Micromechanically-motivated phenomenological Hosford-Coulomb model for predicting ductile fracture initiation at low stress triaxialities

A phenomenological ductile fracture initiation model for metals is developed for predicting ductile fracture in industrial practice. Its formulation is based on the assumption that the onset of fracture is imminent with the formation of a primary or secondary band of localization. The results from a unit cell analysis on a Levy-von Mises material with spherical defects revealed that a Mohr-Coulomb type of model is suitable for predicting the onset of shear and normal localization. To improve the agreement of the model predictions with experimental results, an extended Mohr-Coulomb criterion is proposed which makes use of the Hosford equivalent stress in combination with the normal stress acting on the plane of maximum shear. A fracture initiation model is obtained by transforming the localization criterion from stress space to the space of equivalent plastic strain, stress triaxiality and Lode angle parameter using the material's isotropic hardening law. Experimental results are presented for three different advanced high strength steels. For each material, the onset of fracture is characterized for five distinct stress states including butterfly shear, notched tension, tension with a central hole and punch experiments. The comparison of model predictions with the experimental results demonstrates that the proposed Hosford-Coulomb model can predict the instant of ductile fracture initiation in advanced high strength steels with good accuracy.