Fig. 16: Plot of FCGR in SW (a) NR and TMCP steels in the present study (b) CFCGR curves of α-P steels reported in the literatures
Fig. 16(a) appears to show the partitioning of the FCGR s ofNR and TMCP steels in SW into two domains as demarcated by the arbitrary dash line. The CFCGR s for theNR steel occupy the upper region of the dash line and that of theTMCP the lower region. The result of the NR steel tends to show that decreasing the fatigue load level has more accelerating effect than increasing the load as shown by the 9kN and 12kN under 0.3Hz. Selected results for the G8 steel study by Mehmanparast [59] show that increasing the fatigue load level to 10kN at 0.3Hz did not necessarily increased the CFCGR above that of 9kN. In fact, below about 18.50 MPa√m, the CFCGR of the 10kN decreased below that of 9kN. For the G10 steels, two load levels – 9kN, 10kN and two frequencies 0.3Hz, 0.5Hz were used. At about 20 MPa√m and above, there is no obvious difference in the CFCGR for the test conditions. In general, the CFCGR s of G10 for all the test conditions were slightly lower than that of G8 and the CFCGR s of all theTMCP subgrades were lower than that of the J2N steel. In fact, the dash line shows possibly the line of demarcation, though diffused, between the CFCGR s of the NR and TMCP S355 steel subgrades.
In a similar way to air study, the trend found in Fig. 16(a) is validated by carrying out a comparative study of CFCGR s between NR and TMCP α-Psteels reported in the literature. The steel grades and mechanical properties of the steels compared with the CFCGR data in the present study are shown in Table 7. It must be mentioned that there are limited studies on the corrosion fatigue of marine steels under the conditions listed in Table 7. Fig. 16(b) presents the CFCGRstudies on the steels listed in Table 7. A large separation in theCFGCR s between the NR and TMCP steel is once again observed. It can be observed from Table 7 and Fig. 16(b) thatCFCGR s of even some studies carried out at low stress ratios, such as 0.08 by Musuva [41] where higher than those of theTMCP in Fig. 16(b). It is often reported in the literatures that plasticity-induced, roughness-induced and oxide-induced crack closure can cause crack growth retardation. It is pertinent to note that during fatigue, the surface asperities or roughness is caused by the ductile stretching or tearing of the fractured surface. These features are areas of high energy and would corrode very fast if in contact with corrosive environment, leaving a smooth surface or deep contour in the case of plastically deformed regions. Hence, they would not contribute significantly to crack retardation. The only factor that may likely cause retardation is oxide-induced crack closure and the extent may depend on the flow rate of the corroding medium and the stress ratio.
Table 7: Experimental conditions for the CFCGR data used in this study