DAMAGE MECHANISMS IN ALLOY 800H UNDER CREEP-FATIGUE CONDITIONS

The interaction between fatigue damage (i.e., fatigue crack propagatio n) and internal grain boundary damage (i.e., cavity formation at grain boundaries) has been studied for the Alloy 800H at 750-degrees-C for constant plastic strain ranges but different experimental conditions. Most experiments were performed at constant ranges of alternating tens ile/compression stresses. Symmetrical as well as asymmetrical tests (w ith larger compression stresses) were performed. In comparison to the former tests, asymmetrical tests led to shorter cyclic lifetimes mainl y due to cavity formation which was not observed for symmetrical tests . It could be shown that a fast compressive and a slow tensile half cy cle (at large compressive and low tensile stresses) are ideal conditio ns for the nucleation and growth of cavities. Based on quantitative me asurements of the cavity density from interrupted fatigue tests, a phy sical model is presented which can predict the number of cycles to fai lure. This cycle number is determined only by fatigue crack growth whi ch is controlled by (a) athermal plastic deformation, (b) creep deform ation and (c) rate enhancement by cavitation.