The fibrous and cleavage tensile fracture of an annealed mildsteel was investigated. Round tensile specimens of two geometries,one straight and one with a circumferential notch, were pulled attemperatures between room temperature and liquid nitrogen temperature.Tensile fractures occurred at average strains from 0.02 to0.87. The mechanism of fibrous fracture at room temperature wasinvestigated metallographically. The stress-strain values at whichfibrous and cleavage fractures are initiated were determined.
Many fine microcracks, which are associated with pearlitecolonies and inclusion stringers, develop prior to fibrous fracture.The macrofracture, which leads to final separation of the tensilespecimen, is initiated by the propagation of a microcrack beyond themicrostructural feature with which it is associated. Thus, the fibrousfracture of mild steel does not develop by the gradual growth and coalescenceof voids that are large enough to be visible in the opticalmicroscope. When the microcracks begin to open and propagate,final fracture quickly follows. Axial cracks are a prominent featureof the macrofracture that forms in the interior of the specimen immediatelybefore final fracture.
The Bridgman distribution of stresses is not valid in a notchedtensile specimen. Fibrous and cleavage fractures occur at approximatelythe same value of maximum tensile stress. When the maximumtensile stress that is necessary for cleavage fracture is plottedagainst the corresponding maximum tensile strain, the result is anunique locus.
