Thermomechanical fatigue assessment of inclined film cooling holes in nickel-base single-crystal alloys

Abstract

As advanced aero-engines demand higher turbine inlet temperatures, turbine blades are subjected to increasingly severe thermomechanical conditions, where thermomechanical fatigue (TMF) emerges as a critical failure mode for hot-end components. This study investigates isothermal fatigue (IF) and TMF behaviors of nickel-base single-crystal superalloy structures featuring inclined film cooling holes (FCHs). In IF tests, cracks initiate in Mode I, with subsequent propagation exhibiting a competitive mechanism between Mode I and the hole-path direction under varying stress levels. In TMF tests, stress concentration at FCH edges mitigates interactions among fatigue, creep, and oxidation, rendering fatigue damage the dominant factor in FCH-TMF failure. The stress ratio and concentration effects further modify the out-of-phase TMF mechanism. A non-local life prediction model is developed based on the crystal slip mechanism and critical distance theory, yielding predictions within a threefold scatter band for most specimens. However, significant deviations occur when stress gradients around the critical region of the test specimens are not adequately captured. This work provides novel insights for the structural integrity design of components operating under thermomechanical fatigue conditions.