PSI - Issue 60

Chitresh Chandra et al. / Procedia Structural Integrity 60 (2024) 165–176 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Beyond the hook portion, da/dN vs ΔK varies linearly with a positive slope which has been fitted to generate a logarithmic expression Eq. (1), between da/dN and ΔK . The closeness of the values of Residual sum of square (RSOS), Pearson’s R and R 2 (COD) confirm the accuracy and relevance of the fit as in Table 3. From the table, it can be observed that the slope for the curve corresponding to a dwell period of 600 sec is greater than the one corresponding to a dwell period of 10 sec for the same force ratio. At 0.1 force ratio the slopes are nearly same for both hold time but as the load ratio (R) rises from 0.1 to 0.5, the value of slope increases with the hold time. ( ) = (∆K) (1) Table 3: Details of fitting of (da/dN) vs ∆K plot S. ID R t h (sec) RSOS Pearson’s r R 2 (COD) Adj. R. Square A n 02 0.1 10 0.52493 0.98538 0.97098 0.97007 2.36E-12 5.77 10 0.1 600 0.79297 0.98467 0.96958 0.96888 2.41E-13 6.58 05 0.5 10 0.85655 0.97609 0.95274 0.95169 1.39E-15 8.35 09 0.5 600 0.49090 0.97579 0.95216 0.95085 8.95E-25 15.51

Fig. 7: Relationship between (da/dt) avg and (C t ) avg Fig. 7 shows the variation of (C t ) avg with respect the crack growth rate during hold. From Fig. 7, it can be observed that at the start of the test, both the (C t ) avg and the crack growth rate are high. As the test progresses, both the values start decreasing till they reach a minimum and then again starts increasing. As discussed, in a prior paragraph, P91 steel is creep ductile in nature and hence, the ‘ (C t ) avg ‘ is deemed apt to quantify CFCG behaviour as per ASTM E2760 19. Narasimhachary and Saxena have mentioned that (C t ) avg is affected by ΔK and the dwell time, which renders (C t ) avg more appropriate for representing the CFCG data. For samples subjected to greater dwell period, the average crack growth rates (da/dt) avg are lower than those subjected to lesser dwell periods. This results due to the time-dependent damage originating from the crack tip (Narasimhachary S. B. et. al. 2013). The CFCG data has been found to lie within a narrow scatter band. Similar inferences were found in prior studies on 1.25Cr-0.5Mo (Yoon K. B. et. al. 1993), G115 (Tang Z. et. al. 2018) and P92 steels (Xu L. et. al. 2017).