Effect of Environment on Fatigue Cracks

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Abstract
In this review of the fatigue of metals in controlled gaseous environments, particular emphasis is placed on the mechanism of crack propagation as it is affected by the test variables. The crack growth rates of some metals are accelerated more by oxygen than by water vapor, while for others the reverse is true. Increases of cyclic frequency and of stress decrease the magnitude of the effect of environment. It is generally agreed that the mechanism is more an increase of the rate of crack propagation than of crack initiation. Of the two explanations proposed, the process of corrosive attack of the crack tip is favored over that of the prevention of rewelding of crack surfaces by the formation of oxide layers. Curves of fatigue life, or of crack growth rate, versus gas pressure show regions of little or no dependence, connected by a transition region of steep slope. In a quantitative treatment of the shape of the curve, the significance of the location of the transition region is discussed.