On the simulation of strain induced anisotropy for polymers

The alignment of polymer chains is a well known microstructural evolution effect due to straining of polymers. This has a drastic influence on the macroscopic properties of the initially isotropic material, such as a pronounced strength in the loading direction of stretched films. For modeling the effect of strain induced anisotropy a macroscopic constitutive model is adopted in this paper. As a key idea, weighting functions are introduced to represent a strain-softening/hardening-effect to account for induced anisotropy. These weighting functions represent the ratio between the total strain rate (representing the actual loading direction) and a structural tensor (representing the stretched polymer chains). In this way, material parameters are used as a sum of weighted direction related quantities. In the finite element examples we simulate the cold-forming of amorphous thermoplastic films below the glass transition temperature subjected to different re-loading directions.