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Two nickel-base superalloys, single-crystal PWA 1480 and directionally solidified MAR-M 246+Hf, were studied in view of the potential usage of the former and usage of the latter, respectively, as blade materials in the turbomachinery of the space shuttle main engine. The baseline zero-mean-stress fatigue life behavior of these superalloys was established, and then the effect of tensile mean stress on their fatigue life behavior was characterized. At room temperature, these superalloys have lower ductilities and higher strengths than most polycrystalline engineering alloys. The cyclic stress-strain response was thus nominally elastic in most of the fatigue tests. Therefore, a stress range based fatigue life prediction approach was used to characterize both the zero- and tensile-mean-stress fatigue data. In the past, several researchers, namely, Goodman, Gerber, Soderberg, Smith, and Morrow, have developed methods to account for the detrimental effect of tensile mean stress on the fatigue life for polycrystalline engineering alloys. Applicability of these methods to single-crystal and directionally solidified superalloys has not been established. In this study, these methods were applied to characterize the tensile-mean-stress fatigue data of single-crystal PWA 1480 and directionally solidified MAR-M 246+Hf and were found to be unsatisfactory. Therefore, a method of accounting for the tensile-mean-stress effect on fatigue life that is based on atechnique proposed by Heidmann and Manson was developed to characterize the tensilemean-stress fatigue data of these superalloys. Details of this method and its relationship to the conventionally used mean stress methods in fatigue life prediction are discussed.
advanced materials, fatigue (materials), tensile mean stress, life prediction, polycrystalline alloys, single-crystal superalloys, directionally solidified superalloys
Research engineer, Sverdrup Technology, Inc., NASA Lewis Research Center Group, Cleveland, OH
Research engineer, NASA Lewis Research Center, Cleveland, OH