Accurate constitutive modeling of polymeric fibers presents a difficult and distinct challenge. While significant progress has been made in constructing models applicable for small strains and limited strain-rate and temperature regimes, much less has been made for more general conditions. This is due in part to the complexity of polymeric behavior. In this work, experimental results of uniaxial extension tests on Polyethylene terephthalate (PET) were obtained from Dr. S.Bechtel, were analyzed, and were formulated into a new model which explains the behavior of PET at different temperatures and strains. The biggest impediment in the determining the behavior of polymeric was the difference in the behavior of PET above and below its glass transition temperature. Consequently, well established (from microstructural considerations) constitutive models and concepts for rubber elasticity and plasticity were not directly transferable to modeling PET fibers. In the model, the PET fibers were assumed to be constituted by amorphous and crystallization segments and the response of the material during stretching was the combined response of simultaneous stretching of the amorphous and the crystalline segments. The strengthening mechanism is due to orientation of the amorphous segments during stretching. The model involves a friction element which took account of the plastic behavior below the glass transition temperature. The model was used to predict the response of PET at different temperatures and the results from the model showed good agreement with the experimental data. The results from the research will be further used to increase the overall efficiency of the fiber drawing process.
