MODEL OF DUCTILE FRACTURE INITIATION IN SPHEROIDIZED STEEL

Statistical analysis of the microstructure of spheroidized steel has been harnessed to evolve a new method for assessing the final stage of the ductile fracture process, the coalescence of voids. For a known particle size distribution and a known spatial distribution of second phase particles in the steel matrix, the Garrison and Thompson coalescence crierion is applied to determine the probability density of local fracture deformations, and the dependence of the probability of ductile fracture initiation upon the parameter expressing the distance ahead of the crack tip, x/delta. This newly devised model of ductile fracture has been applied to a low-carbon niobium-microalloyed steel, containing 0.043% by weight of niobium; the results confirm that the probability of ductile fracture initiation ahead of a crack diminishes as the distance from the crack tip grows. As the coalescence of voids ahead of a crack tip governs the onset of stable crack propagation, the new model is expected to prove useful chiefly in the statistical prediction of upper bound of fracture toughness in engineering steels.