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The high-temperature creep-rupture properties of six sheet alloys having application to aircraft design were investigated under conditions of intermittent load and temperature for comparison with their corresponding constant-temperature — constant-load behavior. Included are type 321 stainless steel at 1200 and 1350 F, N-155 alloy at 1350 and 1500 F, Inconel “X” at 1350 and 1500 F, RC-130-A titanium at 800 F, 24S-T3 Alclad aluminum at 300, 450, and 600 F, and FS-1H magnesium at 300 and 450 F. Intermittent-load — constant-temperature tests were conducted with a cycle of 1-hr-load-on, 1-hr-load-off in most cases although several of the alloys were subjected also to an 8-hr-on, 8-hr-off load cycle. Intermittent-temperature — constant-load tests were made using a 2-hr cycle with the specimen at temperature 1 hr and cooled 1 hr. While these relatively simple cyclic conditions do not duplicate the complex load and temperature patterns encountered in aircraft service, the results obtained do provide some qualitative guidance in applying available static creep-rupture data to design of aircraft structural parts. A wide variety of effects were produced by the intermittent-load and temperature conditions. Stability of microstructure was one of the more significant variables controlling the alloy response to the cyclic conditions. Acceleration of creep and rupture was induced by intermittent loading where such processes as overaging, relaxation, recrystallization, and loss of ductility occurred. Retarding of creep and rupture occurred in those alloys where increase in ductility and creep recovery developed because of the intermittent-load cycle. Intermittent heating produced acceleration of creep and rupture in a number of cases, particularly where susceptibility to intergranular oxidation and cracking was aggravated by thermal stresses.
Guarnieri, G. J.
Principal Metallurgist, Cornell Aeronautical Laboratory, Inc., Buffalo, N. Y.