The fibrous and cleavage tensile fracture of an annealed mild
steel was investigated. Round tensile specimens of two geometries,
one straight and one with a circumferential notch, were pulled at
temperatures between room temperature and liquid nitrogen temperature.
Tensile fractures occurred at average strains from 0.02 to
0.87. The mechanism of fibrous fracture at room temperature was
investigated metallographically. The stress-strain values at which
fibrous and cleavage fractures are initiated were determined.
Many fine microcracks, which are associated with pearlite
colonies and inclusion stringers, develop prior to fibrous fracture.
The macrofracture, which leads to final separation of the tensile
specimen, is initiated by the propagation of a microcrack beyond the
microstructural feature with which it is associated. Thus, the fibrous
fracture of mild steel does not develop by the gradual growth and coalescence
of voids that are large enough to be visible in the optical
microscope. When the microcracks begin to open and propagate,
final fracture quickly follows. Axial cracks are a prominent feature
of the macrofracture that forms in the interior of the specimen immediately
before final fracture.
The Bridgman distribution of stresses is not valid in a notched
tensile specimen. Fibrous and cleavage fractures occur at approximately
the same value of maximum tensile stress. When the maximum
tensile stress that is necessary for cleavage fracture is plotted
against the corresponding maximum tensile strain, the result is an
unique locus.</p
