Life Prediction Methodologies for Aerospace Materials Annual Report, 2002

Abstract

Abstract : Three types of models to predict behavior of gamma titanium aluminide (g-TiAl) alloys in a high temperature turbine engine environment were developed and have been refined: (a) a model to evaluate linear elastic response, (b) a crystal plasticity model, and (c) a computational model for analyzing stress variations within polycrystals of g-TiAl. Orientation-imaging microscopy (OIM) has been used to understand the role that crystallographic orientation plays with respect to fatigue crack initiation and growth. Mechanical tests have been conducted on various ceramic matrix composites to determine their viability in applications such as exhaust wash structures, exhaust nozzle flaps and seals and combustor liners. Models have been developed for determining effective thermoelastic properties and creep and damage associated with anisotropy of the creep response in off-axis orientations. Titanium, nickel and aluminum alloys used in fabricating engine components have been examined to assess their behavior under laboratory conditions designed to simulate fretting fatigue, in turbine engine blade attachments, foreign object damage, high-cycle fatigue, and high-cycle/low-cycle interactions under engine operation. The ability to predict turbine-engine-materials behavior under operating conditions is an important facet of the Engine Rotor Life Extension (ERLE) and Phase 1 DARPA Prognostics programs. Various studies have been undertaken to assess: (a) baseline properties for virgin material and material extracted from retired turbine disks, (b) fatigue crack initiation and growth, (c) the role that material defects play in low cycle fatigue crack initiation, (d) periodic overloads on fatigue crack growth behavior, (e) fatigue crack growth under near-threshold conditions, and (f) fracture surface marking for crack location during spin-pit testing. Studies that lead t