抄録
With the same apparatus used in the previous experiment(1), the change of hardness of duralumin was observed during the course of fatigue progress under reversed bending, and the effect of aging on the recovery of hardness of the fatigued specimen was also studied. Duralumin was tested in annealed state and in quenched one.
In the course of fatigue process under continuous repetitions of stress, the hardness decreases at first and after passing through a minimum it increases with the increase of the number of repetitions. As the number of stress repetitions increases furthermore, the hardness change occurs in, the quite same manner as above, that is, it decreases to a minimum value and afterwards increases gradually with the increase of stress repetition. The initial decrease and the subsequent increase of hardness in the early stage of repetitions are caused by the change in the outer layer of the specimen, and those in the next stage are caused by the change in the inner portion of the specimen. The number of repetitions at which the minimum value of hardness occurs becomes less as the intensity of stress increases. Further, the amount of minimum hardness in the early stage becomes less as the stress intensity increases so, that the hardness increases at the first application of a stress, when it is of a comparatively high intensity.
As to the effect of aging at room temperature, on the recovery of hardness of the fatigued specimen, the following results were obtained. After the. application of stress of a constant intensity, the recovery of hardness takes place in two different manners according to the number of repetitions to which the specimen has been subjected, i.e., when the number of repetitions is small, the hardness of a fatigued specimen increases gradually with the lapse of time and becomes constant after a certain time, but when the number of stress repetitions is large, the hardness decreases first quickly and then slowly and at last it attains a constant value. The higher the intensity of stress, the number of repetitions becomes small, by which the decrease of hardness is resulted in the early period of the rest.
The author has explained the above changes of hardness under continuous repetitions of stress by a combined effect of the accumulation of internal stress and the loss of stability of structure. He has also explained the hardness change due to aging by a combined effect of the release of internal stress and the recovery of unstable structure.
