Thermo-Mechanical Fatigue Investigation of Inconel 718 Based on Miniature Specimen Testing

Thermo-Mechanical Fatigue is one of the critical degradation mechanisms of gas turbine components. Testing under the most realistic loading conditions, like TMF, is crucial for these highest-loaded safety-relevant components. In most setups, standard specimens that rarely match the geometrical sizes of the engine components are in use. While there is an ambition to adopt smaller-sized specimens to reduce the material quantity and to allow testing of close to real-shape materials, up to date, only a few setups are capable of cyclic mechanical testing of small-scaled specimens. To the authors knowledge, no test rigs exist for TMF testing under realistic thermal gradients combined with mechanical loads. This paper presents the further development of an existing low-cycle fatigue (LCF) miniature specimen test system for TMF testing, validated with specimens of the nickel-based superalloy Inconel 718. Two different sizes of specimens were compared for this investigation to study the influence of size effects on lifetimes. The cyclic temperature loading with minimum and maximum temperatures of 400°C and 630°C was performed subject to In-Phase and Out-of-Phase loading conditions. Finally, the miniature specimens were investigated by microstructural analysis to study the crack behaviour at different loading conditions and to verify the equivalence to standard-sized specimens.