Modeling photo-oxidation effects in semicrystalline polymer: Boundary-value simulations and experimental multiscale analysis

This study presents a comprehensive computational framework designed to capture the mechanical property changes and damage accumulation in UV-aged semicrystalline polymers. The approach integrates time-dependent continuum damage mechanics with anisotropic plasticity and damage, implemented through user-defined subroutines in commercial finite element analysis (FEA) software. To account for UV-induced photo-oxidation, we introduce an experimentally informed photodegradation kinetics model, seamlessly coupled with the continuum damage mechanics formulation. Our focus is on semi-crystalline Low-Density Polyethylene (LDPE) films subjected to accelerated ultraviolet (UV) aging. Through extensive mechanical and rheological characterizations, we propose competing multi-scale mechanisms to describe the observed material behavior: initial strengthening, driven by chemi-crystallization, is followed by softening due to the formation of chemical cracks as UV exposure increases. The continuous decrease in strain to failure is attributed to oxidation-induced chain scission. The developed model is validated through FEA boundary-value problems modeling the experimentally tested samples. Comparative analysis between the simulated and experimental results highlights the model's accuracy in predicting mechanical behavior, damage progression, and fracture initiation. Overall, the proposed computational framework, along with its finite element implementation, delivers reliable and experimentally validated predictions.