Abstract
In order to obtain fatigue strength data in long life range over 108 stress cycles under multiple two-step varying load conditions with high frequency of Hi/Lo block cycles, an automatic load changing device for uniform rotating bending fatigue testing machine was developed. With this device can be conducted the repeated Hi/Lo varying load fatigue tests with short interval of the higher stress level of about 100 cycles. It was confirmed, through experiments done by using JIS. S45C round bar specimens, that the level of performance of the newly developed load changing device was high. Furthermore, the discussion on the results from the viewpoint of the linear cummulative damage laws showed that, in some cases, the effect of understressing on fatigue lives appeared in two opposite manners, i.e., fatigue lives became considerably longer than estimated values or vice versa, especially when the number of understressing cycles was sufficiently large. To reveal such a inconsistent effect of understressing, and further, to clarify the fatigue behaviors in long life range up to 2×109 stress cycles, the authors intend to conduct a series of experiments systematically.
