The observation and analysis of the dislocation morphology of fatigue crack tips at steady state propagation rates subject to a single peak load

During fatigue crack propagation there is large interaction effects of the fatigue cycle for different loading amplitudes. Two examples are the retarding effect of overload and the accelerating effect of underload on the crack propagation. In the former, the result is explained in terms of residual stress effects associated with the overload, and in the latter, the underload partially annihilates the residual stress built up by the positive load. However, at the microstructure level of material under fatigue, the crack propagation is caused by dislocation action. Assuming this, this paper reports studies of the crack propagation interaction effect by using microstructural examination of crack tips. Observations were made with the Back Electron Images (BEI) of the Scanning Electron Microscope (SEM). The results are: (1) the microstructure of cells close to the crack tips formed into vein or loop patch structures upon the overload, so that the crack propagation was reduced. (2) The region of cells s close to the crack tips become enlarged on the underload and the scale of the other microstructure (such as PSB, vein and loop patch) were also enhanced too, so that the crack propagation was accelerated.