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HIGH-TEMPERATURE FATIGUE IN CONTROLLED ENVIRONMENTS
NAVAL RESEARCH LAB WASHINGTON D C
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The effect of environment on the high-temperature fatigue properties of nickel, Type 316 stainless steel, and Inconel X was investigated by comparing the fatigue life in air to that in vacuum at 1500 F. Sheet-metal specimens of these materials were cycled in reverse bending at their resonant frequency by means of equipment developed to perform high-temperature fatigue tests in controlled environments. The effect of environment on fatigue properties is similar for all three materials. At high strains fatigue lives are superior in vacuum with decreasing strain the vacuum and air curves converge. These results can be explained by a mechanism previously proposed to interpret similar findings in creep. This mechanism involves the simultaneous operation of two competing processes the surface adsorption of gaseous impurities lowers the surface energy and facilitates crack propagation, while oxidation provides a strengthening tendency. Author
APPROVED FOR PUBLIC RELEASE