Comparison in Crack Growth Rate between Impact Fatigue and Non-Impact Fatigue in High Strength Steel

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  • 高強度鋼の衝撃疲労および衝撃でない疲労のき裂進展速度の比較について　　衝撃負荷と応力比および強度レベルの関係
  • コウ キョウド コウ ノ ショウゲキ ヒロウ オヨビ ショウゲキ デ ナイ ヒ
  • 衝撃負荷と応力比および強度レベルの関係

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The impact fatigue tests were carried out by using a rotating disk type impact fatigue testing machine. The influence of impact loading, stress ratio and yield strength on fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered 0.50% carbon steel. Then, the results of impact fatigue tests were compared with those of nonimpact fatigue tests. The results obtained are summarized as follows.<br>(1) For the 423K tempered material, the dominant fracture appearance at impact fatigue was the dimple mode and the cleavage facet while that at non-impact fatigue was the intergranular facet. Such a fracture appearance was called the monotonic one. It was apparent that the acceleration of crack growth rate resulted from the rapid increase in the area percentage of monotonic fracture appearance regardless of the impact fatigue and the non-impact fatigue.<br>(2) For the other tempered materials except the 423K tempered one, the striation was the dominant fracture appearance. However, the fracture appearance of the 573 and 873K tempered ones contained some inclusions and precipitation particles among the striation.<br>(3) The effect of impact loading, Eac, defined by the ratio of impact fatigue striation spacing, Simp, to non-impact fatigue one, Snon, approached to about 1 as the yield strength, σys increased. The effect of impact loading could also be replaced by the equivalent stress ratio, Req. Then, the Req has related to σys as follows, <br>Req=1-α·Eac-0.5=1-1.88×10-3·α·σys0.84.<br>The Req decreased with increasing σys. This tendency was similar to the effect of stress ratio in non-impact fatigue.<br>(4) The impact fatigue crack growth rate at a given ΔK level was larger than that in non-impact fatigue. This difference in crack growth rate increased with increasing tempering temperature. This fact could be well explained by the concept of equivalent stress ratio.

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