Fatigue crack propagation behavior and the corrosion inhibition effect of sodium nitrite inhibitior were investigated from the observation of fracture surface of Cr-Mo cast steel under superposed stress both in air and in fresh water containing NaNO2 at 80°C.
Under zero-tention loading of single wave, the change of crack front profile, a/c (aspect ratio) and the relation between the crack growth rate, da/dN and stress intensity factor, ΔK were represented by the following equations.
a/c=0.90-3.98×10-1(a/D), D: Diameter of specimen.
da/dN=1.04×10-10(ΔK)4.06
Beach mark spacings coincided well with the block of superposed waves, Zp, and they were affected by the stress ratio, m. The beach marks were observed clearly at m=0.3, but not m=0.4 nor 0.5. The change of crack front profile was classified into two types. One was that where the crack propagated with a≅c in a semi-circular or semi-elliptical form. The other was that where the crack propagated with a<c, and afterwards propagated in a semi-elliptical form. The initial cracks always propagated with a<c in fresh water containing NaNO2 at 80°C. The fracture patterns influenced by corrosion were observed at the initial crack propagation region.