弾性履歴と疲れ強さ

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タイトル別名
  • Elastic Hysteresis and Fatigue Strength
  • ダンセイ リレキ ト ツカレ ツヨサ

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Elastic hysteresis, in this paper, means the deviation from the linear elastic line in the stress-strain diagram. Under alternating stress near and above the endurance limit, some magnitude of plastic strain takes place together with the ordinary elastic one in the material. As a result, so-called hysteresis loops are obtained in the stress-strain diagram. The plastic strain in this sense had been considered so far to play an important role for fatigue phenomena of metals, and a number of literatures had been issued discussing the fatigue mechanism relating to the hysteresis loop and the plastic strain, mentioned above. Nevertheless, the magnitude of this plastic strain is so small that it is considerably difficult to be detected during the fatigue test especially in direct stress near the endurance limit. But, it is still now interesting subject to clarify the relation between the fatigue strength and the elastic hysteresis. Recently, we have constructed an equipment to magnify the total elongation of the specimen up to 2000 times, whose principle is same as that of the Martens mirror extensometer, using two rhombuses with a mirror. With this equipment, it is possible to detect the strain of the order 10-6. The performance test proved the good and stable accuracy. The specimens of 0.21% C-steel were tested under direct alternating stress with zero mean stress with Haigh Fatigue Testing Machine at room temperature, and the total strain was measured using the above equipment during the fatigue test. The plastic strain was calculated by deducing the linear elastic strain from the measured total strain. Test results showed that, the plastic strain increased in most cases at very early stage of fatigue, then decreased and became constant at a certain level. Furthermore, the plot of the plastic strain at the constant level against the number of stress cycles to failure in both logarithmic scales indicates the good linear relation.

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