Fig. 5 Fatigue crack aspect ratio vs. fatigue crack depth in sea water.
We compared dl/dN in seawater with reference to that 3/2 in air at Δ K = 5.5 MN/m, defining the ratio as an environmental acceleration factor. Figure 8 plots this factor at each temperature as a function of f. It might be noticed in Figure 8 that this factor exhibited a maximum at a certain f, and peak environmental acceleration factor value tended to increase with decreasing temperature in the examined range for both steel specimens, and increase with increasing of tensile strength of tested steels in order of SWRM10, and SWRH42A. To the authors' knowledge, not many previous works reported peak behavior for dl/dN with respect to f; one of the few examples was a study by Atkinson and Lindley [11] on the through-thickness cracking of A533B-1 steel in distilled water and En56C steel in 3% seawater.
Atkinson and Lindley's [11] results also exhibited a trend of increasing peak value of this factor with decreasing temperature from 80 ℃ to 50 ℃ and to 23 ℃, in agreement with the present observations. Atkinson and Lindley interpreted the observed decreasing trend of the environmental acceleration factor with decreasing f in the f range below the level yielding the peak environmental acceleration factor as being due to crack tip rounding caused by a relative decrease in crack growth rate vs. corrosion dissolution rate of the crack wall in the very low f range.
A report by Shimodaira and colleagues [12] on the through-thickness cracking of HT80 and SUS304 steels in 3% NaCl solutions also indicated a saturation environmental acceleration factor level of approximately 2 observed at around f = 0.3 Hz. Mori and Ohtsuka [13] reported that the rate of HT50-TMC-steel cracking in synthetic seawater was accelerated by a factor of 2〜3 with reference to that in air due to corrosion dissolution of the crack tip in seawater. The order of magnitude of the environmental acceleration factor in seawater in these early reports seems to be in rational accord with those derived in the present study : 1.1 for SWRM10, and 1.8 for SWRH42A at 308K; 1.5 for SWRM10, and 2.1 for SWRH42A at 298K; 1.5 for SWRM10, and 2.5 for SWRH42A at 288K. A report by Komai and coworkers [14] for HT50 steel in synthetic seawater also indicated a trend of decreasing crack growth rate in seawater with decreasing f. Their explanation for the observed trend was a reversed balance between the acceleration effect for crack growth due to corrosion dissolution of the crack tip and a deceleration effect due to crack tip rounding with decreasing f.
There are only a few reports comparing the fatigue crack growth rates of steel species in marine environment. Xing and Song [15] compared the crack growth rate of four low-alloy steels with varying C levels (A537, 15Mn steel, 20MnVB steel, and 45Mn steel) in 3% NaCl solution at ambient temperature; they reported that the crack growth rate increased from low-C steel A537 to high-C steel 45Mn in accordance with the increasing C level ; moreover, on the basis of inspection of the fracture surface, they found that corrosion-dissolution-assisted crack growth on low-C steel changed to hydrogen-assisted crack growth on high-C steel.
Our experimental finding indicate that the crack growth rate in seawater rose with greater steel strength. However, for the time being, we cannot fully explain why the crack growth rate rose with greater steel strength. Although the available reports suggest that the influence of HE may be counted as one of the possible causes, our inspection of the fracture surfaces found no evidence of HE involvement in the cracking, as discussed in some detail further below.
When testing at lower frequencies than those presented in Figure 8, there appeared threshold f, at or below which the crack growth rate in seawater slowed and finally stopped for both steels at 308K, 303K, and 298K, as shown in Figure 9. This threshold f decreased with decreasing temperature from 0.6 Hz at 308K, through 0.3 Hz at 303K to 0.15 Hz at 298K for SWRM10, and for SWRH42A, from 0.2 Hz at 308K to 0.1 Hz at 303K. Crack arrest was reported in earlier works under the following circumstances: in NaCl solutions in Iow-Δ K and low-f ranges [16]; at simulated splash zones using synthetic seawater [17] ; and under cyclic wetting / drying conditions with synthetic seawater for surface cracks [18]. In these earlier works, the cause of crack arrest was identical as crack tip rounding due to an increased corrosion dissolution rate of the crack wall.
