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Substantial modifications have been made to a theory of metal fatigue proposed earlier by Valluri. Modifications are concerned with the nature of crack growth, the appropriate stresses that should be used in discussion of the crack growth, and a discussion of the internal stresses that are presumed to oppose the motion of dislocations. Some results of the modifications are (1) Stress distribution at the tip of the crack is dependent upon the natural plastic stress concentration factor, gross stress, and the crack length; (2) The effect of decrease in internal stresses gives rise to the concept of a “nucleation period” in low stress level fatigue; (3) Rate of crack growth is strongly dependent upon the net section stress, the stress ratio, and the plastic stress concentration factor.