Mechanical Behavior of Poly(Ethylene Terephthalate) During Large Strain Deformations

In this paper, the large strain mechanical behavior of an initially amorphous (only 9% crystalline) poly(ethylene terephthalate) (PET) in the glass transition region is investigated. The temperature and state of strain dependence of the yield and post yield behavior is examined with a set of uniaxial and plane strain compression tests. Differential scanning calorimetry is used to assess crystallinity content prior to and after deformation. A strong state of strain dependence of the strain hardening (strengthening) behavior is observed. It is found that crystallization does not occur during uniaxial compression and therefore strain hardening during uniaxial compression is a result of the orientation of the molecular network alone. However, crystallization is found to occur during plane strain compression and/or shortly afterward resulting in 40% crystallinity after −145% strain. The occurrence of some crystallization during plane strain deformation indicates that, in addition to molecular orientation, a second mechanism to strain hardening may occur depending on the state of strain. The temperature and state of strain dependence of yield and post-yield behavior was found to be well modelled with the Arruda and Boyce elastic-viscoplastic constitutive model for amorphous polymers modified to include the dramatic temperature dependence of yield in the glass transition region. The Arruda and Boyce eight chain network model for the strain hardening behavior was found to predict the plane strain compression behavior based on data obtained in uniaxial compression even though it does not account for the crystallization during straining which was observed in plane strain. This result suggests that even though crystallization does occur, the dominant hardening mechanism is the orientation of the underlying macromolecular network.