PSI - Issue 60

Vivek Srivastava et al. / Procedia Structural Integrity 60 (2024) 233–244 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 10 presents the SEM fractography observations of the ICCP-protected specimen examined at various ΔK levels at suitable magnifications. It was observed that the micro -pits formed due to de-bonding of the inclusions from the matrix do not tend to corrode and exhibited insignificant growth because of the cathodic protection. Further, a well-defined signature pattern of formation of parallel secondary cracks (PSC) was observed, which grew parallel to each other throughout the entire range of ∆K (28 -82), but were perpendicular to the primary crack growth direction (with increasing ∆K). This mechanism can be explained by a schematic drawing (Fig. 11) that illustrates the process of primary crack splitting into multiple branches in as it grows in the CT specimen, which appear as PSCs across the A- A’ cross -sections. It further explains the increasing occurrences and distribution density of PSCs with increasing primary crack growth and hence, ∆K levels. Eventual decrease in the mean distance between two PSCs was also evident. These discussions clearly indicated the phenomena of primary-crack splitting and branching into multiple PSCs, whose propensity increased as the primary-crack advanced further. It has been reported by Zhan et al (Zhan, Hu and Zhang, 2020) in ferritic-bainitic steel (having similar YS to XS steel) that the energy of crack extension process is shared by the formation of SCs, leading to smaller crack driving force and hence retarded crack growth rates. Role of crack branching in reduction of FCGR of ferritic-pearlitic steel (of similar YS to XS steel) has also been reported by Mutoh et al (Y. Mutoh, Akhmad A. Korda, Y. Miyashita, 2007). This phenomenon was seen in the experimental CFCGR curves of the studied XS steel (Fig. 6), where the ICCP-curve was below the FC-curve.

Fig. 10. SEM fractography of ICCP specimen with fractographic features (A- A’ sections in Fig. 11): INC, PSC and SL perpendicular to crack growth direction (or increasing ∆K)