THERMAL FATIGUE OF MAR-M509 SUPERALLOY—II. EVALUATION OF LIFE PREDICTION MODELS

Abstract— Thermal fatigue data on MAR-M509 reported in a companion paper were used to evaluate four life prediction models. The temperature-stress-strain history of the critical element at the thin edge of wedge specimens was computed for this purpose. The analysis method uses a finite element computation of the temperature field and a uniaxial calculation of the stress-strain cycle using a cyclic viscoplastic constitutive equation. The influence of specimen geometry and of maximum temperature on the thermal fatigue life to initiate a macroscopic crack was accounted for by variations in stress and mechanical strain ranges at the thin edge. The accumulated and cyclic creep damage models were found to overestimate thermal fatigue life in all the cases. Models which describe oxidation fatigue crack growth interactions, and which are borne out by metallographic observations on wedge specimens, were found to give reliable predictions of thermal fatigue life.