Plasticity modelling of Post-consumer recycled polypropylene and polyethylene

Abstract

The two most common Post Consumer Recycled (PCR) plastics, isotactic polypropylene (iPP) and high density polyethylene (HDPE), differ in composition and mechanical behavior when compared to their virgin counterparts. This thesis focuses on understanding and modeling the mechanical performance of these two PCR plastics separately. Within this context, the present work implements three finite strain thermoelasto-viscoplastic constitutive models developed by Johnsen et al., Mirkhalaf et al. and Anand et al. proposed in the literature to predict the behavior of PCR-iPP and PCR-HDPE. The models are compared and further developed to take into account the effects of recycling. All the models depend on the fully implicit return mapping algorithm and associated state update procedures.

Given the complexity of the models, this thesis proposes the use of Bayesian optimization to facilitate the material parameter calibration when provided with the experimental data. A two-step procedure is proposed where first the models are calibrated for yielding, and then for post yielding behavior (strain softening and orientational hardening).

The models are assessed considering different experimental tests, including standardized specimens with different radius of curvature. A simple modification is suggested to capture the strain hardening response at large deformations accurately. This work concludes that the model developed by Mikhalaf et al. is capable of accurately reproducing the experimental results obtained in the validation experiments of PCR-PP where as the modified Anand model is capable of accurately reproducing the experimental results of PCR-PE.