Ph.D. Dissertation Defense by Mahesh M. Shenoy
Monday, August 9, 2004
(Dr. David L. McDowell, Chair)
"Constitutive Modeling and Life Prediction in a Directionally Solidified Ni-base Superalloy"
DS GTD111 is a relatively new material which is used primarily in gas turbine blades. The fatigue life in this directionally solidified superalloy depends both on the intrinsic microstructural features such as MC carbide inclusions and extrinsic factors such as the environmental conditions. It is of prime importance to study the relevant damage mechanisms and then relate them to the expected life. The first part of the proposed study consists of an experimental setup to identify the key microstructural features which play an important role in fatigue crack initiation. A physically-based constitutive model is proposed using the single crystal plasticity framework, which can predict the material stress-strain response for temperatures ranging from RT - 1030 ?C. This data will be useful in predicting the fatigue life using a microcrack propagation model, which will be developed based on the individual contributions from fatigue, creep and oxidation damage. The second part of the proposed study comprises the development of a computational framework to quantify the influence of idealized microstructural variables on the crack initiation life. Understanding is sought regarding the most significant microstructure features and how the processing steps might be altered to enhance fatigue resistance using explicit modeling of the microstructure.