Complex Thermo-Mechanical Approaches to Study the Behavior of High-Temperature Alloys

The work discusses the complex thermo-mechanical approaches developed at ONERA within the context of gas turbine component fatigue tests. Emphasis is placed on the general experimental methods to test nickel-based single crystal superalloys used in gas turbine blades under severe thermal and mechanical conditions. First, the successive steps required in combined thermal and mechanical fatigue testing are briefly discussed. Particular focus is placed on the techniques developed to reliably control and measure temperature fields generated by induction heating. The main experimental techniques used for thermo-mechanical testing are then discussed. Here, an axial-torsional thermo-mechanical fatigue (TMF) testing rig used to study the coupling effects of combined thermal and mechanical loads is described. How a thermal gradient is introduced in the TMF specimens to reproduce the internal air cooling technology of modern gas turbine blades is shown. In order to increase the representativeness of in-service conditions, the problem of thermo-mechanical fatigue coupled with thermal gradients is investigated using smooth, multi-perforated, and thermal barrier coated samples. It is shown that such testing capabilities can reproduce realistic thermomechanical loading conditions typical of service and constitute a powerful means to develop and validate fatigue life prediction methods. Furthermore, the effect of an overheating event that can occur during a one-engine-inoperative event during in-service operation is also investigated. This type of engine malfunction is known to lead to a degradation of the microstructure, which can modify the material constitutive behavior and affect the remaining lifetime. Finally, the additional complexities associated with the creep-fatigue interaction, the required constitutive models and the computational challenges are discussed.