Influence of Prior Austenite Grain Size and Stress Ratio on Near Threshold Fatigue Crack Growth Behavior of High Strength Steel

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  • 高強度鋼の下限界値近傍における疲労き裂進展挙動に及ぼす旧オーステナイト粒径および応力比の影響
  • コウ キョウド コウ ノ カゲンカイチ キンボウ ニ オケル ヒロウ キレツ

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The influence of prior austenite grain size and stress ratio on near threshold fatigue crack growth behavior has been studied by means of fracture mechanics and fractography on quenched and tempered Ni-Cr alloy steel where the prior austenite grain size was varied from 21 to 210μm. The results obtained were as follows.<br>(1) For the mid-range of crack growth rates above 5×10-6mm/cycle, the dominant fracture appearance was striation formation regardless of prior austenite grain size and stress ratio. On the other hand, for the near threshold crack growth rate below 5×10-6mm/cycle, the striation-like appearance was mixed with microstructure sensitive fracture appearance such as intergranular facet.<br>(2) The crack growth rate associated with striation formation was insensitive to the change in prior austenite grain size for all stress ratio, and increased slightly as the stress ratio increased from 0.05 to 0.3. On the other hand, no marked difference in crack growth rate appeared for stress ratio of 0.3 and 0.6. The effect of stress ratio was explained by the crack closure phenomenon.<br>(3) The near threshold crack growth rate at low stress ratio was sensitive to the change in prior austenite grain size, while that in high stress ratio was little sensitive. The greatest sensitivity to stress ratio appeared at near threshold crack growth rate regardless of prior austenite grain size.<br>(4) The consequence of coarsening of prior austenite grain size was to reduce ΔKth in all stress ratio. The ΔKth of both prior austenite grain size decreased with increasing the stress ratio. Also, the grain size dependence was observed to decrease with increasing stress ratio.

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