By use of a microscopic fracture model, a computational estimation methodof bending strength of ultra fine-grained cemented carbides was developed onthe basis of micromechanics. A macroscopic breakage of the cemented carbideswas supposed to occur when a matrix crack, which generated from a defect inultra fine-grained cemented carbides, extended unstably. In this paper, ultrafine-grained cemented carbides which contained vanadium carbide and chromiumcarbide were examined, and then 7)-phase in them was paid attention asdefects. In order to evaluate the stability of the extension of the matrix crack,stress intensity factor was introduced. After the amount of the 7)-phase wasrelated to the carbon content of ultra fine-grained cemented carbides under theconcept of material balance, the bending strength was estimated. The estimatedresults were in good agreement with the experimental ones. The effects ofthe carbon content of carbides and the average size of 7)-phase on the bendingstrength were also examined. The results were presented in the figures and itsapplication to material design was discussed.
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