予加工を施した銅および黄銅の疲労強度

抄録

Many metallic materials are put into practical use after strengthening or deforming into intended shapes by cold rolling, cold pressing or cold drawing. However, the fatigue strength does not always increase by cold deformation because of the increase of dislocation density and vacancy density or initiation of microcracks. Therefore, in order to clarify the relation between prestrain and fatigue strength, OFHC copper and Cu-36%Zn alloy (brass) were prestrained to various degrees and then fatigued to failure by the pushpull fatigue-test machine. The microstructures of the prestrained copper and brass were observed by optical and transmission electron microscopy, and the effects of prestrain on fatigue process were investigated.
As a result of the investigation, the fatigue limit of OFHC copper was found to decrease by prestraining of several per cents. This decrease was assumed to be due to the increase of free dislocations. However, by increasing the applied prestrain from few per cents up to 90 per cents, the fatigue limit of OFHC copper increased monotonously because of the formation of cell structures. On the other hand, the decrease of fatigue strength of the brass prestrained by a few per cent was very small, and the fatigue limit dropped down abruptly at the prestrain of about 20 per cents. The cause for this sudden decrease of fatigue strength seemed to be due to the formation of microcracks. The difference of the effect of microcracks on fatigue strength between OFHC copper and brass can be explained by the difference in notch sensitivity.