(Dr. David McDowell, advisor)
"An Internal State Variable Based Constitutive Model for Semi-Crystalline Polymers"
Semi-crystalline polymers are used quite extensively in thermo-forming processes requiring large deformations primarily due to their ability to easily be formed and their enhanced mechanical properties resulting from processing. Some common applications are the blow molding of beverage containers, drawing of film, and vacuum forming of various parts. These desired properties are achieved due to the change in the material morphology which results in complex behaviors and interactions. Current constitutive models reported in the literature do no incorporate the evolution of all of the major morphological components such as chain orientation, number of crystallites, number of entanglements, and degree of crystallinity correctly and fail to include the complex inter-dependencies of the morphology evolution as well as history dependence. The research presented here proposes a model that includes all of these morphology evolution relationships based on the mechanisms of the morphology development rather than purely phenomenological relationships. The model is based on an internal state variable constitutive framework and encompasses the primary effects of morphology such as crystallinity, relaxation, and orientation in such a manner that strain, strain-rate, and temperature dependencies are captured. In its completed form, the model can be effectively utilized in finite element modeling (FEM) software, where it can be utilized to investigate material response during processing as well as the resulting morphology.