24P048-P
Effect of Irradiation Damage on Fatigue Crack Growth Behavior in SteelMaterials
鉄鋼材料の疲労き裂成長挙動に及ぼす照射損傷の影響
Katsuya Yamamoto, Yuki Sato, Shuhei Nogami and Akira Hasegawa
山本雄也, 佐藤佑毅, 野上修平, 長谷川晃
Department of Quantum Science and Energy Engineering, Tohoku University
6-6-01-2, Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8579, Japan
東北大学量子エネルギー工学専攻 〒980-8579 仙台市青葉区荒巻字青葉 6-6-01-2
The effect of irradiation damage on the micro-crack initiation behavior under low cycle fatigue in SUS316L
austenitic stainless steel was investigated using proton irradiation and a low cycle fatigue test at room
temperature in air. The micro-crack initiation (initiation of crack with 10–40 μm surface length) was
observed at the slip band, twin boundary, grain boundary and triple junction in both the unirradiated andproton-irradiated specimens. Significant reduction of the micro-crack initiation life of each initiation site
and increment of the number fraction of the micro-crack initiation at the slip band and twin boundary
occurred due to the proton irradiation.

1. Introduction
It is important to improve the fatigue life
assessment of structural materials to meet thedemands of reliability and economy for fusion
reactors because a fusion reactor material must
support dynamic loads induced by thermal andelectromagnetic stresses under neutron irradiation.
The fatigue life is generally composed of
micro-crack initiation, crack propagation and final
fracture stages, therefore, fatigue life assessment of
the fusion reactor material might be improved byevaluating the micro-crack initiation behavior under
neutron irradiation.
The objective of this study is to investigate theeffect of irradiation damage on the micro-crack
initiation behavior under low cycle fatigue and its
mechanism in austenitic stainless steel.

Low cycle fatigue test was conducted at room
temperature in air using an electromotive testingmachine with a 2 kN load cell. The total strain
range parallel to the loading direction wasapproximately 0.6% at the bottom of the fatigue
specimen’s curve region, which were controlledusing a triangular wave (R = –1). The axial strain
rate was approximately 0.01%/s.
Surface morphology at the bottom of the fatigue
specimen’s curve region was observed using a fieldemission scanning electron microscope (FE-SEM)
and an atomic force microscope (AFM) byperiodically stopping the fatigue test. The electronbackscatter diffraction (EBSD) analysis for thesame region was also conducted using an
orientation-imaging microscope (OIM) equipped tothe FE-SEM.

2. Experimental Procedure
Solution-annealed SUS316L austenitic stainlesssteel was used in this study. The average grain size
was approximately 45 μm. Miniature plate
specimen with a notch was used for the evaluation.
The thickness, width of the minimum cross-section
and radius of the notch were 1.52 mm, 1.25 mm and10 mm, respectively.
2 MeV proton irradiation test was conductedusing the 4.5 MV Dynamitron accelerator ofTohoku University. The proton beam was irradiatedto the bottom of curve region of a specimen. The
displacement damage of the uniformly irradiatedregion was 0.25 dpa, which ranged from thespecimen surface to approximately 12 μm in depth
from the surface. The irradiation temperature wasapproximately 230ºC.

3. Results and Discussion
The micro-crack initiation was observed at theslip band, twin boundary, grain boundary and triple
junction
bothintheunirradiatedandproton-irradiated specimens, where the twinboundary was a Σ3 grain boundary with 60º
misorientation angle between adjoining grains.
Fig. 1 shows the micro-crack initiation life ateach initiation site of the unirradiated andproton-irradiated specimens. The micro-crack
initiation life was defined as the number of cycles
where a crack with 10–40 μm surface length wasobserved. The micro-crack initiation life (Ni) of theunirradiatedspecimenwasapproximately
4000–6000 cycles at all the initiation sites.
Approximately 75–83% reduction of the
micro-crack initiation life at the slip band and twinboundary (Ni = 1000) and approximately 50–67%
reduction at the grain boundary and triple junction(Ni = 2000) occurred due to the proton irradiation.
As shown in Fig. 2, the number fraction of each
initiation site significantly changed due to theproton irradiation. Increment of the number fractionof the initiation at the slip band and twin boundary
was caused by the irradiation.
It is well known that a localized deformationaccompanied by coarse slip formation in the graininterior occurs in irradiated austenitic stainless steel
due to obstruction of the dislocation motion byirradiation defects [1-6]. The localized deformation
occurred due to not only neutron irradiation but also
ion irradiation [1-3]. In the previous study [7], thelarger surface morphology change under low cycle
fatigue in the proton-irradiated SUS316L than theunirradiated one was considered to decrease themicro-crack initiation life at the slip band and twinboundary, probably due to the localized
deformation by the irradiation. Therefore, reductionof the micro-crack initiation life and increment of
the number fraction of the micro-crack initiation at
the slip band and twin boundary due to the proton
irradiation was considered to be induced by thelocalized deformation at a relatively early stage ofthe low cycle fatigue test.
4. Conclusion
The effect of irradiation damage on themicro-crack initiation behavior under low cycle
fatigue and its mechanism in SUS316L austenitic
stainless steel was investigated using proton

irradiation and a low cycle fatigue test at room
temperature in air. The results of this work aresummarized as follows:(1) The micro-crack initiation (initiation of crackwith 10–40 μm surface length) was observedat the slip band, twin boundary, grainboundary and triple junction both in theunirradiated and proton-irradiated specimens.
(2) Significant reduction of the micro-crack
initiation life of each initiation site andincrement of the number fraction of theinitiation at the slip band and twin boundaryoccurred due to the proton irradiation.
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8000

SUS316L
LCF at RT in air
ta=0.6%：Unirrad.
：Irrad.

6000
4000
20000SB

TB
GB
Crack initiation siteTJ

Fig.1. Micro-crack initiation life at each initiation site of
the unirradiated and proton-irradiated specimens (SB:slip band, TB: twin boundary, GB: grain boundary, TJ:triple junction)

Number fraction of crack initiation site [%]Number of cycles to initiation, Ni

10000

100SUS316L
LCF at RT in air, ta=0.6%

TJ
80

60

GB

40

200TB
SB
Unirrad.

Irrad.

Fig.2. Number fraction of each initiation site of theunirradiated and proton-irradiated specimens (SB: slipband, TB: twin boundary, GB: grain boundary, TJ: triplejunction)