Anisotropic Continuum Damage Mechanics Modeling of UV Aged Semicrystalline Mechanical and Fracture Behavior

An anisotropic physical-based continuum damage mechanics framework is proposed to capture the mechanical and fracture behavior of low-density polyethylene (LDPE) films subjected to accelerated Ultraviolet (UV) irradiation. Exposure to UV radiation and oxygen causes photo-oxidation in LDPE, leading to changes in its chemical and physical properties. The degradation in the chemical and mechanical properties under UV irradiation is influenced by two opposing mechanisms: (1) An oxidation-induced strengthening due to chemi-crystallization and cross-linking, leading to increased flow stress and strain hardening at low to medium irradiation doses, followed by (2) an oxidation-induced softening due to chemi-cavitation and chain breakage, resulting in decreased ductility at high irradiation doses. The study investigates the effects of photo-oxidation on the physico-chemical and mechanical properties of of LDPE films, subjected to five different aging conditions. The results were used to develop a polymer constitutive model incorporating the effect of photo-oxidation. The constitutive model was implemented as a user-defined material subroutine in finite element. The proposed model was validated using uniaxial tests performed on aged LDPE films.