Predicting the Lifetime of Polymer Pipes in Corrosive Environments

Low cost polymers are increasingly used for civil infrastructure applications such as pipes for water and natural gas delivery, corrugated drainage pipes, sanitation pipes, and geomembranes. In these applications, the combination of mechanical loading and environmental conditions can accelerate degradation of the polymer and lead to crack formation and propagation referred to as stress corrosion cracking (SCC). The main feature of SCC is the evolution of multiple small cracks with failure ultimately occurring when one of these cracks propagates. The lifetime of polymer components can vary significantly due to the randomness of mechanical and transport properties of polymer materials. In this project, a numerical model will be developed that combines the fracture behavior with the diffusion of the liquid environment and subsequent degradation of the polymer. A series of experiments will be performed in which the mechanical properties of the polymer are characterized as a function of polymer degradation. The statistical variation in these properties is the cornerstone for predicting the variability in polymer lifetime. This research will advance our understanding of probabilistic SCC in polymers. In addition, it will provide engineers a scientific method to design polymer pipes for a prescribed failure risk, which will improve the structural reliability of pipes used for civil infrastructure applications. The educational plan includes activities targeted for a range of constituents at various levels of expertise: a high school camp that utilizes a web-based tool for pipe design, webinars for polymer industry professionals, and university courses for undergraduates and graduate students.

As compared to existing approaches for design of polymer components, which are primarily based on a deterministic framework, this research is anchored by a probabilistic framework. By including the effects of uncertainties in various material properties (i.e., mechanical properties, chemical diffusivity and molecular weight), the model can capture the overall stochastic response of polymers in a corrosive environment. This research will lead to the following important results: 1) a stochastic chemo-mechanical model for SCC in polymers that characterizes the interactions between the chemical diffusion process, chemical-induced material degradation and crack propagation; 2) a methodology and experimental data to quantify the relationship between degradation of mechanical properties and molecular weight; 3) model validation through an experimental study of crack growth in a corrosive liquid environment; and 4) a reliability-based design method for polymer pipes, which will allow engineers to design pipes for a desirable lifetime at a tolerable failure risk.