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:
  1. 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.
  2. 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.
  3. 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.