ひずみ振幅変動に伴って生ずる繰返し応力-ひずみ挙動と疲労寿命

抄録

The effects of slip character in metal polycrystals on the cyclic hardening and softening behaviors were investigated under the multiple repeated tests at two strain levels, and the fatigue lives of copper and α-brass in the double repeated fatigue tests were considered.
The main results obtained are as follows:
(1) The stress range after changing the strain amplitude approaches gradually to the inherent value of the virgin material for S10C and copper which show a wavy slip mode. On the other hand, for α-brass having planar slip character, the stress range approaches to the same value as that of the virgin material as the strain level is increased. However, as the strain level is decreased, the stress range reaches the value larger than the inherent value of the virgin material after the transient increase in the early stage. These results can be explained by considering the slip character of materials and the aging effect in α-brass.
(2) For S10C and copper, the stress range for a given plastic strain range just after the change in the strain amplitude corresponds to the state of hardening just before the change. On the other hand, for α-brass the former does not always correspond to the latter. This characteristic property for the cyclically deformed α-brass can be explained by considering both the slip character and the aging effect of this material.
(3) The cyclic stress-strain relation for α-brass of planar slipping mode is sensitively affected by its strain history, while for copper of wavy slip character this relation is scarcely dependent on it.
(4) The double repeated fatigue tests show that the cumulative cycle ratio does not depend on the stressing sequence for copper, while for α-brass it is larger than one in the High-Low test, but less than one in the Low-High test. These test results correspond well with the cyclic strain hardening and softening behaviors shown in (1) and (2).