Experimental study of stress cracking in high density polyethylene pipes

Stress cracking (SC) is recognized as one of the major concerns for high density polyethylene (HDPE) pipes. SC is a brittle failure that occurs at a stress level lower than the short-term mechanical strength of the material. Many cases of SC have been reported in two types of HDPE pipes: HDPE ducts used in segmental bridges and corrugated HDPE drainage pipes. The causes of SC in these two types of pipes and their stress cracking resistance (SCR) properties are evaluated in this dissertation. Longitudinal cracking was observed in HDPE ducts and it was resulted by circumferential stress inducing from temperature cycles in the field and residual stress in the pipe. Conversely, the circumferential cracking in corrugated HDPE pipes was caused by longitudinal stress from bending and residual stress of the pipe. Majority of the fracture surfaces were covered by the fibril structure indicating that cracks propagated via a slow crack growth (SCG) mechanism. The study confirmed that the notched constant tensile load (NCTL) test can effectively distinguish the SCR of different HDPE ducts and the NCTL test is incorporated into the recommended material specification for quality control of HDPE ducts. Since fatigue lines were observed on the fracture surface, fatigue tests were adopted to estimate the lifetime of the duct under thermal cyclic loading in the field. For the corrugated HDPE pipes, the SCR evaluation focused on the finished pipe in order to incorporate processing effects. Two test methods were developed and evaluated in the study, namely liner and junction tests. The liner test utilizes notched specimens to generate short and consistent failure times, and is good for QA/QC. On the other hand, the junction test challenges the junction where field cracking is observed. Thus, the junction data were used for predicting the long-term SCR of the pipe, and the reliable method was found to be the rate processing method (RPM).