Creep Fatigue Interaction under Different Test Variables: Mechanics and Mechanisms

We present detailed analyses of dwell characteristics of various waveforms of creep fatigue interaction tests performed on the nickel-based superalloy IN 718. We discuss the effects of different dwell modes (strain, stress, and mixed) on creep fatigue properties. Strain dwell tests cause relaxation/accumulation of the mean stress, and stress dwell tests cause accumulation/relaxation of mean strain. True interaction of strain-controlled low cycle fatigue and creep takes place when stress-controlled dwell has been introduced within a strain-controlled cycling. In this article, the effects of pure and mixed modes have been compared in terms of the creep strain accumulation rate at dwell positions. It has been found, irrespective of creep and fatigue, that there is also a second competitive process that goes on, like tensile strain accumulation and compressive strain accumulation. These two processes compete with each other and show a difference in evolution with cycles. Mean strain rates for different stress and mixed control tests have also been determined; by comparing the creep strain accumulation rate and plastic strain rate, the competition of creep and fatigue processes has also been demonstrated in this article. The mechanism of cracking and cavity formation has also been illustrated by computed microtomography and subsequent fractography.