It is known that da/dN vs ΔKeff curve is independent on stress ratio and predicts the most conservative fatigue life of structures, where da/dN is crack growth rate and ΔKeff effective stress intensity range. In corrosive environment, however, determining ΔKeff is difficult because corrosion product within crack promotes the crack closure.
The purpose of this study is to describe an experimental method free from crack closure. Such a method is realized by decreasing ΔK where maximum load is kept constant and minimum load increases with increasing crack length. The experimental results, obtained for S45C and HT80 steels in 3%NaCl water, showed that crack closure did not occur when the test began at an initial condition of ΔK≅7 MPa·m1/2, R≅0.6 and dK/da=-2.3 GPa·m-1/2 and that da/dN continuously decreased down to the region of ΔK≅0.8 MPa·m1/2 and R=0.96. The minimum value of ΔK obtained in 3%NaCl water was one-third than the threshold in air at higher stress ratios such as 0.8. The crack growth curves thus obtained were useful for analyzing crack closure under constant amplitude loading with R=0.1 in 3%NaCl water.
