介在物にトラップされた水素による超長寿命疲労破壊の促進と通常の疲労限度の消滅

概要

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The mechanism for fatigue failure in extremely high cycle fatigue in the regime of N>10^7 is discussed. A special attention was paid to a newly found particular fracture morphology in the vicinity of fracture origin (subsurface nonmetallic inclusions) of a heat treated Cr-Mo-alloy steels (Specimen QT). The particular morphology looks a dark area inside fish-eye mark by optical microscopic observation. Specimens with short fatigue life of the order of N_f=10^5 do not have such dark area, ODA (optically dark area), in fish-eye. To investigate the influence of the hydrogen trapped nonmetallic inclusions on fatigue properties, specimens quenched in a vacuum (Specimen VQ) were prepared. Specimens VQ contained 0.01 ppm hydrogen and on the other hand QT 0.7-0.9ppm hydrogen. Specimens VQ had much smaller ODA than Specimens QT. Hydrogen was detected by Secondary Ion Mass Spectrometry at the inclusion at fracture origin of Specimens QT but not at the inclusion of VQ. Thus, it can be concluded that the formation of ODA is closely related to hydrogen trapped by nonmetallic inclusions. The predictions of fatigue limit by the √<area> parameter model are approximately 10% unconservative for fatigue limit defined for 10^7 cycles. However, if we consider the area of ODA into the fatigue limit estimation, the √<area> parameter model can successfully predict the fatigue limit very accurately.