2004 Fiscal Year Final Research Report Summary
The Effect of Vacuum on the Defect Size Dependence of Fatigue Crack Growth Limit (Aiming at Clarifying the Very High Cycle Fatigue Mechanism of High Strength Materials)
| Project/Area Number |
15560056
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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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 | HOKKAIDO UNIVERSITY |
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Principal Investigator |
NAKAMURA Takashi Hokkaido University, Graduate School of Engineering, Associate Professor, 大学院・工学研究科, 助教授 (30237408)
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| Project Period (FY) |
2003 – 2004
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| Keywords | Interior Originating fracture / Surface Originating Fracture / Very High Cycle Fatigue / Fatigue Limit / High Vacuum / Small Crack / Fractography / Small Defect |
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| Research Abstract |
With respect to fatigue of high strength materials as high strength steel and Ti-alloy, in the very high cycle region over 10^7 to 10^8 cycles, fatigue fractures occur from the interior of the materials. Through our research till now, the research leader indicated that this phenomenon can be explained by assuming that the environment around interior-originating crack tip is similar to a high vacuum environment. To verify this assumption, in this research we conducted fatigue tests of micro-defective materials both in the air and in a vacuum using a vacuum rotating bending machine newly developed by the research leader for the purpose of (1)investigating fatigue behaviors originating from micro defects in a vacuum and clarifying an effectt of vacuum pressure on ΔK_<th>, and (2)investigating environmental effects around interior cracks by comparing initial fracture surface in a vacuum with that of interior cracks.
Testing involved fatigue tests under 10^<-3> -10^<-4> Pa vacuum pressure us
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ing high strength steel specimens with artificial defects of 30μm in diameter and 30μm in depth. The result showed the fatigue life in a vacuum showed approximately 3-4 times longer than those in the air. On the other hand, fatigue limit in a vacuum lowered approximately 50MPa compared with that in the air. In such case as the cracks originating from the very small defects, ΔK_<th> in a vacuum was revealed to be lowered compared with that in the air.
The initial fracture surfaces around micro-defects were analyzed by SEM. It has been known that around origins of interior cracks there are special surface areas consisting of concavo-convex pattern smaller than structure unit ; nevertheless, the surface of high strength steel under vacuum pressure of 10^<-3> -10^<-4> Pa did not show such special fracture surfaces. On the contrary, the experiment under 2x10^<-5>Pa vacuum pressure conducted showed such special fracture surface clearly particularly with Ti-alloy.
From the above, our assumption (underlined sentence described above) can explain the phenomena that interior fractures occur in low stress in the long life region, and at the same time, we assume environmental effects of interior originating cracks over initial crack propagation fracture surfaces, vacuum pressure in particular ; seem to be different based on materials. Less
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