SUBJECT: Ph.D. Dissertation Defense
BY: William Musinski
TIME: Monday, April 28, 2014, 1:00 p.m.
PLACE: MRDC Building, 4211
TITLE: Modeling the Effects of Shot-Peened Residual Stresses and Inclusions on Microstructure-Sensitive Fatigue of Ni-Base Superalloy Components
COMMITTEE: Dr. David L. McDowell, Chair (ME)
Dr. Richard W. Neu (ME)
Dr. Olivier N. Pierron (ME)
Dr. Ken Gall (MSE)
Dr. Julian J. Rimoli (AE)
Dr. Patrick J. Golden (AFRL)


For Ni-base superalloy components used in aircraft jet turbine engines, different competing fatigue failure mechanisms are present depending on applied load, temperature, and environment over time. Typically, the life-limiting features causing failure in Ni-base superalloy components are near surface nonmetallic inclusions (NMIs). Compressive surface residual stresses are often introduced in these alloys to retard near surface fatigue crack growth from NMIs and shift the fatigue crack initiation sites from surface to sub-surface locations, thereby increasing fatigue life. To model the effects of residual stresses, NMIs, and microstructure heterogeneity on fatigue crack driving force and fatigue scatter, a computational crystal plasticity framework is presented that imposes quasi-thermal eigenstrain to induce near surface residual stresses in polycrystalline Ni-base superalloy IN100 smooth specimens with and without NMIs. A fatigue indicator parameter (FIP)-based microstructurally small crack (MSC) growth model incorporating crack tip/grain boundary effects was fit to laboratory air and vacuum experiments and then computationally applied to 3D crack growth starting (1) from a focused ion beam notch in a smooth specimen, (2) from a debonded inclusion located at various depths within notched components containing different notch root radii, and (3) from inclusions located at different depths relative to the surface in smooth specimens containing simulated shot-peened induced residual stresses. Computational predictions in MSC growth rate scatter and distribution of fatigue life were in general accordance with experiments. The general approach presented can be used to advance integrated computational materials engineering (ICME) by predicting variation of fatigue resistance and minimum life as a function of heat treatment/microstructure and surface treatments. In addition, this framework can reduce the number of experiments required to support modification of material to enhance fatigue resistance, which can lead to accelerated insertion (from design conception to production parts) of new or improved materials for specific design applications. Elements of the framework being advanced in this research can be applied to any engineering alloy.