Mechanical Fatigue Testing Under Thermal Gradient and Manufacturing Variabilities in Nickel-Based Superalloy Parts with Air-Cooling Holes

Background As aerospace cast parts move to more complex geometries, the impact of process variabilities must be evaluated more precisely to provide new, less empirical acceptation criteria. However, experimental lifetime assessment with Thermal Gradient Mechanical Fatigue (TGMF) are still complicated to reproduce in a laboratory environment. Objective This paper investigates the impact of geometrical variability on thermo-mechanical fatigue life of Nickel-based superalloy single crystal parts. Methods To this end, hollow specimens with cooling holes drilled in their centre have been produced with parametrized variations of the wall thickness and have been characterized by X-ray tomography. An experimental fatigue setup operating at 1100 degrees C, with a thermal gradient of 50 degrees C across the wall, has been developed to assess the life of each specimen. Results Experimental lifetime assessment were conducted on a sample of specimens containing geometrical defects. Optical observations during the tests have permitted to study crack initiation and propagation in the vicinity of the cooling hole, an area of high thermal and stress gradients where damage localize. Axial strains (in the loading direction) were also gathered and compared to prove the impact of process variabilities on mechanical behaviour. Conclusions Significant mechanical differences are observed during the tests which can related to the geometrical variations of the specimens and their interaction with the shaped hole. Tests results will be further used to calibrate a creep-fatigue model for lifetime predictions.