PSI - Issue 60

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

239

7

Table 3. Outputs of the material parameters obtained from the analysis of CFCGR curves Specimen

Effective threshold stress intensity range, ∆K TH_EFF (MPa √m)

Paris exponent, m (dimensionless)

Paris coefficient, C (MKS units)

Critical stress intensity range, ∆K C (MPa √m)

Critical max. stress intensity range, K max_C (MPa √m)

FC

14 18

2.53 2.58

2.405 E-11 8.816 E-12

78 82

87 91

ICCP

Table 3 lists the output material parameters obtained from the analysis of CFCGR curves of FC and ICCP specimens describing the corresponding corrosion fatigue crack growth behaviours. ∆ K TH of this shipbuilding steel was estimated as 14 MPa √m in FC condition and 18 MPa √m in ICCP condition. Thus, it was found that steel exhibited higher sub-critical or threshold corrosion fatigue crack propagation resistance in ICCP condition as compared to FC condition. Air-environment fatigue crack growth rate (FCGR) curve (at an ambient temperature of 20 degrees Celsius) of DH36 shipbuilding steel, having a similar yield strength of 355 MPa to XS shipbuilding steel, was found in the literature(W. Zhao, G. Feng, H. Ren, B.J. Leira, 2019) . Threshold ∆K TH value of DH36 steel in air was found as 22 MPa √m. To summarize, threshold ∆K TH values were least for FC, intermediate for ICCP, and highest in air for this steel grade. These values give an idea about the relative contribution of the corrosive environment and were justifiably consistent. Stable linear region of CFCGR curves were fitted to power-law relationship to derive material constants (Paris exponent and Paris coefficient) as shown in Table 3 and Fig. 7. Higher correlation coefficient (R 2 ) of ICCP specimen (0.9681) to that of FC specimen (0.8733) indicate lower scatter or more stable crack growth rates in ICCP condition. Critical maximum stress intensity (K max_C ) was estimated from K max at final point of failure having accelerated unstable crack growth rates. K max_C was found to be mar ginally higher in ICCP condition (91 MPa √m) as compared to that in FC condition (87 MPa √m). 3.2 Fractography examination by scanning electron microscopy (SEM) In order to characterize the fracture surface morphology and identify the associated failure mechanisms, fractography of the CFCGR tested FC and ICCP specimens was carried out using SEM. Fractographs were recorded at incremental locations along the direction of crack propagation, characteristic to discrete levels of applied stress intensity ranges (ΔK). Also crack propagation direction is marked whereas the studied fractured surface is the principal stress plane or, plane of maximum normal tensile stress. Fig. 8 presents the SEM fractography of FC specimen examined at various ΔK levels at suitable magnifications. Quasi- cleavage (QC) fracture mechanism was observed for nearly the entire range of ΔK (25 -78). Fatigue striations (FS) were also visible in fracto graph at ΔK = 25. Crack front surface morphology at ΔK = 25 shows the de-bonding or de-cohesing of an inclusion (INC) particle from the matrix under the corrosive effect of chloride solution and associated micro-pit formation. This tendency of micro-pits formation by de-cohesing of multiple inclusions present in the matrix and their joining to form microscopic secondary cracks (SC) is also visible at ΔK = 55 and 75. Triangularly conjugated shear lips (SL) with aligned inclusions appears around the mid-thickness of the CT specimen at final fracture (ΔK = 78). The inclusions were identified as sulphides and oxides by SEM -EDS analysis as shown in Fig. 9. This indicated that aggressive corrosive chloride environment resulted into micro-pit formation by de-bonding or de-cohesion of non-metallic inclusions from the matrix. These micro-pits had further grown to increasing depths and diameters, finally joining to form micro-cracks and resulted into accelerated crack growth rates. It is reported by Martin et al (Martin, Robertson and Sofronis, 2011) that hydrogen-induced fracture in X60 steel was observed with typical fractographic features, such as, quasi-cleavage (QC), cracks at inclusions, secondary cracks (SC) and flat “feature - less” regions. Similarly, Chan et al (Chan, 2010) have reported that fatigue micro-cracks