Giga-cycle Fatigue Properties for Spheroidal Graphite Cast Iron

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  • 球状黒鉛鋳鉄のギガサイクル疲労特性
  • キュウジョウ コクエン チュウテツ ノ ギガサイクル ヒロウ トクセイ

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Giga-cycle fatigue properties were investigated for spheroidal graphite cast irons of FCD400 and FCD800. Fatigue tests were carried out by ultrasonic fatigue testing at 20kHz and conventional servo-hydraulic fatigue testing at 30Hz using specimens with 3 and 6mm diameters under the stress ratio of R ＝ －1. The results of the ultrasonic fatigue testing showed negligible difference from those of the conventional fatigue testing. Accordingly, ultrasonic fatigue testing could be applicable to spheroidal graphite cast iron. Decrease of fatigue strength in the giga-cycle region was in range from 10 to 20%, Namely, fatigue limit was not confirmed even up to 1 × 1010 cycles. Fracture origins were microshrinkage in most cases, while the 3mm specimens of FCD800 showed graphites at the origin as well as microshrinkage. The microshrinkage size was larger in FCD800 than in FCD400, and larger in the 6mm specimens than in the 3mm specimens. The graphite size was much smaller than the microshrinkage, The fatigue strength of FCD800 was lower than that of hot-rolled steels on the fatigue strength versus tensile strength diagram, while that of FCD400 was almost equal to it. Namely, in contrast to FCD800, the fatigue strength of FCD400 was as excellent as that of the hot-rolled steels. This was likely to be caused by difference of the microshrinkage size and the microstructure between FCD400 and FCD800. The effect of specimen size on fatigue strength was not remarkable. However, the fracture sites were graphite and microshrinkage in the 3mm specimens, while all of them were microshrinkage in the 6mm specimens. This difference of the fracture sites was a kind of size effect.

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