| Project/Area Number |
08405012
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| Research Category |
Grant-in-Aid for Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Materials/Mechanics of materials
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| Research Institution | KYUSHU UNIVERSITY |
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Principal Investigator |
MURAKAMI Yukitaka Kyushu University, Faculty of Engineering, Professor, 工学部, 教授 (10038010)
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| Co-Investigator(Kenkyū-buntansha) |
FUKUSHIMA Yoshihiro Kyushu University, Faculty of Engineering, Research Associate, 工学部, 助手 (40156774)
SAKAE Chu Kyushu University, Faculty of Engineering, Associate Professor, 工学部, 助教授 (00253477)
HIRAKAWA Kenji Kyushu University, Faculty of Engineering, Professor, 工学部, 教授 (30264097)
鳥山 寿之 愛媛大学, 工学部, 助教授 (30227681)
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| Project Period (FY) |
1996 – 1998
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| Project Status |
Completed (Fiscal Year 1998)
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| Budget Amount *help |
¥32,800,000 (Direct Cost: ¥32,800,000)
Fiscal Year 1998: ¥1,900,000 (Direct Cost: ¥1,900,000)
Fiscal Year 1997: ¥2,500,000 (Direct Cost: ¥2,500,000)
Fiscal Year 1996: ¥28,400,000 (Direct Cost: ¥28,400,000)
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| Keywords | Superlong fatigue / Nonmetallic inclusion / Fish-eye / Hydrogen embrittlement / Crack growth rate / The ROO<area> parameter model / SEM / AFM / √ areaパラメータモデル / 超寿命疲労 |
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| Research Abstract |
When fatigue life Nf of a specimen of 10mm in size is longer than 10^8, the average fatigue crack growth rate is much less than lattice spacing(- 0.1 _ or 0.01nm or 10^<-11-12>m/cyc1e). In the early stage of fatigue process, the crack growth rate should be much less than the average growth rate and accordingly we cannot assume that crack growth occurs cycle by cycle.
In this study, possible mechanisms for extremely high cycle fatigue are investigated. Of some possible mechanisms, a special focus was put on a newly found particular fatigue fracture morphology in the vicinity of fracture origin (nonmetallic inclusions) of a heat treated alloy steel, SCM435, which was tested up to N* 10^8 The particular morphology observed by SEM and AFM was presumed to be influenced by the hydrogen around inclusions. The predictions of fatigue limit by the ROO< > area parameter model are -10% unconservative for fatigue life of Nf= -10^8, though it successfully predicts the conventional fatigue limit defin
… More
ed for N=10^7. Thus, the fatigue failure for N*10^8 is presumed to be caused by a mechanism which induces breaking or releasing of fatigue crack closure phenomenon in small cracks.
In the vicinity of a nonmetallic inclusion at fracture origin a dark area was always observed inside a fish-eye mark for specimens with long fatigue life. Specimens with short fatigue life of Nf=-10^5 do not have such dark area in fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from usual fatigue fracture surface in martensite lath structure.
Considering a high sensititivity of high strength steels to hydrogen environment and high hydrogen content around inclusions, it may be concluded that the extremely high cycle fatigue failure of high strength steels from nonmetallic inclusions is caused by environmental effects such as hydrogen embrittlement coupled with fatigue.
Similar studies have been also done on an aluminum cast alloy, powder metals and medium carbon steels under tension-compression and reversed torsion. The fatigue fracture mechanism of various metals has been discussed from the viewpoint of superlong fatigue life. Less
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