Effect of thickness on fatigue crack initiation of nickel-based single crystal superalloy micro notched specimens at 850 °C

The effect of thickness on the fatigue performance and crack initiation mechanism of nickel-based single crystal superalloy (Ni-SX) micro-notch specimens were studied. The fatigue lives of specimens under the same nominal stress were obtained by conducting fatigue tests at 850 °C and 950 MPa on bilateral four-notched micro specimens (BFMS) with different thicknesses. The results indicate that the effect of thickness on the fatigue performance of BFMS is not monotonic. Analysis of failure surfaces of BFMS revealed two kinds of crack initiation mechanism: Edge Crack (EC) type initiation mechanism and Subsurface (SC) Crack type initiation mechanism. A damage-coupled macroscopic anisotropic constitutive model was used to conduct a simulation analysis of BFMS with different thicknesses. It was found that EC-type initiation can be characterized by anisotropic equivalent stress, cumulative inelastic strain or damage factor, while SC-type initiation is highly correlated with stress triaxiality. Finally, a machine learning model that considered thickness, stress triaxiality, and crack initiation angle was introduced to predict the fatigue life of BFMS, and the predicted results agree with the experimental results.