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通常，アルミニウム合金ダイカスト材料は，表面が鋳肌のまま使用される．鋳肌近傍には，欠陥が不均一に分布していると考えられ，試験前にその領域は除去されるため，従来の評価において鋳肌近傍の欠陥はほとんど無視されていると思われる．本研究では，鋳肌を有する試験片を用いて疲労試験を行い，疲労寿命について統計的に解析した．また，表面欠陥を3Dで可視化するとともに，疲労き裂発生のその場観察を行うため，シンクロトロ...ン放射光を用いたマイクロトモグラフィを適用した．これにより，短寿命を呈した試験片の鋳肌近傍には，直径10μm程度のミクロポアが並んで存在し，これが早期の疲労き裂発生を引き起こしたことが明らかとなった．疲労破壊のその場観察は，凝集したミクロポアから疲労き裂が発生したことを明瞭に示した．
Usually, aluminum die-cast alloys are used with casting surface, therefore it can be assumed that defects exist in the sub-surface regions. These defects are largely neglected in conventional evaluations as these regions are removed before testing. In the present study, fatigue tests were performed using specimens with casting surface. Fatigue life was statistically analyzed, and synchrotron radiation X-ray microtomography was also applied to visualize surface defects in 3D together with the in-situ observation of crack initiation. The results revealed that specimens with shorter fatigue lives exhibit aligned micropores of approximately 10μm in diameter in a sub-surface region, which inevitably cause premature fatigue crack initiation. The in-situ observation clearly revealed fatigue crack initiation at agglomerated micropores.show more
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