Implementation of Constitutive Model for Thermoplastics with Some Preliminary Results

This paper presents an implementation of a material model for thermoplastics. Such materials have a quite complex behavior involving large elastic and plastic deformations, strong viscous and temperature effects, possible true stress softening and deformation-induced anisotropy. The demand of reliable constitutive models for thermoplastics is increasing, and a promising approach is due to Haward and Thackray (1968), who separated the response in two processes: one flow process related to motion of polymer chain segments, and one extendable spring based on the conventional theory of elastomers. Several researchers have extended the Haward and Thackray model, and in particular, the group headed by Boyce at MIT has worked with modeling of polymers for years. The implementation in this paper is mainly based on a paper by Boyce et al. (2000). Recognizing the large elastic deformations in polymers, we have used a framework with a Neo-Hookean hyperelastic material description. The semi-implicit stress update algorithm is as proposed by Moran et al. (1990). Predictions using the implemented model is compared with results from uniaxial tension and compression tests on polypropylene. The agreement is satisfactory in both cases of loading, as the main features of the force-deformation curve and yield stress difference in compression and tension are reproduced by the model.