# Creep-Fatigue Behavior and Life Prediction of Medium-Si-Mo Ductile Iron
## 4 Conclusions  
In this study, low-cycle fatigue tests and fatigue tests with dwell periods were used to investigate the stress-strain response behavior and life characteristics of medium-silicon molybdenum ductile iron. Combined with creep tests, the relationship between the instantaneous inelastic strain energy density dissipation rate and the inelastic strain energy density at different temperatures was established. Based on these findings, a creep-fatigue damage model was developed using the linear damage summation (LDS) method, which integrates the creep strain energy density dissipation approach with the Ostergren method. The main conclusions are as follows:  
1. In low-cycle fatigue tests, medium-silicon molybdenum ductile iron exhibits cyclic hardening at room temperature and intermediate temperatures (400 \({}^{\circ}\)C), while significant cyclic softening occurs at elevated temperatures. Additionally, the hysteresis loops demonstrate nearly symmetric tension-compression behavior, and this material's plastic deformation capability is enhanced with increasing temperatures.
2. Under intermediate- and high-temperature conditions with dwell periods, the creep-fatigue cycles maintain the softening/hardening characteristics observed in LCF conditions. The stable cyclic yield radius and elastic modulus during loading-unloading phases remain essentially unchanged, indicating minimal alterations in the hysteresis loop shape. However, creep effects cause a slight upward shift in the yield center during a cyclic response.
3. In creep-fatigue interaction cycles, strain amplitude variations do not significantly alter the overall shape of fatigue hysteresis loops but result in a more pronounced upward shift in the yield center at lower strain amplitudes compared to higher ones. At elevated temperatures, since stress relaxation primarily occurs during the  
Figure 16: Life prediction results using the combined T-SEDE and Ostergren methods.  
initial dwell period, the stress values at the ends of the dwell times showed minimal differences across the three tested dwell periods.
4. Accurate characterization of damage caused by stress relaxation during dwell periods is crucial for creep damage calculation. While the TF model showed discrepancies with the experimental results, the combined T-SEDE model and Ostergren method provided excellent creep-fatigue life predictions, with only 2 out of 13 data points falling outside the 1.5\(\times\) scatter band.  
Conceptualization, M.L. and H.F.; methodology, M.L.; software, M.L.; validation, M.L. and P.R.; formal analysis, H.F.; investigation, M.L. and P.R.; resources, H.F. and P.R.; data curation, M.L.; writing--original draft preparation, M.L.; writing--review and editing, H.F.; visualization, P.R.; supervision, H.F.; project administration, H.F.; funding acquisition, H.F. and P.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Vehicle Power Basic Research and Innovation Program of China (grant number 3030021222472) and the National Funded Postdoctoral Researcher Program (grant GZC20233400).  
Not applicable.  
Data Availability Statement: Data are available upon reasonable request from the corresponding authors.  
The authors declare no conflicts of interest.