Transition from ductile tearing to cleavage fracture: A cell-model approach

Abstract

The fracture resistance of ferritic steels in the ductile/brittle transition regime is controlled by the competition between ductile tearing and cleavage fracture. Under typical conditions, a crack initiates and grows by ductile tearing but, ultimately, failure occurs by catastrophic cleavage fracture. In this computational study cleavage fracture is treated by a weakest link mechanism in conjunction with brittle microcrack statistics. The cleavage model also accounts for the competition between the nucleation of voids from the carbide inclusions on grain boundaries and the unstable cracking of these inclusions precipitating catastrophic cleavage fracture. The probabilistic treatment for the transition to catastrophic cleavage is phrased in terms of the Weibull stress, σ_W, reaching measurable material-specific values. The successful application of this cleavage fracture model hinges on an accurate description of the evolution of the stress field during crack growth by ductile tearing. This is accomplished by using cell elements endowed with the micro-separation characteristics of ductile tearing. Load-displacement behavior, ductile tearing resistance and transition to cleavage fracture are investigated for three different test geometries. Crack geometry, microstructure and ductile crack growth exert strong effects on the transition from ductile tearing to cleavage fracture. The model predicts trends in ductile/brittle transition that are consistent with experimental data.