Effect of force ratio on creep-fatigue crack growth (CFCG) of P91 steel

Components operating at elevated temperature are often subjected to conjoint action of cyclic and static loading, rendering crack growth due to creep–fatigue interaction. It is a significant concern during the design and service life of the components made of P91 steel, which finds widespread use in conventional power plants. Creep fatigue crack growth (CFCG) tests have been performed on C(T) specimens with force ratios ranging from 0.1 to 0.8 at a temperature of 600 °C with a dwell period of 60 s. The CFCG results have been presented in terms of stress intensity factor range (Δ K ), ( C t ) avg , C * and ( C t ) SSC parameters. The variation in crack growth rate with Δ K , ( C t ) avg , ( C t ) SSC and C * at different force ratios has been described. In the d a /d N versus ∆ K plot, a point of inflection is observed, where the crack growth rate was minimum for each force ratio, which resulted in a hook-like portion, corresponding to about 17% of the total life cycle of the sample. It can be understood from the plots of (d a /d t ) avg versus ( C t ) avg that, the ( C t ) avg values differ substantially at lower crack growth rates for different force ratios but tend to merge together at higher crack growth rates. Fractographic examination was performed on the fracture surface by dividing into 3 equidistant parts (i.e., A, B & C) of total CFCG portion. Crack growth in the ‘A’ region occupies a majority (60–90%) of the total life-cycle of the sample, and the time spent for growth in the A regime goes on increasing with increase in force ratio, whereas the time spent for crack growth in B regime goes on decreasing. The sample was then subjected to EDAX analysis to chart out the oxidation profile of the fracture surface as a function of time of exposure.