Chemo-mechanical Modeling of Stress Corrosion Cracking of High Density Polyethylene in Bleach Solutions

Abstract

High density polyethylene (HDPE) is increasingly being used in infrastructure applications with a design service lifetime of several decades. In many cases, the member is exposed to a corrosive environment, such as in pipes carrying potable water, where the dissolved bleach selectively attacks the loosely packed amorphous phase of the polymer. The failure mode of HDPE transitions from a ductile to a brittle mode as the corrosion level increases. This leads to subcritical crack propagation that deteriorates the load capacity and long-term behavior of HDPE structure exposed to a chlorinated environment. In this study, we develop a coupled chemo-mechanical model to simulate stress corrosion cracking (SCC) of HDPE members in a bleach solution. The mechanical response of the polymer is described by a constitutive model to considers the individual deformation and damage mechanisms of the amorphous and crystalline phases. The model accounts for the intermolecular deformation and homogeneous void growth in the crystalline and amorphous phases, along with entangled network resistance and craze damage in the amorphous phase. The embrittlement due to corrosion is captured by relating the amorphous phase parameters to the polymer molecular weight which decreases with corrosion level. The proposed model is calibrated using uniaxial tensile tests at different deformation rates, crystallinities, and corrosion levels. The model is used to simulate the double-edge notched (DEN) tension specimens at different corrosion levels. The constitutive model can capture 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. The decrease in the molecular weight of HDPE due to exposure to bleach environment is captured by a reduced-order corrosion kinetics model. The selective diffusion and chemical reaction of bleach into the amorphous phase lead to polymer chain scission that reduces the molecular weight. The corrosion kinetics model describes this diffusion-chemical reaction of bleach and expresses the extent of chain scission as a function of the bleach concentration. The proposed material constitutive model and the diffusion-reaction model are combined in a single finite element (FE) code to investigate the SCC behavior of double edge notched HDPE specimens. The simulation yields the stress-life curves which qualitatively match the measured stress-life data of polymer pipes. The stress-life curve is shown to exhibit different regimes corresponding to distinct failure mechanisms, as indicated by the stress and strain distributions in the specimen. The simulations also provide the fracture kinetics under different environments, which can be used to predict the service life of an HDPE specimen with any geometry and applied load.