Modelling of the transition from mode I to crystallographic crack growth in a single crystal gas turbine blade alloy under service-like loading conditions

•A criterion is proposed to predict crystallographic crack growth in SX superalloys.•TMF crack growth testing is performed to calibrate the criterion.•The influence of high temp hold time on the cracking mode transition is studied.•Pre-test aging is performed to simulate service exposure of gas turbine blades.•Two different methods for crack closure correction are utilized and compared. In fatigue life prediction of single crystal gas turbine blades, the risk of rapid crystallographic crack growth along the close-packed planes poses a large uncertainty. A criterion is proposed to predict the transition from mode I to crystallographic crack growth, which is necessary for reliable prediction of the number of cycles from crack initiation to the onset of crystallographic crack growth. The proposed criterion is calibrated against tests performed under a wide range of conditions representative for a gas turbine blade, including isothermal fatigue crack growth tests and thermomechanical fatigue crack growth tests, some including hold times and pre-test aging.