1988 Fiscal Year Final Research Report Summary
Study on Fatigue Crack Growth Mechanism under Service Loadings with the Aid of Direct Observations by Scanning Electron Microscope
| Project/Area Number |
62550073
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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 | Osaka University |
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Principal Investigator |
JONO Masahiro Faculty of Engineering, Osaka University, Professor, 工学部, 教授 (20029094)
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| Co-Investigator(Kenkyū-buntansha) |
SUGETA Atsushi Faculty of Engineering, Osaka University, Research Assistant, 工学部, 助手 (60162913)
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| Project Period (FY) |
1987 – 1988
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| Keywords | Fatigue Crack / Service Loading / Scanning Electron Microscope / Direct Observation / Fatigue Crack Growth Mechanism / Crack Closure / Crack Tip Opening Displacement / 画像処理 |
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| Research Abstract |
In order to make it possible to record the scanning electron microscope(SEM) images of fatigue crack under service loadings, secondary electron signals of the SEM were taken directly into the micro-computer by using a high speed direct memory access(DMA) type A/D converter. Sequential five frames of SEM images under cyclic loading of 0.1-1 Hz could be stored in a single operation and image processing techniques have been developed.
Fatigue crack growth tests under Hi-Lo two step loadings and repeated two step variable amplitude loadings were performed on a grain opiented 3 % silicon iron, and microscopic and macroscopic behaviors of fatigue crack such as crack growth direction, deformation near crack tip, crack growth rate, crack tip opening displacement(CTOD) and crack closure were investigated to elucidate the fatigue crack growth mechanisms. It was found that the CTOD is the governing parameter of fatigue crack growth under both loading conditions. In the case of former loading, however, fatigue crack was found to grow in a zig-zag manner after load changing, and retardation behavior of crack growth could not be explained even by the effective stress intensity facter, keff. On the other hand, under the repeated variable amplitude loadings, the relationship between CTOD and Keff was found to be similar with that of constant amplitude loading. Therefore, it was conformed that the fatigue crack growth rate under stationary varying loading conditions could be estimated in terms of the effective stress intensity range. In addition, growth mechanisms and estimation method of growth rate of elastic-plastic fatigue crack under variable amplitude loadings in the post-yield region were discussed through the fractographical investigations.
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