Figure 10 shows the inverse relationship between the threshold f leading to eventual crack arrest and seawater temperature derived on the basis of plots given in Figure 9 (open symbol for steady crack growth and solid symbol for crack arrest). It is evident that the threshold f depended on seawater temperature and steel strength, indicating a trend that the threshold f tended to increase with increasing temperature (Arrhenius linear relationship), and decrease with increasing steel strength. For the moment, the physical significance of the observed relationship between f and 1/T in Figure 10 is not clear to US. As described above, crack arrest might be fundamentally understandable in terms of crack tip rounding, and the cause for such tip rounding might be the reversed balance between crack growth rate and the rate of crack wall corrosion dissolution with decreasing f [2]. This model seems to provide a reasonable explanation for the experimental evidence obtained in the present study showing that the threshold f could be defined as reported by the other authors [11, 19], and that it diminished with greater steel strength because the crack growth rate was accelerated as steel strength increased. Figure 11 compares the appearance of fracture surfaces near the crack tip for the specimens tested in seawater at 308K. Among them, (a) and (c) were obtained in the most accelerated crack growth rate conditions in the respective environments.
Fig. 10 Arrhenius plot of critical cyclic frequency in sea water.
Fig. 11 Appearance of fatigue fracture surface in specimen.
