On-line assessment and numerical modelling of thermal fatigue and oxidation damages in turbine airfoils under simulated service conditions

This thesis describes a project on the problem of thermal fatigue cracking in aircraft engine turbine blades and vanes. The goals of the project were to develop and validate a methodology for thermal fatigue life prediction and oxidation damage of these components. The project was divided into an experimental phase, which consisted in reproducing and assessing damage accumulation in airfoil-like specimens tested under well-simulated service conditions, and to a numerical or modelling phase which consisted in establishing the thermal fatigue life and the oxidation damage from the knowledge of the thermal and mechanical histories of the critical elements in the specimens. On the experimental side, eight thermal fatigue tests were conducted on double-edge wedge (DEW) specimens made of DS Rene 80 (coated and uncoated) in a burner rig. The DEW specimens were alternately exposed to a hot gas stream and a cold air jet (at ambient temperature), up to 3000 times.;Initiation and growth of leading edge cracks in the DEW specimens were measured on-line during testing using an advanced alternating current - potential drop (ACPD) technique. Fatigue life of bare DS Rene 80 was determined to be 760 ;A numerical analysis was carried out where the specimen temperature distribution was computed with the ABAQUS-FE code using experimentally determined values of convective heat transfer coefficients and the radiation heat transfer conditions. The stress distribution was also computed by finite element analysis using a thermo-elastoplastic deformation model. A thermo-elasto-viscoplastic analysis based on a modified version of the model of Walker was also used to obtain more realistic values of the inelastic strain and stress relaxation in the most stressed location along the leading edge. A thermo-machanical fatigue model which considers the phasing between the temperature and the mechanical strain as well as the oxidation-fatigue and creep-fatigue damages interactions, demonstrated that the creep damage effect can be neglected for the hold time at high temperature normally found in the thermal fatigue tests carried out in the burner rig.;The alloy surface recession or metal consumption resulting from cyclic oxidation was also modelled. Despite the assumptions made to simplify this complex problem, the predicted alloy surface recession along the leading edge was found to be in very good agreement with experimental profiles. (Abstract shortened by UMI.)...