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.
A correlation of creep and fatigue crack growth in high density poly(ethylene) at various temperatures
The creep crack growth (CCG) and fatigue crack growth (FCG) behaviour of two PE-HD pipe grades was studied based on a linear elastic fracture mechanics (LEFM) methodology. The FCG-tests were performed under a sinusoidal load at a frequency of 1 Hz and R-ratios (Fmin/Fmax) of 0.1, 0.3 and 0.5; the test temperatures were 23 (only FCG), 60 and 80 °C. The results showed that FCG rates in PE-HD are caused by a combination of cyclic-induced and creep-induced damage, depending on the mean stress level. While for given values of KImax (FCG tests) and KI (CCG tests), respectively, at low test temperatures the cyclic component of the applied stress dominates crack growth rates with CCG rates (R = 1) being lower than the FCG rates (R < 1), at high test temperatures the creep component becomes increasingly important in affecting crack growth rates so that CCG rates even exceed FCG rates. The point of inversion from fatigue to creep dominated failure on the temperature scale apparently depends on molecular and morphological characteristics of the PE-HD type and occurs at around 80 °C for PE-HD 1 and around 60 °C for PE-HD 2 in this investigation.