The concept of interfacial layers surrounding inclusions in a host polymer is accepted as a basic source of the scale factor in the simulation of the large-strain deformation and fracture of polymer blends and composites. The essence of the phenomenon is the ductile-brittle transition if the polymer ligament thickness exceeds a critical value, which is determined by the nature of the polymer. Original texture-sensitive constitutive equations have been applied for the simulation of the polymer large-strain deformation. Two periodic structural models of composites have been used. The disperse component (elastomeric or rigid inclusion) has been replaced by a geometrically identical system of pores, which coincides with phenomena of elastomeric inclusion rupture in rubber-toughened plastics and debonding in particulate-filled composites. The local loss of stability of an elastic deformation is used as a composite fracture criterion. The specific properties of the interphase can be caused by a specific texture and by closeness to a free surface. This specificity is stated in the model by an improved plastic ability compared with that of a bulk polymer. Our simulations show that the percolation of an interfacial polymer provides the brittle-ductile transition. (C) 2003 Wiley Periodicals, Inc.