., Yoshikawa, M., Matsuoka, S., 2017. Unified evaluation of hydrogen-induced crack growth in fatigue tests and fracture toughness tests of a carbon steel, International Journal of Fatigue 103, 223 – 233. Ogawa, Y., Matsunaga, H., Yamabe, J., Yoshikawa, M., Matsuoka, S., 2018. Fatigue limit of carbon and CrMo steels as a small fatigue crack threshold in high-pressure hydrogen gas, International Journal of Hydrogen Energy 43, 20133 – 20142. Murakami, Y. and Endo, M., 1986. Effects of hardness and crack geometries on Δ Κ th of small cracks emanating from small defects, Mechanical Engineering Publications, The Behaviour of Short Fatigue Cracks, 275 – 293. Kevinsanny, Okazaki, S., Takakuwa, O., Ogawa, Y., Okita, K., Funakoshi, Y., Yamabe, J., Matsuoka, S., Matsunaga, H., 2019. Effect of defects on the fatigue limit of Ni-based superalloy 718 with different grain sizes, Fatigue and Fracture of Engineering Materials and Structure 42, 1203 – 1213. Ono, Y., Yuri, T., Nagashima, N., Ogata, T., Nagao, N., 2015. Effect of microstructures on high-cycle fatigue properties of Alloy718 plates, IOP Conference Series: Material Science and Engineering 102, 012001. Kondo, Y., Kubota, M., Shishime, K., Yamaguchi, J., 2008. Effect of absorbed hydrogen on the near threshold fatigue crack growth behavior of short crack (examination on low alloy steel, carbon steel and heat resitant alloy A286), Transactions of the JSME (in Japanese) 746, 68-74. Acknowledgments