Influence of notch severity on thermomechanical fatigue life of a directionally solidified Ni-base superalloy

The aim of this work is to understand the influence of notches under thermomechanical fatigue (TMF) in a directionally solidified Ni-base superalloy. Experiments were performed utilizing linear out-of-phase and in-phase TMF loadings on longitudinally oriented smooth and cylindrically notched specimens. Several notch severities were considered with elastic stress concentrations ranging from 1.3 to 3.0. The local response of the notched specimens was determined using the finite element method with a transversely isotropic viscoplastic constitutive model. Comparing the analysis to experiments, the locations observed for crack nucleation in the notch, which are offset from the notch root in directionally solidified alloys, are consistent with the maximum von Mises stress. Various local and nonlocal methods are evaluated to understand the life trends under out-of-phase TMF. The results show that a nonlocal invariant area-averaging method is the best approach for collapsing the TMF lives of specimens with different notch severities.