THE FATIGUE LIFE of structural components is determined by the sum of the elapsed cycles required to initiate a fatigue crack and to propagate the crack from subcritical dimensions to the critical size. Consequently, the fatigue life of structural components may be considered to be composed of three continuous stages: (1) fatigue-crack initiation, (2) fatigue-crack propagation, and (3) fracture. The fracture stage represents the terminal conditions (i.e., the particular combination of σ, a, and KIc) in the life of a structural component. The useful life of cyclically loaded structural components can be determined only when the three stages in the life of the component are evaluated individually and the cyclic behavior in each stage is thoroughly understood.