SUBJECT: Ph.D. Dissertation Defense
BY: Andrew Radzicki
TIME: Thursday, June 16, 2016, 1:00 p.m.
PLACE: Love Building, 109
TITLE: Characterization and Modeling of the Thermomechanical Fatigue Crack Growth Behavior of Inconel 718 Superalloy
COMMITTEE: Dr. W. Steven Johnson, Co-Chair (ME/MSE)
Dr. Rick Neu, Co-Chair (ME/MSE)
Dr. David McDowell (ME/MSE)
Dr. Olivier Pierron (ME)
Dr. Christopher Muhlstein (MSE)


Understanding the impact of the complex and harsh thermomechanical environment aircraft turbine engines subject their components to is critical for designers and operators to ensure the safe and efficient operation of the systems they are responsible for. The combined effects of elevated temperatures and high loads on the components of these engines are not fully understood. The purpose of this project was to better understand this complex interaction. This effort consisted of the characterization and modeling of the performance of Nickel-base superalloys, specifically Inconel 718, used in turbine engine disk applications. Isothermal and thermomechanical tests up to and above the maximum usage temperature for Inconel 718 (650C) were executed to characterize the impact of time spent at elevated temperature on fatigue crack growth rate. Various spectra were designed to evaluate the impact of tensile and compressive holds at elevated temperature. This included evaluating the sequence within the spectrum where the holds occurred. Tensile holds executed at a spectrum's maximum load or immediately following an increase in load were shown to increase fatigue crack growth rate immediately follow the hold. Additionally, a robust series of tests were executed to evaluate the impact of the stress intensity, duration, and temperature of a tensile hold on fatigue crack growth rate. The concept of a thermally affected zone to describe an area ahead of the crack tip weakened by the tensile hold that allows for the crack to propagate faster than expected is discussed. As the stress intensity, duration, or temperature of a tensile hold increase, the temperature affected zone increases leading to an increase in fatigue crack growth rate for cycling following the hold. Observations made during the experimental investigation were used to develop a thermomechanical fatigue crack growth model that accommodates realistic spectra and accounts for the effects of time spent at elevated temperature. The model provides designers and operators with a more thorough representation of how their components will respond in actual application and allow them to make more informed decisions on the safe and efficient implementation of their systems.