PSI - Issue 71

K.M.K. Chowdary et al. / Procedia Structural Integrity 71 (2025) 188–195

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Fig.5 . Stress dependence of minimum creep rate of IN-RAFM steel at 823 K.

Fig.6 . Variation of rupture life with applied stress of IN RAFM steel at 823 K.

At a lower stress, the material deforms slowly towards failure causing a lower minimum strain rate value thereby increasing the λ value. The damage simulation results with respect to normalized creep exposure at various stresses for IN-RAFM steel at 823 K shown in Fig.7. The damage value reaches unity at failure.

Fig.7 . Variation of damage with Normalised creep exposure time of the steel at 823 K.

Fig.8 . Comparison of predicted creep FE-CDM (Sinh model) curves with uniaxial experimental creep curves of IN-RAFM at 823 K.

Chowdary et al., 2024 reported that the properties of modified 9Cr-1Mo steel are comparable to those of IN-RAFM steel, and the creep curves predicted by the KR model exhibit similar deviations from the experimental data. Figure 8 shows the comparison of the predicted creep curves by using the Sinh model with the uniaxial experimental creep curves. It clearly indicates that the sigmoidal bend occurs at the same time period as observed in the experimental creep curve of primary to secondary creep stage transition. However, at this primary to secondary transition stage the predicted creep strain is lower than the actual experimental creep strain at all stresses. The predicted tertiary creep regime is also nearly coherent with the experimental data. A comparative study of CDM-based creep damage analysis using the Kachanov-Rabotnov (KR) model and the Liu-Murakami (LM) model was conducted for IN-RAFM steel at 823 K (Chowdary et al., 2024). The predicted creep curves from the KR model showed greater deviation from the experimental data compared to those from the LM model. However, the prediction accuracy was further improved using the Sinh model. The extensive tertiary creep in IN-RAFM steel is ascribed to the effects associated with microstructural degradation. The microstructural damage that occurs upon creep exposure include coarsening of precipitate particles & martensitic lath, and recovery of dislocation substructures. In consequence to this, the physical damage i.e., intergranular/transgranular creep cavitation occurs followed with mechanical instability (due to neck formation) and failure. The aforementioned microstructural damage processes act alone or collectively to cause the