Fracture prediction in tough polyethylene pipes using measured craze strength

In this study, an empirical model is developed that predicts the time to failure for PE pipes under combined pressure and deflection loads. The time-dependent craze strength of different PE materials is measured using the circumferentially deep-notched tensile (CDNT) test. In agreement with previous research, results indicate that bimodal materials with comonomer side-chain densities biased toward high-molecular-weight PE molecules exhibit significantly higher long-term craze strengths. A comparison of currently available PE materials with CDNT samples taken from a PE pipe that failed by slow crack growth in service clearly indicates the superior performance of new-generation materials. Using measured craze strength data from the CDNT test, a simplified model for predicting failure in buried PE pipes is developed. Extending previous research, the reference stress concept is used to calculate an equivalent craze stress for a pipe subjected to combined internal pressure and deflection loads. Good agreement is obtained between the model predictions and observed failure times in an experimental test-bed study of pipes under in-service loading conditions. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

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