Image analysis of exfoliation fracture in cold drawn steel

This work studies the anisotropic fracture behavior of cold drawn prestressing steels in the form of exfoliation fracture consisting of oriented and enlarged cleavage, its orientation being parallel to the wire axis (cold drawing direction) and with river patterns which are detectable in such a direction. Using a computer-assisted image analysis technique, a geometric relationship was found between this special cleavage and the conventional cleavage topography, which indicates that the fracture micromechanisms in both cases are also similar and allows a definition of the critical fracture unit in the drawn material as the pearlite colony more than the prior austenite grain. Thus the slender pearlitic colonies become the actual microstructural fracture units and determine the size of the enlarged cleavage facets characteristics of the exfoliation fracture in notched samples of heavily drawn steels.