Residual stress distribution in extruded polypropylene pipes

This paper deals with the experimental determination of residual stress and other inherent properties, density, yield stress, specific enthalpy of fusion and melting point temperature, of a polypropylene grade for pipe applications. The most important of these characteristics is the residual hoop (circumferential) stress, because it has a considerable effect on pipe lifetime. The experimental procedure for obtaining the residual hoop stress distribution by measuring the deflection of axially slit ring specimens is described and the data are evaluated by two approaches - a method based on theory of a curved beam and an older method based on theory of a thick-walled cylinder. Numerical modelling by FEM is employed to simulate the behavior of the ring specimens. The experiment was carried out on a set of 8 polypropylene pipes of different dimensions. The results for these pipes are compared, and a general equation describing the residual hoop stress distribution independently of pipe dimensions is proposed. Also, the influence of axial residual stress on the hoop residual stress is studied and, with the help of FEM, a correction of the results for long pipes is derived.