Direct Observation of Fatigue Crack by Scanning Electron Microscope and Its Propagation Mechanism

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  • 疲労き裂進展過程の走査電子顕微鏡による連続観察とき裂進展機構
  • ヒロウ キレツ シンテン カテイ ノ ソウサ デンシ ケンビキョウ ニヨル レ

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Fatigue crack propagation tests were carried out on a pure iron and a grain oriented silicon iron by using a specially designed testing apparatus in a scanning electron microscope. It was found from the tests on the poly-crystalline pure iron that surface cracks showed different behaviors depending on the grains. So it seems difficult to estimate the fatigue crack propagation mechanism under plane strain condition from observations of the surface crack propagation behaviors.<br>On the other hand, in the case of the grain oriented silicon iron nearly plane strain condition could be realized up to the specimen surface by selection of loading axis to the specific orientation of the material. From the sequential observations of crack opening and closure behavior on this material, the mechanism of Mode I type fatigue crack propagation was confirmed to be an alternating slipping-off process at the crack tip apex.<br>Quantitative evaluation of a relation between crack tip opening displacement (CTOD) and crack growth length was given by considering the slip direction, the crack tip angle and the amount of rewelding.

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