Abstract
Fatigue tests of copper single crystals initiated by a ramp-loading program yield a higher saturation stress (32 MPa) than the accepted stress (28 MPa) for the plateau of the cyclic stress-strain curve. In order to understand this difference, the dislocation structures of a ramp-loaded crystal have been studied by transmission electron microscopy, and the effect of frequency on the nucleation of persistent slip bands (PSBs) has also been studied. The loop patches thus found in a ramp-loaded crystal are not much different from those produced in a regular test at constant strain amplitude, except that they contain a higher and more uniform content of secondary dislocations. The higher flow stress observed for ramp loading is explained by training of the loop patches. The nucleation of PSBs was found to be strain rate dependent.
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