Multiaxial Life Prediction System for Turbine Components

Abstract

The objective of this paper is to present a complete three-dimensional life prediction system which was developed for turbine engine components. It will deal primarily with turbine blades and vanes which are subjected to hostile thermal and combustion environments under load which creates cyclic and/or steady multiaxial stress and strain fields. All of the above factors combined are detrimental to the service life of these components and need very careful consideration at the design stage. The developed multiaxial system for a mission includes evaluation of transient metal temperatures, corresponding elastic and inelastic strains, creep strains, and subsequently creep/fatigue lives. The calculations are performed using the ductility exhaustion method. The maximum principal normal strain ranges used in the life analysis are found by a developed procedure for a multiaxial system. The concept is based on analyzing all of the time steps computed in the mission, in order to develop the maximum principal normal strain range whose direction and magnitude is strictly a function of the component geometry and mission loading. The mission creep is then developed by maximizing a cumulative creep function. Directional consistency is maintained in accumulating creep/fatigue damage with respect to the incurred multiaxial stress and strain fields. Also the most damaging mission modes (creep or fatigue) will be separated. Further development in the model includes the capability of analytically obtaining the fatigue curve for any ratio R of minimum to maximum strain using baseline fatigue material properties (R = −1.0). Application of the model to an actual uncooled vane correlates well with test rig development experience.