A mechanical model of crack initiation and propagation, which is based on the actual mechanism of ductile fracture in high strength materials, is proposed. Assuming that a crack initiates when the equivalent stress at a distance p from the crack tip reaches a critical value Of, an equation for predicting fracture toughness JIG is obtained. From comparison between the predicted values and the experimental results, it is found that the distance p corresponds to the spacing of micro-inelusions. The temperature dependence of fracture toughness JK estimated according to the derived equation is given in an Arrhenius form of equation and is nearly consistent with the experimental results. 1. ~TRODUCT~O~ CONSIDERABLE work is available in the technical literature predicting fracture toughness based on a mechanical model of fracture. In the case of cleavage fracture, micromechanisms of cleavage fracture are well known [ 11, and since fracture toughness has been predicted according to them [2,3], it seems that the predictions have succeeded. In the case of ductile fracture, several studies using the condition of critical strain have been carried out[A 51. While there are many complicated factors affecting ductile fracture, it seems that the prediction methods have not yet been su~~iently developed mainIy because little consideration has been given to ductile fracture mechanisms.
In the present study, mechanical models of ductile crack initiation and propagation are investigated. Further fracture toughness is predicted according to a. model which is applicable to high strength materials.