Dislocation structures associated with fracture surface topographies in Stage II fatigue crack growth in copper and 70 : 30 brass

Abstract

AbstractAbstractScanning and transmission electron microscopy has been used to examine the fracture surface topography and the dislocation structure of 70:30 brass and copper in order to study the mechanisms of Stage II fatigue crack growth. In the early period of Stage II growth the fracture surfaces lay mainly in crystallographic planes, the crystallographic features in fractographs being more pronounced in brass than in copper. Beneath the fracture surfaces, there were observed dislocation arrays densely packed in the slip planes in brass and the well developed cells in copper. In the late period of Stage II growth, however, fine contrast of indistinct shape was observed just beneath the striations on fracture surfaces in both the metal and the alloy. Outside such regions, however, numerous twin bands were observed in brass and in the elongated cells in copper. It appears that in the later period of Stage II cracks grow by means of an opening and closing behaviour which is unaffected by their crystallographic orientations.