PSI - Issue 14

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Procedia Structural Integrity 14 (2019) 978–985

2nd International Conference on Structural Integrity and Exhibition 2018 Effects of vacuum-like environment on small internal crack growth processes in very high cycle fatigue Takashi Nakamura a, * and Fumiyoshi Yoshinaka b a Division of Mechanical and Space Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo, Hokkaido, 060-8628, Japan b National Institutes for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan In recent years, very high cycle fatigue (VHCF) has become serious issues in the field of mechanical engineering. The VHCF phenomenon is mainly characterized by an internal fracture in cycles over 10 7 –10 8 . This type of fracture has been observed not only in ferrous metals but also in Ti alloys. Especially in Ti-6Al-4V, a remarkable decrease of fatigue strength is evident in internal fractures compared with surface fractures. However, the mechanism of internal fracture is not yet fully understood. The internal cracks seem to propagate through a vacuum-like environment that is shut off from the air.Therefore, the effects of oxidation or gas adsorption on crack growth could be almost ignored in the internal crack. On the basis of this concept, we conducted small crack growth tests in air and high vacuum and observed the fracture surfaces to clarify the effects of vacuum on crack propagation. In addition, we applied a micro computed tomography (µCT) technique using synchrotron radiation to observe the internal crack growth non-destructively. By comparing the crack growth process in vacuum with those of internal crack detected by µCT, the effects of vacuum-like environment on small internal crack growth processes were investigated. 2nd International Conference on Structural Integrity and Exhibition 2018 ffects of vacuu -like environ ent on s all internal crack gro th processes in very high cycle fatigue Takashi aka ura a, * and Fu iyoshi oshinaka b a Division of Mechanical and Space Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo, Hokkaido, 060-8628, Japan b National Institutes for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Abstract In recent years, very high cycle fatigue (VHCF) has become serious issues in the field of mechanical engineering. The VHCF phenomenon is mainly characterized by an internal fracture in cycles over 10 7 –10 8 . This type of fracture has been observed not only in ferrous metals but also in Ti alloys. Especially in Ti-6Al-4V, a remarkable decrease of fatigue strength is evident in internal fractures compared with surface fractures. However, the mechanism of internal fracture is not yet fully understood. The internal cracks seem to propagate through a vacuum-like environment that is shut off from the air.Therefore, the effects of oxidation or gas adsorption on crack growth could be almost ignored in the internal crack. On the basis of this concept, we conducted small crack growth tests in air and high vacuum and observed the fracture surfaces to clarify the effects of vacuum on crack propagation. In addition, we applied a micro computed tomography (µCT) technique using synchrotron radiation to observe the internal crack growth non-destructively. By comparing the crack growth process in vacuum with those of internal crack detected by µCT, the effects of vacuum-like environment on small internal crack growth processes were investigated. Abstract © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 2019 he uthors. ublished by lsevier . . This is an open access article under the - - license (https://creativeco mons.org/licenses/by-nc-nd/4.0/) Selection and peer-revie under responsibility of Peer-revie under responsibility of the SI E 2018 organizers. 1. Introduction In recent years, very high cycle fatigue (VHCF) and gigacycle fatigue have become serious issues in the field of mechanical engineering (Sakai, 2009). The VHCF phenomenon is mainly characterized by an internal fracture in cycles over 10 7 –10 8 . This type of fracture has been observed not only in ferrous metals but also in Ti alloys (Neal, et al., 1976, Atrens, et al., 1983, Nakamura, et al., 2003, Oguma and Nakamura, 2010). Especially in Ti-6Al-4V, a remarkable decrease of fatigue strength is evident in internal fractures compared with surface fractures; therefore, to clarify its mechanism is strongly needed for the safety use of this material. In addition to the peculiar fatigue properties in the VHCF regime, a unique fracture surface with a fine convexo-concave feature was also discovered. Fig. 1(a) and (b) show an example of the internal fracture surface of Ti-6Al-4V and its magnified view, respectively. A rough surface corresponding to the Stage IIa crack propagation process is observed in the interior of the specimen (Fig. 1(a)), and a few micrometer-size granular feature is clearly recognized around the origin site (Fig. 1(b)). The 1. Introduction In recent years, very high cycle fatigue (VHCF) and gigacycle fatigue have become serious issues in the field of mechanical engineering (Sakai, 2009). The VHCF phenomenon is mainly characterized by an internal fracture in cycles over 10 7 –10 8 . This type of fracture has been observed not only in ferrous metals but also in Ti alloys (Neal, et al., 1976, Atrens, et al., 1983, Nakamura, et al., 2003, Oguma and Nakamura, 2010). Especially in Ti-6Al-4V, a remarkable decrease of fatigue strength is evident in internal fractures compared with surface fractures; therefore, to clarify its mechanism is strongly needed for the safety use of this material. In addition to the peculiar fatigue properties in the VHCF regime, a unique fracture surface with a fine convexo-concave feature was also discovered. Fig. 1(a) and (b) show an example of the internal fracture surface of Ti-6Al-4V and its magnified view, respectively. A rough surface corresponding to the Stage IIa crack propagation process is observed in the interior of the specimen (Fig. 1(a)), and a few micrometer-size granular feature is clearly recognized around the origin site (Fig. 1(b)). The Keywords: Small fatigue crack growth; Vacuum; Gigacycle fatigue; Internal fracture; Fine granular area, Synchrotron radiation light Keywords: Small fatigue crack growth; Vacuum; Gigacycle fatigue; Internal fracture; Fine granular area, Synchrotron radiation light

* Corresponding author. Tel./fax: +81-11-706-6419. E-mail address: nakamut@eng.hokudai.ac.jp * Corresponding author. Tel./fax: +81-11-706-6419. E-mail address: nakamut@eng.hokudai.ac.jp

2452-3216  2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 10.1016/j.prostr.2019.07.079 2452-3216 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 2452-3216 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers.