A morphology based constitutive model for high density polyethylene

High density polyethylene (HDPE) has been widely used in pipe systems for water delivery. Over the course of service, chlorine diffuses into HDPE and causes changes in the material morphology. This process has a significant influence on the deformation and failure mechanisms of HDPE. It has been demonstrated that the failure of HDPE exhibits a ductile-to-brittle transition as the corrosion level increases. Capturing this transition in the material failure mode is essential for predicting the long-term behavior of HDPE structures exposed to a chlorinated environment. In this study, a morphology based constitutive model of HDPE is developed. The model takes into account several essential deformation and failure mechanisms, which include viscoplastic deformation due to intermolecular resistance and homogeneous void growth in the crystalline and amorphous phases, as well as entangled network resistance and craze damage in the amorphous phase. The constitutive parameters are explicitly related to the molecular weight, which varies with the corrosion level. The proposed model is calibrated using uniaxial tensile tests on HDPE samples performed at different deformation rates, crystallinities, and corrosion levels. The model is then used to simulate the mechanical behavior of double-edge notched tension specimens exposed to different corrosion levels. It is shown that the model is capable of capturing the rate-dependent elasto-viscoplastic behavior of HDPE under the unexposed condition as well as the brittle failure behavior after exposure to a highly corrosive environment.