Cyclic stress—strain response of polycrystalline copper under fatigue conditions producing enhanced strain localization

Abstract

It is now established that the cyclic stress—strain curve of polycrystalline wavy slip metal, e.g. copper, does not contain a plateau if the tests are carried out in conventional strain control. Since fracture research indicates that such tests homogenize the strain, and tests initiated by ramp loading appear to give higher degrees of strain localization, an investigation of cyclic response has been made in polycrystalline copper using the ramp-start method. After specimens were ramped to a desired stress level, they were then tested in strain control. The results showed a plateau to occur at 98 MPa, which corresponds to the plateau shear stress for monocrystals corrected by the Taylor factor. Observations of persistent slip band behavior on the gage surfaces, and of the dislocation structures by transmission electron microscopy (TEM) support the existence of the plateau and demonstrate that it is associated with higher degrees of strain localization induced by starting the tests by ramp loading.