A constitutive model for thermoplastics intended for structural applications

This paper presents a hyperelastic-viscoplastic constitutive model for thermoplastics [1]. It is partly based on a model proposed by Boyce et al. [2]. The model involves a hyperelastic- viscoplastic response due to intermolecular resistance, and an entropic hyperelastic response due to re-orientation of molecular chains. A Neo-Hookean material model is selected for describing large elastic deformations. Moreover, the Raghava plastic yield surface [3] is introduced to capture the pressure sensitivity behaviour, and a non-associative visco-plastic flow potential is assumed for volumetric plastic strain control. The strain-rate effects are formulated in a format well-suited for structural applications. Finally, the intramolecular stiffness is represented with Anand’s stress-stretch relation [4]. The model is developed within a framework developed for finite elastic and plastic strains, using a multiplicative decomposition of the deformation gradient. It is implemented as a user-defined model in LS-DYNA [5]. The material model requires 10 parameters which are easy to identify from true stress-strain curves obtained from uniaxial tension and compression tests. In this paper, the parameters are determined from experimental tests on a polyethylene material with high density (PEHD). Subsequently, the model is employed in numerical simulations of the uniaxial tension test and a quasi-static test on a centrally loaded plate. The numerical model gives satisfactory predictions when compared to the observed experimental behaviour.