Abstract
Continued from the 1st and 2nd reports, we studied the fatigue processes for the high (0.82% C) and low (0.08% C) carbon steels with 86% reduction of area in cold drawing.
The results obtained are as follows:
(1) The fatigue process in case of the repeated stress below the fatigue limit.
In high carbon steel, the progress of fatigue does not occur with increase of number of stress cycles. In low carbon steel, when the repeated stress is much lower than the fatigue limit, it does not occur, but when the stress is near the fatigue limit, it occurs up to the stage of formation of submicroscopic cracks.
This formation of submicroscopic cracks was found in tension-worked or strain aged low carbon steel (See the 2nd report). Therefore, we may suppose that this formation of submicroscopic cracks by the repeated stress below the fatigue limit occurs in the cold worked low carbon steel alone.
(2) The fatigue process in case of the repeated stress above the fatigue limit.
This process is characterized by the same four stages as mentioned concerning the annealed steel (see the 1st report). But the relative duration of these stages differs according to the kind of carbon steel, high or low.
(3) The cycle ratio at the time of formation of submicroscopic cracks.
a) It shows a nearly constant value regardless to the size of repeated stress. (In annealed steels mentioned in the 1st and 2nd reports, it became small with increase of repeated stress.)
b) It is much smaller in low carbon steel than in high carbon steel. That is, the former shows a value of about 10%, and the latter about 85%.
(4) The comparison with the result of study by the electro-magnetic method (previous reported).
These two results closely coincide with each other. Therefore, we could confirm the reliability of our method of static torsional test.
