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.