重複荷重疲労強度に関する一考察

抄録

Rotary bending fatigue tests were carried out with specimens of 0.25% carbon steel, varying stress amplitude in two stages. After receiving any stress cycles at the first stage, the stress amplitude was changed to the second stage and fatigue tests were carried out till the test pieces were fractured. When the stress amplitude of the second stage is higher than that of the first stage, the cumulative cycle ratio exceeded unity, sometimes reaching infinity. When the stress amplitude of the second stage is lower than that of first stage, the cumulative cycle ratio exceeded also unity or was equal to unity.
Now, suppose that a material which received the repeated stress of the first stage by N₁ cycles, was fractured after receiving the same stress amplitude by N₂ cycles, then we are compelled to consider that the material was damaged by the repeated stress of the first stage and the residual life was shortened. In the present experimental results, the material was not only damaged but also strengthened by the repeated stress of the first stage if the repeated stress amplitude of the second stage is higher than that of the first. Therefore, we can not predict whether the repeated stress of the first stage damaged the material or not if the repeated stress amplitude of the second stage to be followed is not known.
Such a phenomena will be explained if we consider the process of fatigue as follows. The material is hardened gradually by the repeated stress, i. e. the material becomes brittle and proceed the way to fracture, reducing the residual life. At the same time, the material which has been hardened by the repeated stress receives artificial strain ageing by the action of the repeated stress and the material is strengthened, prolonging the life before cracks originate. These two actions coexist in the process of fatigue. By the change of the repeated stress from the first stage to the second which is, e.g. higher than the first, both weakening and strengthening actions are increased. If, in this case, the increment of both actions are the same, the cumulative cycle ratio is equal to unity, but if the increment of the strengthening action is larger than that of the weakening action, cumulative cycle ratio will exceed unity.