Abstract
To study the behavior of cracks in the earlier stage of fatigue in copper, observations were made on both longitudinal sections and fracture surfaces using optical and scanning electron microscopes.The observation on longitudinal sections showed that transcrystalline crack paths along the slip bands were predominant at low fatigue stresses, but intercrystalline crack paths were frequent at high fatigue stresses.Two kinds of fracture surfaces were observed beneath the specimen surface; one is lying in a slip band and the other along a grain boundary. The former was usually said to be featureless, but steps of about one micron in the direction of crack growth as well as clear slip traces were found on the flat surface. Matching of the steps on mating fracture surfaces was fairly good in such manner that hills on one surface fit into valleys on the other, but matching of the slip traces was not good. On the smooth fracture surfaces lying along grain boundaries, slip traces were observed in most areas, but clear and regular striations were found in some cases. Such topography observed on the fracture surface of intercrystalline crack was similar to that observed within the interior.The observations above suggest that both unslipping and the local plastic blunting process are responsible for the growth of the crack lying in the slip bands, and that intercrystalline cracks grow in tensile mode even in the earlier stage.
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