Fatigue Crack Growth Rates and Their Thresholds of High Strength Steels in Sea Water at the Zinc Potential

Abstract

Fatigue crack growth rates, da/dN, and their thresholds, ΔK_th, have been examined in sea water at zinc potential, sea water, and air, using seven kinds of 102-165 kgf/mm² grade steels at the frequency of 0.167 Hz and 20 Hz and at the stress ratio of 0.10 and 0.70. The following results are mainly obtained.
1) At the zinc potential, the effect of corrosion control against fatigue crack growth of 100-120 kgf/mm² grade steels is observed to be beneficial when ΔK <55-60 kgf/mm³/² at the stress ratio of 0.10 and at the frequency of 0.167 Hz. The value of ΔK, however, decreases to 30 kgf/mm³/² for the 165 kgf/mm² steel.
2) At the zinc potential, the effect of hydrogen on da/dN-ΔK curves is classified into two; one is that for the plateau behavior (intergranular failure) which is sensitive to temper brittleness and the other is for the threshold ΔK_th (trangranular failure) which is insensitive to that.
3) The value of ΔK_th gradually decreases with increase of yield strength. At the zinc potential, the value decreases catastrophically at the yield strength of more than about 120 kgf/mm² due to hydrogen.