1988 Fiscal Year Final Research Report Summary
Correlation between Behavior of Small Cracks and Variation of Cumulative Fatigue Damage under Variable Amplitude Loadings
| Project/Area Number |
62550079
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
材料力学
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| Research Institution | KYUSHU UNIVERSITY |
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Principal Investigator |
MURAKAMI Yukitaka Dept. of Mechanics & Strength of Solids, Faculty of Engineering, Kyushu University, 工学部, 教授 (10038010)
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| Co-Investigator(Kenkyū-buntansha) |
TSURU Hideto The same as the head investigator, 工学部(昭和62年度), 助手 (10038060)
KOBAYASHI Masaki The same as the head investigator, 工学部, 助手 (90037789)
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| Project Period (FY) |
1987 – 1988
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| Keywords | Fatigue / Small Crack / Variable Amplitude Loadings / Critical Crack Initiation Stress / 累積疲労被害 |
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| Research Abstract |
The correlation between behavior of small fatigue cracks and variation of cumulative fatigue damge under the multiple repeated two-step stress loading were investigated using specimens with an initial small crack. Various combinations of the overstress h and the understress l cause various values of cumulative fatigue damage. The crtical stress wi for fatigue crack initiation from a small crack is the important measure to evaluate the contribution of l on fatigue damage.
The behavior of crack propagation showsmarked difference depending on whether the understress l is lower or higher than wi. Therefore, the values of cumulative fatigue damage D based on the Modified Miner Rule become much smaller than 1.0 for a certain combination of h and l . The minimum value of D obtained in the present study is D=0.18. Although wide scatter of D has been reported by many studies, it is not caused by statistical scatters of a material but caused by various combinations and sequence of h and l in relation to crack size at ever moment. The possible minimum value of D(Dmin) fro a definite cycle ratio of h and l under various combination of sequence of loadings can be predicted by the method proposed in the present study and D min will be useful for the design under variable amplitude loadings.
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