INFLUENCE OF LONG-TERM ANNEALING ON RESIDUAL STRESS DISTRIBUTION AND QUASI-BRITTLE FAILURE PROPERTIES OF TALCUM REINFORCED PIPE GRADE POLYPROYPLENE
In this study a polypropylene material with talcum reinforcement used for sewer pipes has been subjected to an annealing procedure at 80°C, roughly 60°C above the actual application temperature, in air for a time period of 18 months. As expected, examination of the material showed no significant decrease in mechanical or fracture mechanical properties due to the temperature exposure. However, samples stored at higher temperature showed better resistance against quasi-brittle failure in fatigue tests compared to unconditioned samples. This could mainly be attributed to the decrease of residual stress in the pipe wall. Even though pipes have been annealed for very long times above Tg, residual stress could not be totally relaxed within 18 months.
Residual stress distribution in extruded polypropylene pipes
Abstract 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.
The influence of process history on the ductile failure of polyethylene pipes subject to continuous hydrostatic pressure
Failure times of plastic and metal pipes subjected to hydrostatic pressure at various levels assist pipe manufacturers to not only design pipes for certain applications, but also to give them an indication of the useful service lifetimes of these pipes. In this work, a medium density polyethylene was converted into various pipes by varying extrusion parameters. These pipes displayed low levels of lamellar orientation and a skin-core gradient in morphology. The hydrostatic pressure performance (ductile failure regime at 23°C) of the pipes was observed to depend on morphology, which was inferred by the systematic dependence of the failure times on the tensile behavior of the pipes. It was also found that pipes annealed at 80°C prior to testing displayed 400% longer failure times; annealing was found to increase the crystallinity of the pipe only marginally. Consequently, this performance improvement was attributed to relaxation of some of the frozen-in residual stresses in the pipe. © 2005 Wiley Periodicals, Inc. Adv Polym Techn 24: 226–232, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20044