Modeling of Stress Corrosion Cracking in Plastic Pipes

Stress corrosion cracking (SCC) is commonly observed in the form of a microcrack colony within a surface layer of degraded polymer exposed to mechanical stresses and a chemically aggressive environment. SCC results from strongly coupled mechano-chemical processes. We distinguish four stages of SCC: 1) localized material degradation leading to crack initiation, 2) individual stress corrosion (SC) crack propagation, 3) multiple crack interaction and crack clusters formation, and, 4) instability and dynamic growth of individual crack or a cluster of cracks leading to the ultimate failure. The duration of the last two stages of SCC is relatively short in comparison to the total time of SCC related failure process. Therefore, the duration of the first two stages of SCC serve as a conservative estimate of total time before failure. In this paper, we present a model with consistent results of the second stage of SCC. The proposed model improved the estimation of failure time. The scatter of crack initiation time presents the major part of uncertainty in failure time. Individual SC crack propagation is much more reproducible phenomenon than fracture initiation. Deterministic Crack Layer (CL) theory is employed to model the kinetics and duration of slow SC crack growth. SC crack growth is modeled by a system of nonlinear differential equations, which calls for a numerical solution. Comparison of cracks driven by SC and stress only is presented. Conventional plot of SC crack growth rate vs. the stress intensity factor is constructed and analyzed. In conclusion, an algorithm is discussed for a conservative estimate of the life of engineering thermoplastic submitted to a combine action of mechanical stresses and chemically aggressive environment.