A Physical State-Dependent Model for Semicrystalline Polymers: Modeling the Transition from Elasto-Plasticity to Viscous Flow
Sandra Köhler 1*, André Hüllmann 2, Prativa Giri2, André Leonhardt 1, Andreas Seefried 2, Till Clausmeyer 1, Birgit Awiszus 1
1 Professorship Forming Technology, Institute of Machine Tools and Production Processes, Chemnitz University of Technology, 09107 Chemnitz, Germany; 2 Professorship of Polymer Technology, Institute of Material Handling and Plastics, Chemnitz University of Technology, 09107 Chemnitz, Germany
Keywords: hot plate butt welding, FE simulation, thermomechanical coupling, high-density polyethylene (HDPE), temperature-dependent material model, thermo-mechanical simulation, polymer welding
This paper presents a multi-regime framework designed to capture the complex thermomechanical behavior of high-density polyethylene (PE-HD) across the melting transition. Assuming a homogeneous, isotropic material and volume constancy (incompressibility), the proposed approach integrates non-linear elasto-plastic hardening for the solid state and viscous behavior for the molten state into a unified numerical environment. The mathematical core of the model relies on a temperature-dependent state transition criterion based on the melting temperature . The governing equations are partitioned as follows: (1) An elasto-plastic formulation utilizing the Hensel-Spittel law – traditionally employed for metallic materials – is adapted to describe the solid state flow behavior of the polymer. Flow curves are derived from temperature-controlled flat compression tests, accounting for thermal softening and strain hardening. (2) A viscous model based on the rate-power-law, where the constitutive relations are informed by oscillatory rheometry and converted using the Cox-Merz rule to define the rate-dependent viscosity. A comprehensive material card was developed, incorporating additional experimentally determined temperature-dependent thermal properties (heat capacity, thermal conductivity). A key aspect of this work is the implementation through a user-defined subroutine into the commercial FE software Simufact Forming. This enables a seamless numerical transition at the solid-liquid interface, which is critical for solving the underlying moving boundary problem. The model’s predictive capability is validated through a dual approach: accuracy is verified via flat compression tests and measurement of the melt layer thickness in a heating experiment, which is tracked using tactile penetration depth analysis and embedded thermocouples. The results demonstrate that the proposed framework accurately predicts the material behavior across the melting transition.