Francis J. Clauss, James W. Freeman
An experimental study was made of the changes in the stress-rupture life, ductility, hardness, and microstructure of S-816 and Inconel 550 specimens that had been exposed to varying amounts and conditions of thermal fatigue. Tensile specimens of S-816 and Inconel 550 were alternately heated and cooled while constrained in a manner that prevented their free axial expansion and contraction. Before failure by thermal fatigue occurred, the thermal cycling was discontinued so that the effect of the number of cycles on the properties of the material could be measured. The thermal cycling covered a range of maximum cycle temperature, temperature difference in cycling, and cyclic exposure time at the maximum cycle temperature. A few specimens were first run for short periods of time in stress-rupture and were then failed by thermal fatigue. Exposure to thermal-fatigue conditions strengthened S-816 in stress rupture and weakened Inconel 550. Under the most damaging contitions studied, Inconel 550 lost 98 percent of its original stress-rupture life as a result of prior thermal fatigue, even though the number of cycles was only one-half of that required for failure by thermal fatigue alone. The stress-rupture life of S-816 was increased by about 50 percent. When specimens were first exposed to stress-rupture conditions and then run to failure in thermal fatigue, the thermal-fatigue life of S-816 was sharply reduced, whereas that of Inconel 550 showed a slight increase. The results can be interpreted by extending existing theories of mechanical fatigue and creep-rupture to thermal fatigue.
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