In this paper, an experimental analysis for determining the fatigue strength of PE-100, one of the most used High Density Polyethylene (HDPE) materials for pipes, under cyclic axial loadings is presented. HDPE is a thermoplastic material used for piping systems, such as natural gas distribution systems, sewer systems and cold water systems, becoming in a good alternative to metals, as cast iron or carbon steel. One of the causes for failures of HDPE pipes is fatigue, due to pipes are under cyclic loading, such as internal pressure, weight loads or external loadings on buried pipes, which generate stress in different directions: circumferential, longitudinal and radial. HDPE pipes are fabricated using an extrusion process, which generates anisotropic properties. By testing in the Laboratory a series of identical specimens obtained directly from PE-100 HDPE pipes in longitudinal and circumferential directions, the relationships between amplitude stress and number of cycles (S-N curves) for two values of test frequency (2 and 5 Hz.) and stress ratio (R = 0.0 and R = 0.5), are established. For each case, three sets of survival probability data (90%, 50% and 10%) and coefficients of Basquin’s equation for the Ps = 50% curves, were obtained. The results obtained are in good agreement with the literature results, showing that stress direction in the pipe, tests frequency and stress ratio affect the fatigue strength of HDPE grade PE-100 pipes.
Evaluation of local friction and pore-water pressure evolution along instrumented probes in saturated clay for large numbers of cycles