In predicting the limited real fatigue life of structural parts at high temperature, it is generally accepted that the creep and/or environmental damages affect the fatigue life. And as a result, the concept of them is included in the life prediction methods. However, both of them are characteristically time dependent and thermally activated processes. Again, materials, which are used for the structural parts, such as type 304L stainless steel, usually contain several allowing elements to compose the complex compounds as the hardening components. And the behavior of such compounds is also known to be one of the time dependent and thermally activated processes by nature within the matrix. One of the examples is the dynamic strain ageing (DSA) which is generally observed as the serrated flows in the load-elongation curves. Simply and syllogistically, it is believed that the effects of alloying elements on the fatigue behavior, especially, at high temperature should be also probed. Because it is another aspect of time dependent and thermally activated process which can influence the fatigue behaviors at high temperature.
For these reasons, it is necessary to study the fatigue behavior of the structural materials. And the objectives of this study is to propose another consideration in the methods of fatigue life prediction including the concept of creep and environmental damages.
Based on the above discussions, high temperature low-cycle fatigue behavior of type 304L stainless steel was investigated. The testing temperature was set at 873K(0.48Tm) and 973K(0.54Tm), as these range of temperatures are known and proved to be related with DSA for this material. Within these temperature ranges, the boundary of the strain rate for the occurrences of DSA is also proved to be $4 × 10^{-3}/s$. For these reasons, the upper and lower boundaries of strain rate for the fatigue tests were simply set to include the strain rate of $4× 10^{-3}/s$. In order to find out and to extrapolate the fatigue behavior of structural parts, the effect of symmetrical strain rate was first probed at each temperature. The variations of the symmetrical strain rate were from $8 × 10^{-3}/s$ to $3.8 × 10^{-4}/s$, as the strain rate of $8 × 10^{-3}/s$ is the upper limit of the testing equipment. Another interesting experimental procedure was also applied in order to find out the systemmatic influence of DSA on the fatigue behavior. That is to vary the wave form maintaining the constant duration time of 110 sec. for one cycle, such as slow-fast or fast-slow non-symmetrical cycling.
From the above experiments, following conclusions are drawn. The effect of DSA on the fatigue behaviors is proved to increase the fatigue life during symmetrical cycling. For the non-symmetrical cycling, the behaviors are different depending on test temperatures. At 873K, the abnormally increased fatigue life was discovered. However, at 973K, the abnormally decreased one was detected. This strongly suggests that the influence of DSA on the fatigue behaviors should be considered in the life prediction method including the concept of creep and environmental damages.
