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 retard ing effect of overload and the accelerating effect of underload on the crac k propagation. In the former, the result is explained in terms of residual stress effects associated with the overload, and in the tatter, the underlo ad 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 repo rts studies of the crack propagation interaction effect by using microstruc tural examination of crack tips. Observations were made with the Back Elect ron 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 w as reduced. (2) The region of cells close to the crack tips become enlarged on the underload and the scale of the other microstructure (such as PSB. v ein and loop patch) were also enhanced too, so that the crack propagation w as accelerated. (C) 2001 Elsevier Science B.V. All rights reserved.