Fatigue tests under superposed stress have been considerably conducted in the past. The present authors also carried out the fatigue tests under the superposed stress on many engineering materials and proposed a fatigue life estimation method. But most of these investigations under the superposed stress have been concerned with the fatigue life reduction, and the studies on the mechanism of fatigue failure under the superposed stress are few.
It is the purpose of this paper to clarify the behavior of stress-strain response and plastic strain amplitude under the superposed stress.
Fatigue tests were conducted on two kinds of specimens made of cast steel under sinsoidal and superposed stress, and the stress-strain behavior was measured during the fatigue tests.
The experimental results are summarized as follows:
(1) When the plastic strain amplitude caused by the maximum stress amplitude (primary stress+secondary stress) and that by the secondary stress amplitude are taken into consideration, the experimental results under superposed stress agree with those under sinsoidal stress. But further investigations under the condition that the secondary stress amplitude is less than that of primary stress will be necessary to draw final conclusion.
(2) When the material elongates to the tension side under fatigue test (such as creep-wise), it is necessary to take account of such behavior into fatigue life estimation.
