- Numerical studies of the ductile-brittle transition are described that are based on incorporating physically based models of the competing fracture mechanisms into the material's constitutive reletion. An elastic-viscoplastic constitutive relation for a porous plastic solid is used to model ductile fracture by the mucleation and subsequent growth of voids to coalesoence. Cleavage is modeled in terms of attaining a temperature and strain rate independent critical value of the maximum principal stress over a specified material region. Various analyses of ductile-brittle transitions carried out within this framework are discussed. The fracture mode transition emerges as a natural outcome of the initial/boundary value problem solution.