Abstract
Fatigue tests under superposed stress were carried out in air and in fresh water containing sodium nitrite inhibitor (NaNO2) at 80°C using a relatively large Cr-Mo cast steel specimen. The applicability of the equivalent frequency method (ELF method) and the range pair mean method (RPM method) was investigated. The modified Goodman's method (RPM method I) and Yamada's method (RPM method II) were used to evaluate the effect of mean stress on stress amplitude.
The best estimation of fatigue life among those three methods was RPM method II. The ratio of the experimental life, N to the estimated life, Nes tended to become low at the high life side for all the three methods. However, when RPM method II was applied on the basis of the completely reversed (stress ratio R=-1) S-N curve, in which the values under the fatigue limit were corrected, N/Nes became within the range of 1/3≤N/Nes≤5. It was found that NaNO2 was effective in the prevention of corrosion fatigue. However, the experimental life in fresh water containing NaNO2 at 80°C became a little bit lower than that in air. The relation between Nes and the cumulative damage, D was represented as follow.
Nes=C/D
C=1.00 (in air)
C=0.80 (in fresh water containing NaNO2 at 80°C)
