Predicting the Effects of Overloads on Sustained-Load Crack Growth in a High-Temperature Superalloy.
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH SCHOOL OF ENGINEERING
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This thesis investigates methods of modeling the effects of overloads on high-temperature sustained-load crack growth. In addition to a model previously developed for this specific problem, a computer program developed for low-temperature, high-frequency cyclic load applications was evaluated. Sustained-load hold times were converted to equivalent fatigue cycles to analyze a load spectrum, consisting of sustained-load with periodic overloads. The CRACKS crack growth program was used with the Wheeler and Willenborg models used to account for crack growth retardation due to overloads. Predictions were compared with experimental test data generated on specimens of Inconel 718 at 650 C with periodic overloads of either 20 or 50 percent. Crack measurements were made using a electric potential system. The application of the electric potential system to crack growth measurement following overloads was extensively evaluated. It was concluded that the system had to be recalibrated after each overload due to a sudden advancement in crack length.
- Metallurgy and Metallography