Mechanisms of Fatigue in the Creep Range

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Abstract
The mechanisms of deformation and fracture in creep and fatigue are surveyed. Interactions between time- and cycle-dependent mechanisms are discussed from the standpoint of cumulative damage laws for lifetime prediction. The major roles of temperature and time in the fatigue process are shown to be an increase in the homogeneity and irreversibility of slip, cavitation of particle-matrix interfaces, and the growth of surface oxide layers to a thickness that can affect crack initiation; the principal effect of cyclic loading on creep fracture is the acceleration of crack propagation resulting from resharpening of microcracks. One of the major areas of uncertainty in cumulative damage is concluded to be the influence—either beneficial or deleterious—of compressive loading. Cavities initially produced in tension may be sintered by diffusion or rewelding in compression; on the other hand, sharp microcracks generated in compressive creep can severely accelerate fracture. It is suggested that these effects may be understood by considering whether the deformation is planar or wavy.