1987 Volume 51 Issue 12 Pages 1139-1144
The mechanism and critical conditions for crack initiation of ductile fracture of hypo-eutectoid graphitic steels were studied in the round bar specimens pulled in tension.
In the centre of the necked part of a tensile specimen, voids which nucleated at the interface between the matrices and graphite nodules grow both in longitudinal and transverse directions because of a triaxial tensile stress.
In the next stage of tension, a central crack is formed by coaleacence of voids due to the microvoids formation in the matrix between large voids. This crack spreads rapidly in the transverse direction, but near the surface the crack propagates in a direction inclined at about π/4 radian to the tensile axis.
A critical condition for the central crack initiation is expressed as follows:
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\
oindentwhere σm⁄\barσ is a stress triaxiality in the centre of the necked part, lV and ρV are a spacing and a radius of curvature of the large voids located in the centre of the necked part, respectively, and C is constant.
A dependence of the ductile fracture strain upon microstructure may be attributed to the void dependency mentioned above.
