Abstract
Hot sections in turbine engines are subjected to large variations in temperatures and mechanical/thermal loadings. As such, accurate predictions of fatigue crack growth must account for many physical phenomena: temperature dependent crack growth behavior, load interaction history effects, time at temperature effects, temperature history effects, and the effects of stress/temperature/time on the materials. Through extensive experimental work on superalloys, a very definite “temperature history effect” on the resulting crack growth behavior has been identified and modeled. This work also identified a Temperature Affected Zone (TAZ) that occurs ahead of the crack tip and affects subsequent crack growth rates. The size of the TAZ is dependent on temperature, hold time, and stress state. Measurements of the TAZ were made under various conditions. The changes that occur in this TAZ are a combination of oxidation and material microstructure evolution. Various simplified “hot section” engine spectra (changing temperatures and stress levels) were tested to determine resulting crack growth behavior. Correlation between the experiments and model predictions were good and generally conservative.
Highlights
The world is dependent on jet turbine aircraft for travel and commerce
This paper focus on the latter experimental study
This increase in fatigue crack growth rate (FCGR) due to a Temperature Affected Zone (TAZ) was on the order of three to five times faster, not the three hundred times increase observed by Lundstrom et al [13]
Summary
The world is dependent on jet turbine aircraft for travel and commerce. The technology is relatively mature, there are still unexpected failures as a recent high publicity engine failure highlighted: an American Airlines flight in Chicago, Illinois in 2017 [1]. Final reporting blamed an undetected manufacturing flaw in one of the alloy 718 turbine disks that grew by low cycle fatigue until catastrophic failure [1]. The typical spectra turbine disks are exposed to are complex combinations of thermal and mechanical loading. Using a damage tolerant approach, where an initial flaw is assumed, predicting fatigue crack growth rate (FCGR) in these situations is especially difficult. The authors sought to develop a robust model for predicting crack growth in high temperature turbine engine alloys, in nickel-base superalloys
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