Anisotropic fatigue limit estimation for a notched single crystal superalloy based on the theory of critical distances

The notch effect on the high-cycle fatigue strength of the Ni-based single-crystal superalloy CMSX-4 was investigated, focusing on the influence of crystallographic anisotropy. High-cycle fatigue tests were conducted at room temperature on smooth and notched specimens, each with two different secondary crystal orientations. The experimental results for the notched specimens revealed that while their crack initiation sites and propagation paths differed, their fatigue strengths were comparable for the two orientations. Subsequently, finite element analysis was performed on models simulating the notched specimen to predict the experimentally observed fatigue limit based on the theory of critical distances (TCD). It was demonstrated that the anisotropic fatigue limit could be accurately predicted by determining the critical distance based on the observed crack initiation and early propagation modes. This high accuracy was achieved using both the axial normal stress field from an elastic analysis and the shear stress field on the slip plane from a crystal plasticity analysis. The former prediction provides a practical tool for engineering applications, while the latter offers a mechanistically-based rationale for the crystallographic notch effect in fatigue.