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.