Kiyotsugu Ohji1, Keiji Ogura1 and Yoshiharu Mutoh2
1Department of Mechanical Engineering, Faculty of Engineering, Osaka University, Suita
The processes of initiation, growth and coalescence of voids in tensile ductile fracture of 0.15% and 0.35% carbon steels were investigated with reference to the effects of compressive prestrain, pearlite colony morphology and hydrostatic pressure.
Initiation sites of voids in these steels were found to be at cracks generated in pearlite particles during deformation. In the annealed steels, the cracks began to appear at a relatively small strain level and the number of the pearlite colonies with cracks increased with deformation. Growth of these cracks into voids was rather slow until the stage just before fracture; the formation of voids was rapidly completed at this stage. Coalescence of these voids was found to be triggered and accelerated by microvoids which were nucleated at the final stage of fracture. This triggering action of the microvoids, which, in reality, was one of the most important processes in determining fracture and accordingly fracture strain of a specimen, was quite similar to that previously observed in copper.
Lamellar pearlite particles seemed to be cracked more easily when the lamellae were oriented in the direction of the tensile axis. When the pearlite particles were spheroidized, their cracking or separation from ferrite matrix was remarkably delayed. Under the hydrostatic pressure, retardation of all the foregoing processes, i.e., cracking of pearlite particles, growth of these cracks into voids, mechanical interaction of these voids and nucleation of microvoids, brought about an increase in fracture strain.
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