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An investigation of the cyclic stress-strain behavior of forged Inconel 718 is described. The study was conducted at 294, 616, and 839 K, and Rε (εmin/εmax) = 0, −1. The material showed moderate amounts of cyclic strain softening, with the extent and rate of softening increasing as the temperature increased. It was expected that the cyclic stress-strain curve defined by the amplitude of the load-displacement hysteresis loops would be independent of Rε; specimen-to-specimen scatter prevented such a conclusion, although the data tended to support it. A second cyclic stress-strain curve, defined by the locus of the tips of the hysteresis loops, incorporates the mean stress of the loop and was dependent on Rε. The presence of the mean stress in this definition of the cyclic stress-strain curve produced a curve with a hump; after reaching a maximum value at about 0.8 percent strain range, the stress decreased as strain range increased further. This hump shape is not the result of transient behavior, but represents the stable cyclic curve. The transient behavior and the stable state reached by the material during the test is discussed.
nickel-base superalloys, Inconel 718, mechanical properties, cyclic stress-strain analysis, fatigue (metals), cycles, stress, strain, cyclic softening, tests, evaluation, low cycle fatigue
Engineer, Materials Life and Methods, General Electric Company, Cincinnati, Ohio