6 DISCUSSIONS
The strengthening effect has been observed in many investigations. However, the existing fatigue life prediction models are mainly concerned with the strengthening effect caused by loads lower than the fatigue limit41. There is limited attention to loads higher than the fatigue limit. In this paper, we conducted two-level load tests in the low-high sequence. Then a new fatigue damage model based on the Chaboche model was developed to take into account the strengthening effect. The main conclusion of this paper is the effect of preloading cycle life fraction and load magnitude on the strengthening effect. It’s consistent with the test results of various materials in the literature44,45,49.
A limitation of the model is that only considering the preload conditions. The mechanism that yields the strengthening effect is not incorporated. In fact, there is still lack of a widely accepted explanation for the strengthening effect54,58. In many cases, the pre-applied low amplitude loads only cause damaging rather than strengthening46,47. Hence the increase in cycle life should be considered as a special case when specific materials and loading conditions are combined. The model is suitable for describing the phenomenon instead of predicting whether it occurs. In addition, most of the tests were conducted under two-level loading. The applicability of the model to multi-level loads needs further validation.
In general, our work incorporated the strengthening effect into the fatigue damage accumulation theory. Although the mechanism was not fully understood, this study illustrated that the strengthening effect is independent of the crack initiation mechanism. The tests in this study showed that the strengthening effect is significant in specific situations. For future research, a full understanding of the mechanism will be valuable for cycle life design.