Fracture Mechanics Approaches for Characterizing Creep-Fatigue Crack Growth

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Consideration of crack growth under creep-fatigue conditions can be the dominant factor in determining the allowable maximum stress and temperature as well as the design and remaining lives of elevated temperature components. Several advancements in the concepts of linear and nonlinear fracture mechanics have occurred over the past fifteen years which address creep-fatigue crack behavior growth. These developments are reviewed in this paper. The available data strongly demonstrate the importance of loading frequency and waveform on the creep-fatigue crack growth behavior of high temperature alloys. There is considerable data and analytical support in the literature to show that crack growth under gross plasticity conditions can be treated by the ΔJ-integral. Further, (Ct)avg and ΔJc parameters are needed if creep deformation accompanies crack growth. Several examples of creep-fatigue crack growth data correlated with (Ct)avg and ΔJc parameters are shown. It is also shown that (Ct)avg and ΔJc are equivalent parameters when extensive creep conditions occur. Under small-scale and transition creep conditions, ΔJc is not rigorously defined but (Ct)avg is related to the average creep zone expansion rate. Methods for determining (Ct)avg and ΔJc are described. Experimental methods for obtaining creep-fatigue crack growth data are also described and critically evaluated. Models for predicting creep-fatigue crack gowth in components are also described. It is shown that in some materials, (Ct)avg does not uniquely characterize the behavior during early crack growth. The reasons for these trends are discussed. Several recommendations for future research in this area are made.

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