The influence of strain state on the nucleation and early growth of fatigue cracks was investigated. Thin-walled tubular specimens of Inconel 718 were subjected to combinations of axial and torsional loading in the low-cycle fatigue regime.
The changes in crack growth rates as a function of crack size have been examined at effective strain amplitudes of 0.01 and 0.005. The crack growth rates under pure torsion, combined loading, and axial loading were found to be similar for the same crack size. Crack growth rates were interpreted based on an elastoplastic fracture mechanics characterization. A strain intensity range parameter was defined based on the shear and normal strain calculated on the crack plane. For mixed-mode loading conditions, similar crack growth rates were observed at a given range of strain intensity.
Extensive crack surface rubbing has been observed due to facets formed along the crack. Results indicate that relative changes in the ratio of Mode II/Mode I strain intensity components did not alter the crack growth rates notably at an effective strain amplitude of 0.005. However, at higher strain levels a relative decrease in Mode II/Mode I ratio promoted higher crack growth rates. If the magnitude of Mode I component of loading is high enough to result in crack opening beyond the size of the facets, then Mode II and Mode III components may be fully effective in propagating the crack.