<I>S</I>-<I>N</I> Curves of the Impact Fatigue (On the Formation and Propagation of Fatigue Cracks)

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  • 衝撃疲労における<I>S</I>-<I>N</I>曲線（疲労割れの発生と進展について）
  • 衝撃疲労におけるS-N曲線(疲労割れの発生と進展について)
  • ショウゲキ ヒロウ ニ オケル S-N キョクセン ヒロウ ワレ ノ ハッセイ ト シンテン ニ ツイテ

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Usually, U(energy)-N curves are used to study the impact fatigue, so we cannot compare the results with other impact fatigue data or non-impact fatigue data. So we tried to show a few example of S(stress)-N curves of the impact fatigue, based on the results of our previous experiments in which the relation between impact stresses and impact energy were studied. And we carried out the experiments on the stand-point of the formation and the propagation of fatigue cracks, and whether the stresses applied are tensile, compressive or tensile-compressive stresses alternately is very important to study fatigue phenomena. We used Matsumura’s impact fatigue testing machine and notched test pieces (annealed 0.2% and 0.3% carbon steels). We can compare the fatigue data under two different conditions. From S-N curves for impact fatigue (S is stress at the bottom of notch), N0 (number of blows at which the fatigue crack begins to propagate) is the largest for the Case C, and the smallest for the Case T-C. (Case C means that the stresses at the bottom of the notches are compressive stress, Case T means tensile stresses, and Case T-C means tensile and compressive stresses alternately). For the Case C, the relation between l (length of fatigue cracks) and N (number of blows), and V (velocity of propagation of cracks) and I are as follows. l=K logN/N_0  V=0.4343 K/N K is constant and means tangent of l−logN curves. We applied the above formula to the results of the Cases T and T-C approximately, and compared K and V for each case. K in the Cases T and T-C were almost the same, and K in the Case C was smaller. V in the Case T-C was the largest and in the Case C was the smallest.

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