Initiation and Propagation of Low-Cycle Fatigue Cracks in Fe-Ni-C Martensitic Steel

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  • Fe-Ni-Cマルテンサイト鋼における低サイクル疲労クラックの発生と伝ぱの観察
  • Fe-Ni-Cマルテンサイト コウ ニ オケル テイサイクル ヒロウ クラック ノ ハッセイ ト デンパ ノ カンサツ

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This investigation has been undertaken to determine the influence of microstructural factors (prior austenite grain boundaries, lenticular martensite leaves, non-metallic inclusions, etc.) on the characteristics of low-cycle fatigue cracks longer than 10μ (initiation period and sources, growing rate, propagation paths, etc.) in Fe-25% Ni-0.17% C martensitic steel, by conducting optical microscopic observations of the surface of smooth specimens. The fatigue tests were carried out at 20°C under a constant total-strain range of ±1.2% by alternating bending. The results obtained are summarized as follows:<br>A number of microcracks originate mostly from the prior austenite grain boundaries (γB), boundaries of martensite leaves (α'B) and non-metallic inclusions, and some of these microcracks spread out by means of growing and joining. The growing rate of cracks along α'B is almost equal to that along γB, but the propagating manners of these cracks are different each other. That is, a microcrack occurred in γB tends to grow into larger size than that occurred in α'B which often remains as a non-propagating. As for the propagation path, the path along γB is slightly longer than that along α'B, and the total length of these two paths accounts for above 80% of the whole crack length.

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