Effect of Oxygen and of Water Vapor on the Fatigue Life of Nickel at 300 C Available to Purchase

The fatigue behavior of high-purity nickel, vibrated in reversed bending, was studied at 300 C in oxygen and in water vapor at pressures from 10^-9 to 10 torr. Fatigue life was reduced in both oxygen and water vapor, but the variation in life with pressure differed somewhat for the two. The curve of fatigue life versus oxygen pressure is step-shaped with little or no dependence of life on pressure at very low and high pressures, the major reduction taking place at intermediate pressures. In contrast, the fatigue life in water vapor drops off gradually throughout the entire high-pressure range, but the maximum reduction is much less than in oxygen. Evidence obtained by residual gas analysis shows that a hot ionization gage filament may change the composition of the environment which, in turn, may affect fatigue life. Metallographic examination of failed specimens disclosed a relation between deformation and cracking characteristics and oxygen pressure. A calculation of the pressure required to saturate the crack surfaces at an estimated rate of crack growth showed that the predicted pressure agreed with the highest transition pressure, at which the reduction in fatigue life approaches a maximum.