Abstract
The dislocation structures produced by fatigue crack tip deformation in copper specimens of two different grain sizes were observed by means of transmission electron microscopy. While the crack propagation at higher rates in macroscopically mainly non-crystallographic and the structure surrounding the crack tip is a cell structure, the near-threshold propagation is crystallographic and the crack tip structure consists either of large cells or of ladder-like persistent slip bands embedded in dislocation veins. The threshold stress intensity is rationalized in terms of the grain boundary blocking of the persistent slip bands emitted from the crack tip.
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