The loading at a crack tip varies during fatigue crack propagation. As a result, overloading caused retardation of crack propagation, and under-loading caused the acceleration of crack propagation. In addition, reducing the load range by changing either the minimum load or maximum load could cause a reduction or retardation of crack propagation to occur. Examining a fatigue cracked specimen made of polycrystalline Cu with back-scattered electron images in a scanning electron microscope revealed that (a) the dislocation structures close to the crack tips gradually evolved from a cell structure into a new loop patch structure during the crack retardation period which follows after reducing the maximum load, (b) restoring the crack propagation rate was a result of re-establishing the cell structure from new loop patches or persistent slip bands and (c) the evolution of the dislocation structure at the crack tip due to the maximum loading reduction was affected by residual active slip systems.

The Observation of Dislocation Reversal in Front of Crack Tips of Polycrystalline Copper after Reducing Maximum Load. H.L.Huang, N.J.Ho: Materials Science and Engineering A, 2003, 345[1-2], 215-22