Anisotropic fatigue crack initiation and propagation around stress raisers in a single crystal Ni-based superalloy

Abstract

The effect of crystal anisotropy on the threshold of the stress intensity factor range ΔKth was investigated in a single crystal Ni-based superalloy. First, fatigue crack propagation tests were conducted in air at room temperature using four types of test specimen with different crystal orientations in the loading direction and crack propagation direction. In the four types of test specimen, three types had fatigue cracks that propagated along the slip plane, whereas the other type showed an opening mode cracking without following the slip plane. ΔKth calculated from the projected crack length differed for the four types of test specimen affected by crystal anisotropy. In order to investigate the effect of crystal anisotropy, a 3D finite element models were created to reproduce the crystal orientation and crack propagation path, and the slip deformation near the crack tip was calculated according to the crystal plasticity theory. It was found that the damage parameter based on the plastic shear strain on the slip surface can evaluate ΔK_th for the three types of test specimens in which the crack propagated along the slip plane.