過大過小応力下の疲労き裂伝ぱと開閉挙動および転位組織

概要

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Fatigue crack propagation by periodic overstressing, i. e. by intermittent application of a very small number of cycling of overstress during a very large number of cycling of understress below K_<th>, was studied with plate specimens under the in-plane reversed bending stress condition. This condition is a more sophisticated test condition from the standpoint of fracture mechanics than the rotating bending test previously reported. Remarkable acceleration of crack propagation amounting to more than one hundred times was observed in the present study as in the previous ones, indicating the validity of the previous results. Crack closure levels were not affected appreciably by the periodic overstressing in most of the tests. An extra-ordinarily well developed dislocation cell structure was observed in thin foil specimens containing a fatigue crack by means of ultra-high voltage electron microscopy. The acceleration could be attributed not to mechanical causes such as crack closure but to microscopic processes such as the change of dislocation structure observed in the present study or the change of crack propagation mechanism revealed by fractography in the previous studies.
1979-09-15