Stage I fatigue crack propagation along crystallographic slip planes in a Ni-base single crystal superalloy, NKH-304, was investigated. At first, crack propagation tests at room temperature were carried out using four types CT specimens with different combinations of primary and secondary orientations. A series of experiments revealed the Stage I cracks in the single crystal superalloy were mixed mode containing mode I, II and III components, and the <100> primary orientation or the <110> secondary orientation resulted in higher crack propagation rate. In order to interpret such effect of crystal orientations on the Stage I cracking, elastic-plastic FEM model was developed taking account the 3-D geometry of the mixed mode crack plane and elastic anisotropy in the CT specimen. It was found from fracture mechanics analysis that the stress intensity factor derived from the shear stress components on the co-planar slip plane provided a reasonable explanation on the effects of crystal orientation on the Stage I crack propagation rate.
