Issue 48

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 48 (2019) 545-553; DOI: 10.3221/IGF-ESIS.48.52 545 Focussed on “Crack Paths” Evaluation of fatigue properties under four-point bending and fatigue crack propagation in austenitic stainless steel with a bimodal harmonic structure Shoichi Kikuchi Shizuoka University, Japan kikuchi.shoichi@shizuoka.ac.jp, http://orcid.org/0000-0003-1127-8748 Yuta Nakatsuka, Yoshikazu Nakai Kobe University, Japan 161t347t@stu.kobe-u.ac.jp nakai@mech.kobe-u.ac.jp, http://orcid.org/0000-0002-4146-6670 Masashi. Natakani, Mie. Ota. Kawabata, Kei. Ameyama Ritsumeikan University, Japan rm005034@ed.ritsumei.ac.jp mie-ota@fc.ritsumei.ac.jp, https://orcid.org/0000-0002-3991-274X ameyama@se.ritsumei.ac.jp, https://orcid.org/0000-0002-4479-854X A BSTRACT . Austenitic stainless steel (JIS-SUS304L) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. Four-point bending fatigue tests and K - decreasing tests were conducted in air at room temperature under a stress ratio R of 0.1 to investigate fatigue crack propagation in SUS304L. The fatigue limit of this harmonic-structured material is higher than that of the material with a homogeneous coarse-grained structure. This is attributable to the formation of fine grains by mechanical milling and to the suppression of pore formation. In contrast, the threshold stress intensity range,  K th , for the harmonic- structured material is lower than that for the homogeneous coarse-grained material, while the crack growth rates, d a /d N , are higher at comparable  K . These results can be attributed to a reduction in the effective threshold stress intensity range,  K eff,th , due to the presence of fine grains in the harmonic structure. K EYWORDS . Fatigue; Fracture mechanics; Crack closure; Grain refinement; Powder metallurgy; Stainless steel. Citation: Kikuchi, S., Nakatsuka, Y., Nakai, Y., Nakatani, M., Kawabata, M.O., Ameyama, K., Evaluation of Fatigue Properties under Four-point Bending and Fatigue Crack Propagation in Austenitic Stainless Steel with a Bimodal Harmonic Structure, Frattura ed Integrità Strutturale, 48 (2019) 545-553. Received: 12.11.2018 Accepted: 22.12.2018 Published: 01.04.2019 Copyright: © 2019 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.