AN AUTOMATED-METHOD FOR THE MEASUREMENT OF RESIDUAL-STRESS IN MELT-EXTRUDED PLASTIC PIPES

The residual stress in an isotropic plastic pipe is an inherent charac teristic of the conventional manufacturing process via melt extrusion and subsequent rapid cooling. This paper describes an automated test m ethod developed for the measurement of residual stress in a MDPE/HDPE pipe. It is based on the concept of monitoring the deflection of an ax ially slit pipe ring as a function of time. The slit ring creeps in a typical time dependent viscoelastic fashion. The four parameter Voigt model is employed to characterize such behavior and to calculate the c reep parameters including the average through-wall resultant stress. A comparative study of the residual stresses in MDPE pipe samples produ ced under different processing conditions was conducted and its result s demonstrate the relationship between the severity of treatment of th e molten polymer and the level of residual stress in the end use produ ct. (C) 1998 Elsevier Science Ltd. All rights reserved.