7 CONCLUSIONS
This paper investigated the strengthening effect of low amplitude loads
over the fatigue limit on residual cycle life in two-level loading.
Two-level load tests with different preload cycles under elevated
temperatures were conducted on FGH96 smooth specimens. The results show
that the fatigue life of the specimen was increased after subjecting
low-amplitude loading. Then test data from various materials were
collected to investigate the influence of the preload cycle life
fraction and the stress level of the preloading on residual cycle life.
Finally, a novel cumulative damage model based on continuum damage
mechanics was proposed to consider the strengthening effect under
low-high loading sequences. The conclusions are as follows:
- In the low-high loading sequence, the residual fatigue life of FGH96
under high-amplitude loads can be higher than the life of virgin
specimens. The residual life first increases and then decreases with
preload cycles.
- It is assumed that the strengthening effect decreases with the stress
level of the pre-applied load, and this assumption was verified on
test data for various materials. For many materials, the strengthening
effect increases with the ratio of fatigue life corresponding to
preloading and subsequent stresses.
- By introducing a strengthening factor into the Chaboche model, a novel
cumulative damage model was proposed to take into account the
strengthening effect. Compared to existing models, the proposed model
provides a better description of the experimental results of FGH96. In
addition, for the collected experimental results of various materials,
the proposed model can provide accurate predictions using a set of
universal parameters instead of material-dependent parameters.