FCGRs in NR and TMCP S355 steels in air

The formation of the microstructural phase morphologies described in Section 2 created different local microstructural properties. The presence of a hard phase (P ) in a soft matrix (α ) is expected to influence localFCG behaviour in steel [38]. The crack paths in these microstructures are also expected to vary. Steel microstructure influences fatigue crack propagation primarily by changing the mode of crack propagation. The mode can change from ductile striation to cleavage mode and the combination of the two modes increases crack propagation rates. There are many studies that claimed that microstructure has negligible effect at intermediate FCGR s (Paris Region) [3][5][6][11][12] [13][14][18]. Some other study concluded that grain size affected sub-critical crack growth to the extent to which it affected fracture toughness,K Ic and the onset of rapid fracture, and that grain size did not affect the FCG threshold,ΔK th [4]. Zerbst [6] in his recent publication reported, I quote, ‘… Perhaps the most astonishing observation is that the curve, in the Paris Region, is only slightly influenced by the microstructure of the material within whole material classes such as steels or aluminium alloys .’ In fact, it is commonly reported that microstructure has little or no influence in the Paris Region of the da/dN sigmoidal curve in air. Part of the presentation in this paper is to show what appears to be an influence of the FCGR by microstructural phase morphologies in the Paris Region of the da/dN vs. ΔK curve.
In the recent studies, the authors evaluated the resistance of the NR and TMCP steels given in Table 2 to FCG . The details of the experimental methods have been discussed in Refs [22][36][39]. The results of that experiment - for FCGR tests in air are shown in Fig. 7. Shown also are the microstructures of the J2N and TMCP(G10 & G8) steels examined. In Fig. 7, it appears there is a demarcation between the FCGR of NR and TMCP in the Paris Region. The arbitrary line drawn as shown appears to be the separation line. The FCGR in G8 and G10 steels are essentially the same and significantly lower than that of the J2N steels under the same experimental conditions. Generally, the FCGR increased with increase in load level from 10kN to 12kN in J2N and also from 9kN to 10kN in TMCP . This is the expected trend in air, where increase in the fatigue load level increases the FCGR . We can also observe that the FCGR in J2N for 10kN under 2Hz and 5Hz are essentially the same. This tends to confirm that frequency has no effect on theFCGR of steel in air under the same loading conditions. To validate this finding, a comparative study of FCGR in NRand TMCP was carried out. The steel grade, chemical composition in wt% (approximated to two decimal places) and mechanical properties of the steels studied and compared with the data in this study are included in Table 4. The composition range for the steels in Table 4 is shown in Table 5. It is pertinent to note that BS4360 Grade 50D, Q345, RD480 and EH36 are equivalent to S355J2+N hot-rolled normalized steel. Q345 is common structural steel in China and used in the nuclear industry and petrochemical plants. Laurito studied FCGR in intercritical-isothermally-quenched grain-modified steel designated as RD480. Though the chemical composition is comparable to other steels, the properties of the microstructure generated may differ significantly due to this heat treatment. Two microstructures of the same ABS grade EH36 steel produced by NR and TMCP are presented. This steel is often used in shipbuilding. Comparing these two microstructures of the same alloy will clearly support this study. Table 6 lists all the studies and test conditions considered. All the steels are designed to be used in both onshore and offshore structures. They are fundamentally the same. The only difference perhaps is that the steels were produced under different proprietary methods by different steel plants. All the tests were done under constant-amplitude sinewave, except Cheng [40] who performed his test under random waveform. The general range for temperature of the studies is about 27 oC to 0oC.
From the conclusions in the literatures [41][42][43][44][45][46][47], stress ratio (R) in the range 0.1- 0.85, frequency in the range 0.005Hz – 50Hz and waveforms appear to have little or no effect on the FCGR in theα-P steels in air. Thus, the inferences here are within these limits.