Issue 48

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 48 (2019) 545-553; DOI: 10.3221/IGF-ESIS.48.52 550 structure at near-threshold levels is attributable not to crack closure but to grain size, i.e., the effect of slip-band growth resistance by grain boundaries and misorientations [37, 38]. This same effect, in which a bimodal harmonic structure affects  K eff,th , has also been observed for a Ti-6Al-4V alloy [29]. To compare the small and long crack propagation behaviors of SUS304L, the crack growth rate, d a /d N , under four-point bending is calculated using the data shown in Fig. 5. Fig. 6(b) plots the dependence of the crack growth rate, d a /d N , on  K for small and long cracks in the untreated series. For the small crack, the d a /d N values are slightly higher than those for the long crack under comparable  K . In addition, a small crack in the untreated series propagates at  K values lower than the  K eff,th for a long crack. The results show that the  K th value for small cracks in the untreated series is lower than that for long cracks with the elimination of crack closure. The same effect has been observed for metallic materials [39, 40]. Consequently, the harmonic-structured SUS304L exhibits a lower resistance to fatigue crack growth owing to grain refinement, and the formation of pores is suppressed by MM. In the future, the three-dimensional fatigue crack shape [41- 46] and the misorientations near the crack initiation site [47-49] will be measured for SUS304L with fatigue damage using synchrotron radiation to describe the mechanism of fatigue crack initiation and propagation in greater detail. C ONCLUSIONS he effect of a bimodal harmonic structure on the fatigue properties under four-point bending and fatigue crack propagation in austenitic stainless steel (JIS-SUS304L) was investigated. The main conclusions obtained can be summarized as follows: (1) Mechanical milling suppresses the formation of pores in SUS304L during the subsequent sintering process, owing to the plastic deformation of SUS304L powder surface. (2) The harmonic-structured SUS304L exhibits a higher fatigue limit than the sintered compact prepared from as-received powders, which had pores and a coarse microstructure. (3) The d a /d N values for harmonic-structured SUS304L are higher than those for steel fabricated from as-received powders with coarse grains for comparable values of  K . (4) The  K th for harmonic-structured SUS304L is lower than that for steel fabricated from coarse-grained as-received powders. This difference is attributable to a reduction in  K eff,th , resulting from the presence of fine grains in the harmonic structure. A CKNOWLEDGEMENT his work was supported by JSPS KAKENHI Grant Number JP15K05677 and JP18H05256. R EFERENCES [1] Baddoo, N.R. (2008). Stainless steel in construction: A review of research, applications, challenges and opportunities, J. Constr. Steel Res., 64(11), pp. 1199-1206. DOI: 10.1016/j.jcsr.2008.07.011. [2] Kikuchi, S., Sasago, A., Komotori, J. (2009). Effect of simultaneous surface modification process on wear resistance of martensitic stainless steel, J. Mater. Process. Technol., 209(20), pp. 6156-6160. DOI: 10.1016/j.jmatprotec.2009.04.024 [3] Kikuchi, S., Nakahara, Y., Komotori, J. (2010). Fatigue properties of gas nitrided austenitic stainless steel pre-treated with fine particle peening, Int. J. Fatigue, 32(2), pp. 403-410. DOI: 10.1016/j.ijfatigue.2009.07.019. [4] Kikuchi, S., Nakahara, Y., Dobashi, K., Komotori, J. (2012). Effects of FPP/gas nitriding hybrid surface treatment on fatigue properties of austenitic stainless steel (SUS316), J. Soc. Mater. Sci., Jpn., 61(8), pp. 680-685. DOI: 10.2472/jsms.61.680. [5] Kikuchi, S., Yasutake, Y., Komotori, J. (2012). Effect of fine particle peening on oxidation resistance of austenitic stainless steel, J. Solid Mech. Mater. Eng., 6(6), pp. 431-439. DOI: 10.1299/jmmp.6.431. [6] Kurashina, Y., Hamano, T., Miyata, S., Komotori, J., Koyama, T. (2014). Proliferation of calf chondrocyte on stainless- steel surfaces with different microtopography, J. Jpn. Inst. Met. Mater., 78(4), pp. 170-176. T T