1989 年 38 巻 434 号 p. 1316-1321
Low-cycle fatigue tests were conducted in order to investigate the effects of strain wave shape, strain hold time and mean stress on the low-cycle fatigue properties of modified 9Cr-1Mo steel at 550°C in air.
The life reduction of fast-slow and compressive-strain hold cycling was observed at the same degree as slow-fast and tensile-strain hold cycling, respectively. But the life of both tensile- and compressive-strain hold cycling with the same hold time approximately equaled that of fast-fast cycling.
We concluded that the life reduction was caused by the time-dependent damage which was induced by the difference of the tension going period from the compression going period.
However, the mean stress effect may be another cause for the life reduction, because the positive mean stress was noticeable for fast-slow cycling.
A creep-fatigue damage model connected with the overstress was applied to the life prediction. By analyzing the stress-strain response under unloading, the degree of overstress was experimentally determined as a function of the inelastic-strain rate. Based on our test results, the time-dependent damage parameter of the damage model was modified. The predicted lives by this modified damage model agreed well with the experimental results.