Thermo-mechanical fatigue crack propagation experiments in Inconel 718

An experimental test facility was developed to perform thermo-mechanical fatigue crack growth experiments. The thermal cycles were generated using hot and cold air flows distributed by a nozzle onto the test specimen. With the equipment it was possible to obtain TMF cycles with cycle times of 120s with less than 10°C temperature variation over the measurement length of the sample, cycling between 200°C and 550°C. The equipment allows TMF crack propagation tests up to, at least, 600°C. The crack length and the crack closure level were determined using the potential drop technique. Thermo-mechanical fatigue crack propagation experiments were performed in-phase and out-of-phase with various R-values on samples of Inconel 718. Cylindrical specimens with a small starter crack were cycled in nominal total strain control. The crack propagation rate was determined and the correlation to the effective J-integral range, ΔJeff, corrected for crack closure presented. The fracture surfaces showed a dominance of trans-granular crack propagation with striations, indicating a low degree of time dependency in the procedure.