Constitutive and Fracture Modeling of Biaxially Oriented Polyethylene Terephthalate Film and Its Application to Polymer-Coated Sheet Metal Forming

Abstract

Polyethylene terephthalate (PET) films produced by rolling and tentering have biaxial orientation and thus exhibit anisotropic mechanical properties such as yielding, hardening, and fracture. The anisotropy of film makes it difficult to analyze deformation and failures occurring during processing such as metal forming of polymer coated sheet metals. In this work, the anisotropic yielding and hardening of the PET film were measured and an anisotropic constitutive model is developed to represent the anisotropic behavior. In addition, fracture limits of the PET film were measured using the Nakajima test. To describe anisotropic fracture behavior of the film, an anisotropic fracture criterion based on the strain energy density and the theory of isotropic equivalent material was proposed. For validation of the developed constitutive model and the fracture criterion, a deep drawing test of the PET coated sheet metal was conducted. The film cracking predicted by the anisotropic fracture criterion agreed well with measurements.