Advancement of Experimental Methods and Cailletaud Material Model for Life Prediction of Gas Turbine Blades Exposed to Combined Cycle Fatigue

Abstract

The European project PREMECCY has been conducted to enhance predictive methods for combined cycle fatigue (CCF) of gas turbine blades, i.e. interaction of low cycle fatigue (LCF) and high cycle fatigue (HCF). While design of CCF feature tests, comprising specimen and test rig design, has already been reported, this paper presents experimental HCF/ CCF test results and progress in life prediction. Besides standard lab specimen tests for characterization of single crystal and conventional cast material, also advanced specimens representing critical rotor blade features were tested in a hot gas rig. Based on these experimental data an extended Cailletaud material model for stress-strain analysis has been calibrated and combined with a modified ONERA damage model for creep-fatigue interaction to estimate the lifetime of the advanced test specimens. The model extensions address the effect of ratcheting, which is typical for CMSX-4 at asymmetric cyclic loading at elevated temperature. Caused by limitations of the Armstrong-Frederick kinematic hardening rule regarding ratcheting, three models for improved ratcheting simulation of isotropic material were adopted to anisotropic material. In addition multiple Norton-flow rules for the viscous part of the model are combined with time recovery terms in the kinematic hardening evolution to represent the behaviour of single crystal material in high temperature environment at a wide range of strain rates. Hence, an improved model for stress-strain and lifetime prediction for single crystals has been developed.