Coupled Thermomechanical Fatigue Tests for Simulating Load Conditions in Cooled Turbine Parts

Modern heavy-duty gas turbines operate under hot gas temperatures that are much higher than the temperature capability of nickel superalloys. For that reason, advanced cooling technology is applied for reducing the metal temperature to an acceptable level. Highly cooled components, however, are characterized by large thermal gradients resulting in inhomogeneous temperature fields and complex thermomechanical load conditions. In particular, the different rates of stress relaxation due to the different metal temperatures on hot gas and cooling air exposed surfaces lead to load redistributions in cooled structures, which have to be considered in the lifetime prediction methodology. In this context, the paper describes coupled thermomechanical fatigue (CTMF) tests for simultaneously simulating load conditions on hot and cold surfaces of cooled turbine parts (Beck et al., 2001, “Experimental Analysis of the Interaction of Hot and Cold Volume Elements During Thermal Fatigue of Cooled Components Made From AISI 316 L Steel,” Z. Metallkunde, 92, pp. 875–881 and Rau et al., 2003, “Isothermal Thermo-mechanical and Complex Thermo-mechanical Fatigue Tests on AISI 316 L Steel—A Critical Evaluation,” Mater. Sci. Eng., A345, pp. 309–318). In contrary to standard thermomechanical fatigue (TMF) testing methods, CTMF tests involve the interaction between hot and cold regions of the parts and thus more closely simulates the material behavior in cooled gas turbine structures. The paper describes the methodology of CTMF tests and their application to typical load conditions in cooled gas turbine parts. Experimental results are compared with numerical predictions showing the advantages of the proposed testing method.