Mechanical component failure is usually caused by metal fatigue originating from small defects in metallic materials. Thus, it is important to precisely capture the fatigue properties of materials containing small defects. Fatigue tests of materials with artificial surface defects introduced by drilling have been conducted. Using the resulting data, an equation for predicting the material fatigue limit has been proposed on the basis of the √area parameter model, and its effectiveness has been confirmed for various materials. However, for additive manufactured (AM) materials that contain internal defects resulting in failure, controlling the size of the defect where the fracture originates is extremely difficult. Therefore, verification of the predictive ability of the √area parameter model for AM materials is impossible, in contrast with other materials that fail because of surface defects. In this context, developing a technique to intentionally introduce internal defects with arbitrary sizes at arbitrary locations can provide insights that help predict the fatigue limit of AM materials. This study aimed to establish a technology for quantitatively evaluating the effect of internal defects on the fatigue properties of AM materials by introducing internal defects with arbitrary sizes at arbitrary locations via AM. Specimens with different defect sizes and locations were prepared. Prior to the fatigue tests, the defect sizes and locations were measured non-destructively via X-ray computed tomography (CT). The fatigue tests were conducted in air at room temperature. All the specimens failed because of the intentionally introduced internal defects, and the fatigue lives became shorter with increasing defect sizes, except for the specimens with defects adjacent to the surface. In those cases, fatigue cracks easily reached the surface; therefore, the fatigue lives were speculated to be shorter than those of the specimens with the same defect sizes. Moreover, the defect sizes determined from the fracture surfaces by scanning electron microscopy were nearly consistent with those determined by X-ray CT.

1. [1] M. Seifi, M. Gorelik, J. Waller, N. Hrabe, N. Shamsaei, S. Daniewicz, and J. J. Lewandowski, “Progress towards metal additive manufacturing standardization to support qualification and certification,” JOM, Vol.69, No.3, pp. 439-455, 2017. https://doi.org/10.1007/s11837-017-2265-2
2. [2] K. Egashira, T. Furumoto, K. Hishida, S. Abe, T. Koyano, Y. Hashimoto, and A. Hosokawa, “Formation mechanism of pores inside structure fabricated by metal-based additive manufacturing,” Int. J. Automation Technol., Vol.13, No.3, pp. 330-337, 2019. https://doi.org/10.20965/ijat.2019.p0330
3. [3] M. Panchenko, E. Melnikov, V. Moskvina, S. Astafurov, G. Maier, K. Reunova, V. Rubtsov, E. Kolubaev, and E. Astafurova, “The effect of hydrogen-charging on mechanical properties of austenitic CrNi steel fabricated by wire-feed electron beam additive manufacturing,” E3S Web Conf., Vol.225, 01011, 2021. https://doi.org/10.1051/e3sconf/202122501011
4. [4] M. S. Pham, B. Dovgyy, and P. A. Hooper, “Twinning induced plasticity in austenitic stainless steel 316L made by additive manufacturing,” Mater. Sci. Eng. A, Vol.704, pp. 102-111, 2017. https://doi.org/10.1016/j.msea.2017.07.082
5. [5] T. R. Smith, C. S. Marchi, J. D. Sugar, and D. K. Balch, “Effects of extreme hydrogen environments on the fracture and fatigue behavior of additively manufactured stainless steels,” Proc. of the American Society of Mechanical Engineers (ASME), Pressure Vessels and Piping Conf., V06BT06A036, 2019. https://doi.org/10.1115/PVP2019-93903
6. [6] Y. J. Yin, J. Q. Sun, J. Guo, X. F. Kan, and D. C. Yang, “Mechanism of high yield strength and yield ratio of 316L stainless steel by additive manufacturing,” Mater. Sci. Eng. A, Vol.744, pp. 773-777, 2019. https://doi.org/10.1016/j.msea.2018.12.092
7. [7] Y. M. Wang, T. Voisin, J. T. McKeown, J. Ye, N. P. Calta, Z. Li, Z. Zeng, Y. Zhang, W. Chen, T. T. Roehling, R. T. Ott, M. K. Santala, P. J. Depond, M. J. Matthews, A. V. Hamza, and T. Zhu, “Additively manufactured hierarchical stainless steels with high strength and ductility,” Nat. Mater., Vol.17, No.1, pp. 63-71, 2018. https://doi.org/10.1038/nmat5021
8. [8] R. Silverstein and D. Eliezer, “Hydrogen trapping in 3D-printed (additive manufactured) Ti-6Al-4V,” Mater. Charact., Vol.144, pp. 297-304, 2018. https://doi.org/10.1016/j.matchar.2018.07.029
