二相ステンレス鋼における大き裂と微小き裂の疲労き裂進展特性

概要

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Growth characteristics of large and small fatigue cracks have been investigated in dual-phase stainless steel consisting of ferrite and austenite phases. The results of large crack-growth tests revealed the extremely high crack-growth resistance of the present material, and showed high threshold-stress intensity ranges, ΔK_<th>, of 15 to 17 MPa √<m>. This behaviour is attributed primarily to significant roughness-induced crack closure resulting from Mode III displacement. Small cracks grew much faster than large cracks subjected to the same nominal stress intensity ranges, because the pronounced crack closure observed for large cracks was restricted for small cracks. This was supported by the close correspondence of small crack results for cracks larger than 100μm in length with large crack-growth rates plotted in terms of ΔK_<eff>. Small cracks mainly initiated at ferrite phase, and showed a growth perturbation due to microstructure in the region below 100μm. Therefore, the cracks smaller than 100μm could be considered to be microstructurally small cracks, for which the use of the continuum mechanics approach is inappropriate.
1990-04-25