This paper describes a special preparation technique of thin foil containing fatigue cracks created in the early stage of fatigue in copper bulk specimens and some results of direct observations of dislocation structures around the cracks by means of a 2000kV electron microscope.
Ladder-like structures were observed ahead of the crack tips in the early stage of crack growth (so called Stage I). These structures seemed to be formed prior to the crack initiation. The dislocation density in these structures was very low, and cracks initiated within them and propagated either in the middle of the structures or along the interface between the matrix and these structures. Fine cells, dislocation entanglements and voids were not observed ahead of the crack tips in the present examination.
From these observations, it is suggested that dislocation motion in ladder-like structures (the zone of low dislocation density) is responsible for Stage I crack growth in copper. The plastic blunting process along the band is considered unrealistic in the earlier stage of fatigue.