9. [9] Z. Zou, M. Simonelli, J. Katrib, G. Dimitrakis, and R. Hague, “Microstructure and tensile properties of additive manufactured Ti-6Al-4V with refined prior-β grain structure obtained by rapid heat treatment,” Mater. Sci. Eng. A, Vol.814, 141271, 2021. https://doi.org/10.1016/j.msea.2021.141271
10. [10] H. Xiao, S. Li, X. Han, J. Mazumder, and L. Song, “Laves phase control of Inconel 718 alloy using quasi-continuous-wave laser additive manufacturing,” Mater. Des., Vol.122, pp. 330-339, 2017. https://doi.org/10.1016/j.matdes.2017.03.004
11. [11] A. Hirayama, M. Kimura, M. Kusaka, and K. Kaizu, “Microstructure and mechanical properties of AlSi12CuNi alloy fabricated by laser powder bed fusion process,” Int. J. Automation Technol., Vol.15, No.4, pp. 388-395, 2021. https://doi.org/10.20965/ijat.2021.p0388
12. [12] B. Barkia, P. Aubry, P. Haghi-Ashtiani, T. Auger, L. Gosmain, F. Schuster, and H. Maskrot, “On the origin of the high tensile strength and ductility of additively manufactured 316L stainless steel: Multiscale investigation,” J. Mater. Sci. Technol., Vol.41, pp. 209-218, 2020. https://doi.org/10.1016/j.jmst.2019.09.017
13. [13] Y. Murakami, H. Masuo, Y. Tanaka, and M. Nakatani, “Defect analysis for additively manufactured materials in fatigue from the viewpoint of quality control and statistics of extremes,” Procedia Struct. Integr., Vol.19, pp. 113-122, 2019. https://doi.org/10.1016/j.prostr.2019.12.014
14. [14] Kevinsanny, S. Okazaki, O. Takakuwa, Y. Ogawa, Y. Funakoshi, H. Kawashima, S. Matsuoka, and H. Matsunaga, “Defect tolerance and hydrogen susceptibility of the fatigue limit of an additively manufactured Ni-based superalloy 718,” Int. J. Fatigue, Vol.139, 105740, 2020. https://doi.org/10.1016/j.ijfatigue.2020.105740
15. [15] R. Biswal, X. Zhang, A. K. Syed, M. Awd, J. Ding, F. Walther, and S. Williams, “Criticality of porosity defects on the fatigue performance of wire + arc additive manufactured titanium alloy,” Int. J. Fatigue, Vol.122, pp. 208-217, 2019. https://doi.org/10.1016/j.ijfatigue.2019.01.017
16. [16] L. Carneiro, B. Jalalahmadi, A. Ashtekar, and Y. Jiang, “Cyclic deformation and fatigue behavior of additively manufactured 17–4 PH stainless steel,” Int. J. Fatigue, Vol.123, pp. 22-30, 2019. https://doi.org/10.1016/j.ijfatigue.2019.02.006
17. [17] A. H. Chern, P. Nandwana, T. Yuan, M. M. Kirka, R. R. Dehoff, P. K. Liaw, and C. E. Duty, “A review on the fatigue behavior of Ti-6Al-4V fabricated by electron beam melting additive manufacturing,” Int. J. Fatigue, Vol.119, pp. 173-184, 2019. https://doi.org/10.1016/j.ijfatigue.2018.09.022
18. [18] G. Meneghetti, D. Rigon, and C. Gennari, “An analysis of defects influence on axial fatigue strength of maraging steel specimens produced by additive manufacturing,” Int. J. Fatigue, Vol.118, pp. 54-64, 2019. https://doi.org/10.1016/j.ijfatigue.2018.08.034
19. [19] R. Molaei, A. Fatemi, N. Sanaei, J. Pegues, N. Shamsaei, S. Shao, P. Li, D. H. Warner, and N. Phan, “Fatigue of additive manufactured Ti-6Al-4V, Part II: The relationship between microstructure, material cyclic properties, and component performance,” Int. J. Fatigue, Vol.132, 105363, 2020. https://doi.org/10.1016/j.ijfatigue.2019.105363
20. [20] J. W. Pegues, S. Shao, N. Shamsaei, N. Sanaei, A. Fatemi, D. H. Warner, P. Li, and N. Phan, “Fatigue of additive manufactured Ti-6Al-4V, Part I: The effects of powder feedstock, manufacturing, and post-process conditions on the resulting microstructure and defects,” Int. J. Fatigue, Vol.132, 105358, 2020. https://doi.org/10.1016/j.ijfatigue.2019.105358
21. [21] E. Pessard, M. Lavialle, P. Laheurte, P. Didier, and M. Brochu, “High-cycle fatigue behavior of a laser powder bed fusion additive manufactured Ti-6Al-4V titanium: Effect of pores and tested volume size,” Int. J. Fatigue, Vol.149, 106206, 2021. https://doi.org/10.1016/j.ijfatigue.2021.106206
22. [22] Y. Y. Sun, S. L. Lu, S. Gulizia, C. H. Oh, D. Fraser, M. Leary, and M. Qian, “Fatigue performance of additively manufactured Ti-6Al-4V: Surface condition vs. internal defects,” JOM, Vol.72, No.3, pp. 1022-1030, 2020. https://doi.org/10.1007/s11837-020-04025-7
23. [23] Y. Murakami, “Metal fatigue: Effects of small defects and nonmetallic inclusions,” 2nd Ed., Academic Press, 2019.
24. [24] K. Komai, H. Matoba, and J. Kikuchi, “Fatigue crack growth and closure behaviors of high-tensile strength steel in vacuum,” J. Soc. Mater. Sci. Japan, Vol.33, No.368, pp. 566-571, 1984 (in Japanese). https://doi.org/10.2472/jsms.33.566