Abstract:

The low cycle fatigue lifetimes of nickel-base alloy GH4169 were tested under different stress amplitudes, with a sheet specimen containing a central hole. The effects of the stress concentration factor nearby the central hole on the fatigue lifetime were compared on the same maximum stress amplitude. The initiation and propagation of fatigue cracks in the stress concentration field were investigated with SEM. It was found that the initial fatigue crack nucleated at the cross edge of the hole and surfaces of the specimen by sliding. The initiation periods of specimens were predominantly dependent on the stress amplitude near the hole and less dependent on the diameter of the hole. But the crack propagation rates exhibited a correlation with the stress distribution near the hole. Under the same stress amplitude, the higher the stress concentration factor nearby the hole, the faster the crack propagation rate. The scatters of fatigue lifetime of bigger hole diameter specimens were slightly lower than those of smaller hole specimens. Cracks grew in the form of an edge crack in the short fatigue crack and developed in the form of a penetrated crack in the long fatigue crack. In the steady propagating stage, the fatigue crack extended in a toughness tearing way of crystal penetrating. Near to unstable propagating, the crack fast stretched out by quasi-cleavage. The connection and coalescence of short fatigue crack clusters were not observed.

Key words: stress concentration, crack initiation, rupture analysis