FATIGUE LIFE BEHAVIOR OF COPPER SINGLE CRYSTALS. PART II: MODEL FOR CRACK NUCLEATION IN PERSISTENT SLIP BANDS

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Abstract
A crack nucleation model is constructed based on the random slip of the dislocations within the fatal persistent slip band (PSB). From such a model, we derive the notch‐peak probability function. The crack is considered to nucleate when a critical notch depth is attained. This depth is constant for applied strains corresponding to the plateau of the cyclic stress‐ strain curve because the stress is constant. The PSBs behave in such a way that the more concentrated their localized slip the fewer the cycles required to attain the critical notch depth. Using the experimental relation between the applied strain and the magnitude of the slip offsets (reported in Part I) in conjunction with observations of the notch distributions along the gage surface, we have obtained the relation between the applied strain and the cycles for crack nucleation. A log‐log plot of this relation shows a slightly different slope from that of the Coffin‐Manson life data, previously reported. However, by allowing for cycles spent in Stage II propagation, good agreement is obtained.