Early stage crack tip dislocation morphology in fatigued copper

Abstract

Observations of dislocation structures around the fatigue cracks in the early stage in polycrystalline copper were made through an ultrahigh voltage electron microscope operating at 2000 kV. Ladder like structures and initial cracks within them were fre-quently observed. These dislocation structures were formed prior to the crack initia-tion, and the crack was initiated and propagated within these structures accompanied by no appreciable dislocation entanglements in the earliest stage. On the other hand, cell structures were observed around the cracks which appeared to have grown into the sub-sequent stage (the stage between Stages I and II). Cell size ahead of the crack tip was not so small, and it seemed that the crack was able to propagate across the cells. On the basis of these observations, the mechanisms of fatigue crack initiation and propagation in the early stage in copper bulk-specimens are briefly discussed.