Abstract
Fatigue crack initiation and early growth have been studied in low carbon weld-simulated steel microstructures consisting of martensite, bainite and acicular ferrite. Cracks nucleated at slip bands which formed very rapidly at the surface. The crystallographic direction of the slip lines corresponded to slip on the primary glide plane. The slip lines tended to be aligned along the needles in the microstructure but were frequently oriented differently, crossing several interfaces. The slip bands developed most easily in the relatively weak microstructures of bainite and acicular ferrite. Apart from serving as crack nucleation sites, the slip lines were also preferred paths for crack propagation in the surface. Crack growth between the slip bands was frequently associated with a high plastic activity, generating fine microslip from the crack. If possible, the cracks tended to avoid propagation through martensite.
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